Damp, Condensation
& Home Maintenance

Every house gets wet: rain falls on it, the ground under it is damp, and the people inside breathe, cook, wash and dry clothes. A healthy house is not a dry box but a building that dries out faster than water can do harm. Almost every damp problem comes down to water arriving faster than usual or leaving more slowly than it used to. So start by knowing how your house was designed to handle water, because the right answer for an 1890s terrace is often the wrong one for a 1990s estate house.

1.1 UK Building Types and How Each One Manages Water

British housing falls into four broad families. Many houses mix them, such as a Victorian terrace with a 1980s extension, so look at each part separately.

Solid-wall houses (mostly built before the 1920s)

Older houses usually have solid walls of brick or stone, typically one brick (about 225mm) thick or more, built with lime mortar and lime plaster. The whole wall is porous: in wet weather the outer part soaks up rain, and in dry weather it gives the water back to the air from both faces. Its thickness is what keeps rain from reaching the inside.

The 2022 joint position statement by RICS, Historic England and the Property Care Association calls these traditional buildings: solid walls of permeable materials such as brick, stone and lime-based mortars, plasters and renders, which absorb moisture but let it evaporate when conditions become drier. A traditional wall works because it can dry. Seal it with cement render, plastic paint or waterproof plaster and it stops drying, which lies behind a large share of the damp in old houses.

Cavity-wall houses (from the 1920s onwards)

From the 1920s, and almost universally after the Second World War, external walls were built as two leaves with a gap between them. The outer leaf is expected to get wet; water that gets through runs down its inner face and cannot jump the gap. Metal ties hold the leaves together and should slope slightly towards the outside so water cannot track along them.

The Energy Saving Trust's guidance for telling them apart: houses built after the 1920s are likely to have cavity walls; cavity brickwork shows all bricks laid lengthways, while solid walls show some brick ends (headers); and if the brick is hidden, a brick wall more than about 260mm thick at a door or window opening probably has a cavity.

Timber-frame houses

Many houses from the 1970s onwards have a timber-frame inner structure with a brick or rendered outer skin. Insulation sits between the studs, with a vapour control layer on the room side, a breathable membrane outside the sheathing, then a ventilated cavity. The timber must stay dry, so avoid making large holes in the inside linings and never fill or block the cavity.

Modern airtight houses

Recent houses are much more airtight, so they must be ventilated on purpose. Approved Document F describes natural ventilation with trickle vents and intermittent fans as suitable only for "less airtight" dwellings (design air permeability above 5m³/(h·m²) at 50Pa); tighter dwellings need continuous mechanical ventilation. Block the vents or switch off the system in a new house and condensation follows quickly.

1.2 Breathability, Vapour and Liquid Water

"Breathable" is a loose sales word, so be precise. Water moves through building materials in two forms.

• Liquid water moves by gravity (rain running down a wall), by wind pressure (driving rain forced into cracks) and by capillary action, the wicking effect that draws water through the fine pores of brick, stone, mortar and plaster in any direction, including upwards.
• Water vapour is water as an invisible gas mixed in the air. It moves from where there is more of it to where there is less, which in winter means from the warm, moist inside of a house outwards through the walls and roof.

A vapour-permeable material lets vapour through. Lime plaster and render, limewash and mineral paints are highly permeable; dense cement render, vinyl and acrylic paints and bitumen coatings much less so. Lime and soft brick also move liquid water back to the surface to evaporate. Hard cement render does the opposite: rain gets in through hairline cracks and cannot get out again, so the wall behind ends up wetter than with no render at all. This matters little in a cavity wall and a great deal in a solid one.

1.3 Relative Humidity, Dew Point and Condensation

Warm air can hold far more water vapour than cold air. Relative humidity (RH) is the vapour in the air as a percentage of the most it could hold at that temperature. When moist air touches a cold surface it cools, its RH rises, and at 100% water condenses out. That temperature is the dew point. Any surface colder than the room air's dew point collects condensation: cold glass, an external corner, the wall behind a wardrobe, the underside of a roof.

Worked example: a living room in January

Take a living room at 20°C and 60% RH, with 0°C outside. Using the standard Magnus approximation for vapour pressure, the room air contains water vapour at a pressure of about 14.0 hPa, and its dew point works out at about 12°C. Any surface in the room that falls to 12°C or below will get wet.

A wall's temperature factor (f_Rsi) describes how warm its inside surface stays, from 0 (as cold as outside) to 1 (as warm as the room). BRE Information Paper IP 1/06, cited in Approved Document L, recommends at least 0.75 at junctions in dwellings. With 20°C inside and 0°C outside:

The first wall looks dry, but air at 15°C carrying 14.0 hPa of vapour is at about 82% RH. Mould needs persistently damp surfaces, not liquid water. Approved Document F's criterion for the inside of external walls is a "surface water activity" (surface RH divided by 100) below 0.75 averaged over a month, 0.85 over a week and 0.95 over a day. That is why mould grows on walls that never look wet, concentrated in corners and behind furniture where surfaces are coldest.

The example shows the three levers that recur throughout this handbook:

1. Reduce the moisture in the air (extract it at source, ventilate, dehumidify). At 20°C and 50% RH the dew point falls to about 9°C.
2. Raise the surface temperature (heat the room more steadily, insulate, let warm air reach the corner by moving furniture away).
3. Avoid cold spots (thermal bridges such as concrete lintels, uninsulated reveals and gaps in loft insulation).

1.4 How Humid Should a Home Be?

There is no legal humidity limit for an existing home. The best official yardstick is Approved Document F, which says its moisture criteria are likely to be met in new dwellings if, during the colder months, the moving-average RH in a room stays below:

It is a design criterion for new homes rather than a legal limit, but a sensible benchmark for any house. Short spikes after a shower are harmless if they clear within an hour or so; the average is what matters. A bedroom sitting at 70% every winter morning is heading for mould. Measure over days, not at one moment (section 3.3).

"Damp" is a symptom, not a diagnosis. Four mechanisms account for nearly all damp in British homes, and they need completely different remedies: an injected damp-proof course does nothing for condensation, and a dehumidifier will not stop a leaking gutter. Work out which you have, remembering that a house can have more than one at once.

2.1 The Four Types at a Glance

2.2 Condensation

Condensation is one of the commonest causes of damp and mould in British homes, as the government's 2023 guidance on damp and mould notes. Moisture from breathing, cooking, washing and drying clothes meets a surface colder than its dew point (section 1.3). It appears in predictable places: external rather than internal walls, north-facing rooms, corners, the top of walls where loft insulation stops short, and behind furniture against outside walls. It is seasonal, worst in cold weather and often gone in summer, and it carries no salts.

2.3 Penetrating Damp

Penetrating damp is rain getting in from outside. It needs a lot of water arriving at one place (usually from a blocked or leaking gutter, downpipe or hopper, or from driving rain on an exposed wall) and a route through, such as cracked render, eroded pointing, a failed flashing, a blocked sill drip or a gap round a frame. The giveaway is the link with weather. It can appear at any height, which is why a damp patch high on a wall or on a ceiling is never rising damp. See chapter 5.

2.4 Rising Damp and Other Ground Moisture

Rising damp is ground water drawn up into the base of a wall by capillary action where there is no effective damp-proof course (DPC). Evaporation limits how high it climbs, so it only ever affects the lower part of a wall. It is real, but diagnosed far more often than it occurs. Damp at the base of a wall is often caused instead by ground or paving built up above the DPC, leaking gutters or downpipes, condensation on the cold bottom of a wall, hygroscopic salts left by an old problem, or impermeable render or plaster that has stopped an old wall drying. Chapter 6 covers the evidence.

2.5 Plumbing Leaks

Escaping water from pipes, tanks, radiators and sanitary ware causes a patch that grows regardless of the weather. Leaks below baths and showers often come from failed sealant or grout and run only when the shower is used. Quick checks:

1. With every tap and appliance off, read the water meter, wait an hour without using water, and read it again. Movement means a supply-side leak.
2. On a sealed (combi or system) heating circuit, a steady fall in boiler pressure suggests a heating leak. See the UK Plumbing Handbook, section 7.3.
3. For a suspected shower leak, stop using the shower for a week and see if the patch dries.

Good diagnosis relies on many small pieces of evidence pointing the same way, not on one instrument reading. The 2022 joint position statement on investigating moisture in traditional buildings (RICS, Historic England and the Property Care Association, also adopted by SPAB and the IHBC) asks surveyors to take a whole-building approach, look for the source of moisture, and not rely on potentially misleading data. A householder should do the same.

3.1 A Step-by-Step Method

Work through these steps in order, writing down what you find with dated photographs.

1. Map the pattern. Sketch the wall and mark the damp, staining or mould. A band along the base, a patch high up, a ring round a window, a stripe under a pipe, or spots in corners? External, internal or party wall?
2. Go outside to the same spot and look up and down: gutter and downpipe, pointing, render, sill, ground level, airbricks, pipes and overflows. Many damp problems are solved at this step.
3. Note the season and weather. Worse after rain, in cold weather, after showers or cooking, or regardless? Photograph it weekly for a month.
4. Look for salts and tide marks. White crystals, bubbling paint and powdery plaster mean water has been moving through masonry and evaporating. Condensation leaves no salts. A brown tide mark is typical of ground moisture or a long-running leak.
5. Look for mould. Black spotty mould in corners and behind furniture is the classic sign of condensation, though mould grows on any damp surface.
6. Check the services (section 2.5).
7. Measure the air for a week (section 3.3). High humidity points to condensation; normal humidity with a wet wall points elsewhere.
8. Only then measure the wall, understanding the instrument's limits (section 3.2).

3.2 Why Pin Moisture Meters Mislead on Masonry

Hand-held "damp meters" measure electrical resistance between two pins or capacitance through a pad. Neither measures water directly. They are calibrated for timber; on brick, plaster and mortar they report conductivity as if it were moisture, and many things other than water make masonry conductive:

• Salts. Walls near the ground or once damp accumulate soluble salts, which conduct even when fairly dry. BRE Digest 245 notes that salts can accumulate in old walls without a damp problem, so high meter readings alone are not conclusive. The RICS, Historic England and PCA joint statement expects surveyors to understand how salts affect resistance meters.
• Hygroscopic salts also pull moisture from the air. Common salt (sodium chloride) starts absorbing water vapour above about 75% relative humidity, so a salty wall can be genuinely damp because of the room's humidity alone.
• Carbon and foil. Some old mortars and plasters made with ash or soot contain carbon, and foil-backed plasterboard or wallpaper sends meters off the scale.
• Surface moisture. Condensation on the surface gives a high reading that says nothing about the wall behind.

Meters are still useful comparatively, run along a wall at one height to map where readings change, and on timber such as skirtings. They cannot prove rising damp.

3.3 Better Ways to Measure

Drilled samples: carbide and gravimetric testing

The reliable method is to sample the masonry itself. A surveyor drills into a mortar joint or brick with a slow-speed bit at several heights and at least two depths, and collects the dust.

• Calcium carbide meter. A weighed sample is sealed in a pressure vessel with calcium carbide; water in it produces acetylene gas, and the gauge reads moisture content by weight. Results in minutes on site.
• Gravimetric analysis. Sealed samples are weighed, oven-dried and reweighed. A second test at a set high humidity measures the hygroscopic moisture content, separating water arriving from a source from water the salts simply attract. BRE Digest 245, Rising damp in walls: diagnosis and treatment, describes this method.

The profile tells the story. Rising damp gives a base that is wet deep inside and drier with height. Condensation gives a wet surface over a drier core. Penetrating damp is wettest near the outside face and the defect.

Hygrometers, data loggers and surface temperatures

An inexpensive digital thermo-hygrometer is accurate enough to show trends. A data logger (many cheap ones log to a phone app) left in the problem room for a week shows the overnight rise in a bedroom, the spike after a shower and how fast each clears; compare the averages with section 1.4. An infrared thermometer or phone-mounted thermal camera finds cold spots. If a corner's surface temperature is at or near the room's dew point (many hygrometers display it), you have found a condensation risk.

3.4 Common Symptoms and What They Usually Mean

Condensation is the damp problem most within a householder's control. The cure is rarely one product: it is removing moisture at source, giving the rest a way out, and keeping surfaces warm enough that the vapour stays in the air.

4.1 Where the Moisture Comes From

Breathing, cooking (especially pans without lids), showers and baths, drying clothes indoors and unvented tumble dryers all put water into the air every day. Paraffin and bottled-gas heaters add more as they burn, and a newly plastered or flooded house releases a great deal as it dries. The government's damp and mould guidance (section 4.5) is clear that everyday cooking, bathing and laundry unavoidably produce moisture, and that the fundamental causes of damp and mould are building deficiencies, inadequate ventilation, inadequate heating or poor energy efficiency, not tenants' normal activities. The aim is not to stop living normally but to remove moisture as fast as the household produces it.

4.2 Ventilation: Getting Moist Air Out

Extract at source

The most effective ventilation removes moist air where it is made. Approved Document F, Volume 1 (2021, Table 1.1; unchanged in the 2026 edition) sets these minimum intermittent extract rates for new work:

The same document says a recirculating cooker hood on its own does not provide compliant ventilation (it returns the moisture to the room), that fans should be as high as practicable and within 400mm of the ceiling, and that in a room with no openable window an intermittent fan should run on for at least 15 minutes after the room is vacated. In practice:

• Test fans by holding a sheet of toilet paper to the grille; a working fan holds it there. A weak fan may have a blocked grille, a crushed duct or a failing motor.
• Humidity-controlled fans or fans with an overrun timer work far better than ones that stop with the light.
• Shut the bathroom or kitchen door while showering or cooking, with the fan on, and leave it running until surfaces clear.
• Ducts through a cold loft should be insulated, so moisture does not condense inside them and drip back.

Background ventilation

Trickle vents in window heads give a steady trickle of fresh air that dilutes moisture day and night. Keep them open, especially in bedrooms. Approved Document F expects replacement windows to keep existing trickle vents at no smaller a size, and where there were none, not to leave ventilation worse than before; one accepted way is trickle vents of at least 8,000mm² equivalent area in habitable rooms and kitchens and 4,000mm² in bathrooms. Never block airbricks, which ventilate suspended timber floors.

PIV and MVHR

Positive input ventilation (PIV) is a fan, usually in the loft, that blows filtered and often slightly warmed air into the house through a landing ceiling diffuser; the slight positive pressure pushes moist air out through vents and gaps. It can help older houses with persistent condensation. Mechanical ventilation with heat recovery (MVHR) continuously extracts from wet rooms and supplies fresh air to living spaces, recovering heat between the two. It is designed for airtight houses and is rarely a sensible retrofit in a leaky older one. Clogged MVHR filters are a common cause of condensation in new homes; clean or replace them as the manufacturer specifies.

4.3 Dehumidifiers

A dehumidifier is very effective in a closed room, for drying washing or getting a damp room under control while you fix the causes, but it treats the symptom and costs electricity to run.

Sizing. Extraction rates in litres per day are measured at warm, very humid test conditions (manufacturers commonly quote 30°C and 80% RH), so a unit in a British room at 18°C will remove far less. Use the figure to compare models, not to predict performance. A small unit suits one room; choose a larger one for drying out after a leak. Look for a humidistat (55% is a sensible setting) and a continuous drain option, and check the operating temperature range: manufacturers generally advise desiccant models for rooms below about 15°C.

4.4 Heating Patterns and Drying Clothes

Short bursts of heat in a cold house warm the air but leave the walls cold, so when the heating goes off, moist air condenses on them. A steadier, lower background temperature keeps surfaces warmer. Shutting off an unheated spare room makes it the coldest room, and moisture from the rest of the house condenses there; keep its door closed and its radiator on low. Where cost is the constraint, insulation and heating controls are the long-term answer (chapter 9). Bleeding radiators is covered in the UK Plumbing Handbook, section 7.4.

A load of wet washing puts all its water into the room. In order of preference: dry outside when possible; use a vented tumble dryer ducted outside, or a condenser or heat-pump dryer; otherwise dry on an airer in one room with the door shut and a window open, the extractor running or a dehumidifier on. Avoid draping clothes on radiators in living rooms and bedrooms.

4.5 Health: What the Official Guidance Says

Understanding and addressing the health risks of damp and mould in the home was published on GOV.UK in September 2023 by the Office for Health Improvement and Disparities, the UK Health Security Agency and the Ministry of Housing, Communities and Local Government, following the coroner's report on the death of two-year-old Awaab Ishak in December 2020 from prolonged exposure to mould in his home. It was last updated in 2026. Its key points:

• Damp and mould produce allergens, irritants, spores and toxins; dampness alone, even without visible mould, can increase health risks.
• Effects mainly involve the airways and lungs (cough, wheeze, breathlessness, infections, worse asthma and allergic rhinitis), and can affect eyes, skin and mental health.
• Those at greater risk include people with asthma, COPD, cystic fibrosis or heart conditions, weakened immune systems, pregnant women and new mothers, children, older people and those who are bedbound or housebound.

Speak to a GP or pharmacist about symptoms. Landlords should not wait for medical evidence before acting.

4.6 Landlords' Duties and Awaab's Law

Under the English social housing rules a landlord must: investigate and make safe an emergency hazard within 24 hours (or offer alternative accommodation); investigate a significant hazard within 10 working days; give the tenant a written summary within 3 working days of the investigation concluding; and carry out safety work within 5 working days, beginning any further preventative work in that time or physically starting it within 12 weeks. Report damp in writing, keep copies and dated photographs, and if a social landlord fails to act, use its complaints process and then the Housing Ombudsman.

4.7 Treating Mould Safely

Cleaning removes the symptom; mould returns unless the cause is fixed. Clean it promptly anyway, following the government guidance:

1. Wear a mask, gloves and goggles, whatever the size of the area. People with asthma or other vulnerabilities should not do the cleaning.
2. Shut the door to the rest of the house and open the window, during and after.
3. Do not dry-brush mould or vacuum it with an ordinary vacuum; both spread spores.
4. Use a mould and mildew product on hard surfaces, following the label; the guidance prefers these to bleach for health and safety reasons. Never mix cleaning products.
5. Wipe with disposable cloths, bag them and bin them, and let the surface dry completely.
6. Mouldy carpets, soft furnishings and ceiling tiles may have to be thrown away. Clothes can usually be washed.
7. The guidance expects householders to handle condensation and very small amounts of mould; larger areas should be dealt with by professionals.

Before redecorating, see the Painting & Decorating Manual, section 2.3, on fungicidal washes and stain-blocking primers. Anti-mould paint buys time; it does not cure condensation.

Penetrating damp is almost always a maintenance problem, which is good news: the cure is usually a visible repair, not a treatment. Water can travel inside a wall or along a timber before it shows, so start immediately outside the damp patch and work upwards and sideways.

5.1 Gutters, Downpipes and Hoppers

In a storm, a blocked gutter or leaking joint pours a huge volume of water down one small area of wall. Leaking and blocked rainwater goods are among the commonest causes of penetrating damp in old houses, and among the cheapest to fix. Look, ideally in heavy rain, for:

• Water overflowing the front of a gutter (blockage, or a sagging gutter falling the wrong way), or dripping from joints.
• Green algae streaks or dark staining on brickwork below gutters and round downpipes, which show on dry days where water runs.
• Cracked downpipes, often at the back where you need a mirror to see; cast iron rusts through on the face against the wall.
• Downpipes discharging onto the ground at the wall foot or into a blocked gully, splashing the base of the wall.
• Hoppers full of leaves.

Repairs, gutter falls and Approved Document H are covered in the UK Plumbing Handbook, section 8.4, and blocked gullies in section 8.2. Painting gutters and fascias is in the Painting & Decorating Manual, section 7.2.

5.2 Pointing: Lime Mortar and Cement Mortar

Sound pointing sheds rain; eroded joints let water collect on brick edges and soak in. Houses built before cement came into general use were built with lime mortar, which is deliberately softer and more permeable than the bricks: moisture leaves the wall through the joints, and frost and salt damage happen in the mortar, which can be renewed, rather than in the bricks, which cannot. Historic England's Repointing Brick and Stone Walls is clear that cement-based mortar should not be used to repoint historic walls because it is too strong, inflexible and impermeable. Repoint a soft old wall in hard cement and water is forced out through the brick faces instead, which then crumble (spalling) around proud grey joints.

• On pre-1920s solid walls, repoint in a lime mortar matched to the original in colour, texture and strength. A conservation builder will choose between natural hydraulic lime and lime putty mortars for the masonry and exposure.
• Rake out only loose or eroded joints, by hand rather than with an angle grinder, to a depth of at least twice the joint width.
• Finish flush or slightly recessed, never as a raised "strap" joint, and keep mortar off the brick faces.
• Protect lime work from frost, hot sun and drying wind while it cures.
• On modern cavity walls, use a cement-lime mortar weaker than the bricks.

5.3 Render Cracks and Failed Sealant

Cracked render lets driven rain in behind it, where it is trapped. Hard cement render on an old solid wall is the worst case (section 6.3). Tap it with a screwdriver handle: a hollow sound means it has come away. Fine cracks in sound render on a modern wall can be filled with flexible exterior filler and repainted; hollow or widely cracked areas need cutting back and patching or re-rendering. On a pre-1920s wall consider replacing cement render with lime, or leaving the masonry exposed if it was built to be.

Sealant round frames and at junctions of materials hardens, shrinks and pulls away with age. If it is brittle or has lost its grip, cut it all out, clean the joint and apply a new exterior sealant suited to the materials (check the data sheet). Never run new sealant over old.

5.4 Chimneys and Flashings

Chimney stacks are fully exposed and often neglected. Damp on a chimney breast, especially upstairs or in the loft, usually comes from:

• Flashings: the lead apron, side steps and back gutter where the stack meets the roof can crack, lift or slip; old cement fillets crack away.
• Stack pointing and flaunching: the sloping mortar round the pots cracks and lets water straight into the brickwork.
• Open flues: fit a ventilated cap to disused flues, and leave a vent at the fireplace end too, so the flue stays ventilated and dry. Never seal a flue airtight.
• Salts from past fires: tars and hygroscopic salts cause brown staining that comes and goes with humidity long after the last fire; the plaster may need replacing once the stack is sound.

Flashing and stack repairs are work for a roofer.

5.5 Window Sills and Drips

An external sill must slope away from the window and project beyond the wall face, with a drip (a groove or edge) on its underside near the front. Water running round the front edge cannot cross the groove, so it drops clear instead of running back under the sill and down the wall.

• Drip filled with paint, filler or sealant: clear it with a screwdriver tip or narrow chisel and repaint without bridging it again.
• No drip, common on replacement or rendered-over sills: a drip bead or batten can sometimes be fitted, or the sill replaced.
• Sill too short to project past the wall face, often after external render or insulation is added: an extended sill or sill cover is needed.
• Cracked or rotten sills and failed sealant at the sill ends, both of which let water into the wall below. Timber sill repair is in section 8.2.

Copings, parapets and projecting string courses need drips for the same reason.

5.6 Ground Levels Against Walls

Approved Document C expects an external wall's damp-proof course to be at least 150mm above the adjoining ground, keeping rain splash and wet soil away from it. Over the years flower beds, relaid patios and resurfaced drives often close that gap, and the base of the wall gets damp in a way that looks exactly like rising damp. Walk round and check:

• Can you see the DPC (slate, bitumen felt or plastic in a joint, or a course of dense engineering bricks), and is it about two brick courses above the ground?
• Does paving fall away from the house?
• Are airbricks clear and above ground?
• Is render carried down over the DPC? Cut it back to stop just above, ideally on a bell-mouth or stop bead.

Where ground is too high, lower it, or form a shallow gravel-filled channel against the wall that drains to a proper outlet. Do not dig below or undermine the foundations. Houses without a DPC benefit just as much. Soakaways are in the UK Plumbing Handbook, section 12.2.

5.7 Cavity Wall Insulation Problems

Cavity insulation saves a great deal of heat and is trouble-free in most homes, but it removes the gap that stopped water crossing to the inner leaf. Approved Document C says suitability should be assessed using its map of exposure to driving rain (Diagram 12) and table, or a recognised calculation, with the work done under appropriate certification. Warning signs are damp patches on the inside of the most exposed walls appearing after installation, patches at window edges or along mortar courses, and outer-leaf defects letting water saturate the fill.

Repair external defects and rainwater goods first, since these are often the real fault. If damp persists, an independent surveyor can inspect the cavity with a borescope; defective insulation may have to be extracted. Many installations carry a 25-year guarantee from the Cavity Insulation Guarantee Agency (CIGA) that passes to later owners. The Welsh Government has published guidance on issues arising from cavity wall insulation.

No area of home maintenance generates more argument, or more unnecessary spending, than rising damp. The remedial treatment industry, represented by the Property Care Association (PCA), holds that rising damp is a genuine problem in walls without an effective damp-proof course and that a correctly specified remedial DPC is an effective cure. Conservation bodies including Historic England and the Society for the Protection of Ancient Buildings (SPAB) accept that it exists but argue that it is much rarer than it is diagnosed, that most damp at the base of old walls has other causes, and that injected DPCs and waterproof replastering often do more harm than good in traditional buildings. This chapter sets out the evidence.

6.1 What Rising Damp Is

Ground water in contact with the base of a porous wall is drawn up by capillary action until the rate of rise is balanced by evaporation from the wall faces. The height depends on the materials, how wet the ground is and how freely the faces dry; anything that stops the faces drying, such as cement render, waterproof plaster or impervious paint, pushes the damp higher. Since the late nineteenth century, walls have usually had a DPC near ground level (slate or engineering bricks, later bitumen felt and plastic), and Approved Document C still requires one at least 150mm above the adjoining ground.

6.2 The Controversy and the Evidence

Several points are widely accepted, including in the remedial industry's own professional standards:

• Moisture meters are calibrated for timber and strongly affected by salts (section 3.2); BRE Digest 245 warns that high readings alone are not conclusive.
• Many other sources wet the base of a wall: ground and surface drainage, leaking rainwater goods, condensation, bridged DPCs, leaks and hygroscopic salts. The building pathologist Tim Hutton, writing on the Building Conservation website (buildingconservation.com), argues that defective ground and surface drainage is the most common source.
• Water-based injection products may only form an effective water-repellent band in a wall that is reasonably dry when treated.
• The 2022 joint position statement by RICS, Historic England and the PCA expects surveyors to understand how salts affect resistance meters, take a whole-building approach, find the source of moisture, and declare any conflict of interest where the diagnoser also sells the remedy.

That the PCA, RICS and Historic England signed a common statement matters. The disagreement now is less about whether rising damp exists than about how often it is the real cause and how to treat traditional buildings. For a householder, rising damp should be a conclusion reached after other causes have been ruled out with evidence, not a starting assumption.

6.3 The Real Causes of Damp at the Base of Walls

Bridged damp-proof courses

A DPC only works if water cannot go round it. Common bridges: soil, paths or patios at or above the DPC (section 5.6); render carried down to the ground; internal plaster carried down onto a solid floor; mortar droppings piled in the cavity above DPC level; and a new solid floor whose membrane is not lapped to the wall DPC.

Raised ground on houses without a DPC

Many older houses never had a DPC. They coped because the ground outside was lower and well drained, and lime plasters, limewash and timber floors let the wall base dry. A century of raised paths, concrete yards and tarmac changes that. Lowering the ground and improving drainage often solves the problem without treating the wall.

Impermeable renders, plasters and paints on solid walls

This is the most widespread self-inflicted cause of damp in old houses. Cement render outside, dense sand-and-cement or gypsum plaster inside, plastic masonry paints and concrete floors each reduce a solid wall's ability to dry, so moisture builds up, rises higher and concentrates salts at the edge of the impermeable layer. A tide mark just above a band of cement replastering, often from an earlier damp "treatment", is typical. The cure is to reverse the change where possible: lime render and plaster, breathable paints (Painting & Decorating Manual, section 6.1), and a breathable floor when the chance arises. Case study 3 (section 12.3) follows an example.

6.4 Injected DPCs: What They Can and Cannot Do

An injected DPC is a water-repellent fluid or cream inserted into holes drilled along a mortar course near the wall base. BRE Digest 245 describes chemical injection, and physical membranes inserted into a cut slot, as the options where a new barrier is genuinely needed; the code of practice is BS 6576. Before agreeing to one, understand that:

• It addresses only one cause. If the water comes from a gutter, high ground, condensation or salts, it will not help.
• Replastering is usually part of the package, typically stripping plaster to a height of around a metre and replastering in a salt-resistant cement-based or specialist plaster. That hides symptoms whether or not the injection works, and on a traditional wall reduces its ability to dry. Historic England and SPAB advise against injected DPCs, cement plasters and waterproofing treatments on historic buildings.
• The wall still has to dry, which takes months.
• Guarantees usually cover only the injection and may require you to maintain ground levels and gutters. Check whether they are insurance-backed and transferable.

In a modern house with a failed or bridged DPC, a remedial DPC may be right once bridging and drainage are fixed. In an old house with lime mortar and no original DPC, start with ground levels, drainage and breathable finishes, and take conservation advice before anyone drills the wall.

6.5 Getting an Independent Opinion

Separate the diagnosis from the sale. A free survey by a company that sells damp-proofing has an obvious conflict of interest, however honest the surveyor. A fee for an independent report usually costs far less than an unnecessary treatment.

Ask any damp surveyor in advance: will you inspect outside as well as in; will you measure humidity and surface temperatures; will you take drilled samples rather than rely on a meter; do you or your firm sell any remedy you might recommend; and will the report explain the evidence and list non-invasive remedies first? If a mortgage lender asks for a "damp and timber report", ask whether a report from an independent surveyor will satisfy it.

The roof takes more weather than any other part of the house. Most inspection can be done safely from the ground and from inside the loft; repairs on the roof itself are for a roofer.

7.1 Inspecting a Roof Safely from the Ground

Never climb onto a roof to inspect it. Binoculars, a zoomed phone camera and a walk round the house (and across the road) will show most of what you need:

• Slipped, cracked or missing slates or tiles. Slates hang on nails that corrode with age ("nail sickness"); many slipped slates suggest the whole roof is near the end of its life.
• Ridge and hip tiles with cracked or missing mortar.
• Flashings at chimneys, abutments, dormers and roof windows (section 5.4), and valleys choked with debris or split.
• Heavy moss, which holds water and blocks gutters. Avoid pressure washing roofs, which can drive water under slates and damage tiles.
• Sagging ridges or slopes, which need a surveyor.

Flat roofs on extensions and dormers can often be seen from an upstairs window: look for ponding, blisters, splits, gaps at upstands and blocked outlets.

7.2 From the Loft Side

On a dry day and again after heavy rain, go into the loft with a torch, walking only on boards or joists. Look for daylight through the covering (in an unlined roof), stains on rafters, battens and sarking, torn or sagging underlay, damp brickwork or timber round the chimney, gaps in the party wall (a fire risk), and signs of rot or fresh insect bore dust.

7.3 Loft Condensation and Ventilation

A well-insulated loft is cold, and warm moist air leaking into it from the house can condense on the underlay, timbers and nail heads. Signs are droplets on a cold morning, rusty nails, black mould on sarking boards and damp insulation. There are two defences, and both matter:

1. Stop moist air reaching the loft. Fit a draught-stripped, insulated hatch (Approved Document L says hatches in a roof insulated at ceiling level should be designed and installed to ensure optimum airtightness). Seal gaps round pipes, cables and the soil stack. Make sure kitchen, bathroom and dryer extracts discharge outside, not into the loft; disconnected ducts are a common hidden cause.
2. Ventilate the cold roof space, usually at the eaves, sometimes with ridge or tile vents. Do not let insulation block the eaves: eaves ventilator trays hold an airway open while the insulation is taken out to the wall head. With a vapour-permeable underlay, follow its manufacturer's ventilation requirements.

7.4 Loft Insulation Depth

The government's damp and mould guidance singles out improving loft insulation to a minimum of 270mm as a particularly effective way of keeping homes warmer, and 270mm of mineral wool (typically about 100mm between the joists and the rest laid across them) is the depth widely recommended by the Energy Saving Trust.

Approved Document L, Volume 1 (2021, incorporating 2023 amendments) expresses the standard as a U-value: 0.16 W/(m²·K) for a renovated roof in an existing dwelling, with a typical construction of at least 250mm of mineral or cellulose fibre between and across the joists. Other materials reach the same U-value at different thicknesses; check the data sheet. The 2026 edition, taking effect on 24 March 2027 for most building work in England, keeps the same 0.16 value for existing roofs.

• Topping up loft insulation is not in itself one of the works Approved Document L treats as renovating a roof, but keep the eaves ventilation clear.
• Do not cover recessed downlights unless they are rated for it; lay cables on top of the insulation, not buried under it, where they can overheat.
• To board for storage, use raised loft legs so the insulation is not squashed.
• Wear gloves, long sleeves, goggles and a dust mask when handling mineral wool.

7.5 Water Tanks and Pipes in the Loft

An insulated loft gets very cold, so anything above the insulation can freeze.

• Do not insulate the ceiling directly under the cold water tank; a little rising heat helps stop it freezing. Insulate the tank's sides and top instead and bring the loft insulation up to meet it.
• The tank needs a close-fitting lid that is not airtight and a screened overflow (UK Plumbing Handbook, section 1.2).
• Lag every pipe in the loft, including overflow and vent pipes, and tape joints and bends.
• Check old galvanised tanks for corrosion and make sure the ballvalve shuts off properly; an overflowing tank can soak a ceiling.

External maintenance is mostly about keeping paint on timber, joints sealed, pointing sound and the ground draining away from the house. Painting methods are in the Painting & Decorating Manual; this chapter covers what to look for and why.

8.1 Brickwork, Spalling, Efflorescence and Cracks

Spalling is the flaking or crumbling of a brick or stone face, from frost or salt crystallisation in saturated masonry. The cause is nearly always too much water at that spot: a leaking gutter, a missing coping, splashing at ground level, or hard cement pointing or render forcing evaporation through the brick faces (section 5.2). Fix the water first; then a bricklayer can cut out and replace badly spalled bricks. Do not patch with cement or seal the wall with water-repellent coatings or plastic paints, which trap moisture and salts.

Efflorescence is the white salt deposit left as water evaporates from masonry. On new brickwork it usually fades as the salts wash out. On older walls it marks where water is moving: below a leaking gutter, at the wall base, round a chimney. Brush it off dry; washing just carries the salts back in. Persistent efflorescence means a persistent water source. The Painting & Decorating Manual covers both before painting in section 6.2.

Cracks in mortar joints are common and often harmless. Cracks wider than a few millimetres, diagonal cracks through bricks, cracks that taper or keep growing, or cracks with sticking doors and windows can indicate movement and should be seen by a chartered surveyor or structural engineer. Monitor with dated pencil marks or a plastic crack monitor, and tell your insurer early if you suspect subsidence (section 11.4).

8.2 Timber Windows and Doors

Timber joinery lasts a very long time while its paint film is intact. Rot starts where water sits: sash bottom rails, bottom corner joints, sills and the foot of door frames.

• Inspect each spring and autumn, pressing a bradawl into sills, bottom rails and frame feet; rotten wood is soft.
• Keep the paint unbroken, touching up cracks and bare patches before water gets in. Follow the paint manufacturer's repaint intervals; south- and west-facing elevations usually need attention first.
• Glazing putty that is cracked or missing lets water rot the glazing bars. Replace it and paint over it, lapping the paint slightly onto the glass.
• Small areas of rot can be cut out and filled with two-part wood filler or pieced in with new timber; a joiner can make new sections. Repair is often cheaper, and better for an old house, than replacement.

See the Painting & Decorating Manual, section 7.1 (exterior woodwork) and section 4.3 (windows).

8.3 uPVC Windows and Doors

• Clean with warm soapy water and a soft cloth; avoid abrasives and solvents unless the manufacturer approves them.
• Drainage slots along the outside of the bottom frame let water out of the frame chambers. Clear them with a pipe cleaner; blocked slots often cause water on the inside sill.
• Hinges and locks: clean and lubricate once or twice a year as the manufacturer specifies. A stiff multipoint door lock often means a misaligned door; adjust the hinges rather than forcing the handle.
• Gaskets harden with age and can be replaced. Misting between panes means a failed sealed unit, which can be replaced without changing the frame.

8.4 Fascias, Soffits and Bargeboards

Rotten timber fascias let gutters sag and leak; check them when clearing gutters. Painting them is covered in the Painting & Decorating Manual, section 7.2.

8.5 Fences, Sheds and Decking

• Fence posts fail at ground level. Use gravel boards so panels do not touch the soil, and keep soil from piling against them.
• Panels and sheds need an exterior wood treatment every few years; check shed bases drain and roof felt is intact.
• Decking grows slippery algae: clean it each spring, check for loose or rotten boards, and make sure it does not cover airbricks or rise above the house DPC.
• Boundaries: responsibility for a fence depends on the deeds, if they say anything; the "fence on your left" idea is not a legal rule.

8.6 Paths, Drives and Front Gardens

Hard surfaces should fall away from the house and stay below the DPC (section 5.6). In England, householder permitted development rules mean:

• No planning permission is needed for a new or replacement front-garden drive of any size if it uses permeable surfacing (gravel, permeable block paving, porous asphalt) or drains to a lawn or border.
• Planning permission is needed if more than five square metres is covered with impermeable material that does not drain to a permeable area.

Listed buildings, flats and some conservation areas differ, and a dropped kerb needs separate permission from the highway authority. GOV.UK's Guidance on the permeable surfacing of front gardens gives practical advice. Wales, Scotland and Northern Ireland have their own rules.

8.7 Trees Near Houses

On shrinkable clay soils, particularly common in London and south-east England, tree roots dry the soil in summer and shallow foundations can subside; removing a large established tree can cause the opposite, heave. Trees also fill gutters, shade walls and can damage drains.

• Take advice from a structural engineer or arboriculturist before removing a large tree close to a house on clay.
• Trees under a Tree Preservation Order need council consent for most work. In a conservation area you must give the council six weeks' notice (a section 211 notice) before cutting down or working on most trees over 75mm in diameter measured 1.5m above ground.
• Cut ivy at the base and let it die back rather than ripping it off an old wall, which can pull out the mortar.

A warmer house is generally a drier one, but energy-saving work changes how a building handles moisture. Approved Document F warns that many existing dwellings are ventilated by infiltration through gaps, and that energy efficiency measures can reduce it and leave the house under-ventilated; the government's damp and mould guidance likewise notes that improving airtightness may require better ventilation to avoid raised humidity and condensation. Every time you reduce heat loss, ask what happens to the moisture.

9.1 Draught-Proofing Without Losing Ventilation

A draught is uncontrolled cold air where you do not want it; ventilation is controlled air exchange where you need it. Block the first and keep the second.

9.2 Insulation Basics

Roughly in order of cost-effectiveness: loft insulation (section 7.4), cylinder and pipe insulation, draught-proofing, cavity wall insulation where suitable (section 5.7), underfloor insulation, then solid wall insulation. Each affects moisture:

• Loft insulation makes the loft colder; keep the eaves ventilated and moist air out.
• Cavity insulation counts under Approved Document L as renovating a thermal element, so it must comply with the Building Regulations, through building control or an installer registered with a competent person scheme covering the work.
• Underfloor insulation must not block the airbricks that keep suspended floor joists dry.
• Replacement windows make a house tighter; they must not leave ventilation worse than before (section 4.2).

9.3 Insulating Solid Walls: the Condensation Risk

Approved Document L treats internal dry-lining and external cladding or rendering as renovating a thermal element. Where more than half of a wall is renovated, the whole wall should reach a U-value of 0.30 W/(m²·K) if technically, functionally and economically feasible; a lesser standard may be acceptable, and the document specifically allows for this where needed to comply with Part C on interstitial and surface condensation.

Internal wall insulation keeps the room warm but makes the masonry behind it colder and slower to dry. Vapour from the room can condense unseen on the cold masonry (interstitial condensation), embedded joist ends are at risk of rot, and rain absorbed from outside dries more slowly. Vapour-closed systems need a perfectly sealed vapour control layer; vapour-open, capillary-active systems such as wood fibre let moisture move and dry. On traditional walls, Historic England's guidance generally favours vapour-permeable materials compatible with the existing fabric. In all cases the wall must first be sound and dry, and insulation must be continuous at reveals, floors and ceilings, or the edges become cold spots.

External wall insulation keeps the masonry warm and dry and saves room space, but changes the appearance (it may need planning permission and is rarely acceptable on listed buildings), and its detailing at eaves, sills, ground level and openings is critical. Sills must be extended to keep a working drip (section 5.5), and insulation must stop above the DPC unless designed for ground contact.

9.4 Energy Performance Certificates

An EPC rates a home's energy efficiency from A to G and lists suggested improvements. In England and Wales one is needed when a home is built, sold or rented, and it is valid for 10 years; you can look any home up on the government's EPC register. Private landlords currently may not let a home rated below E without a registered exemption, and in January 2026 the UK government confirmed it intends to raise this to C for all private tenancies by 1 October 2030, with a £10,000 cost cap, subject to legislation. An EPC is a standardised model, not a measurement, and says nothing about damp or ventilation; check any recommendation for solid wall insulation against section 9.3.

Most damp and decay starts with a small defect left unnoticed for a year or two. A routine of seasonal checks, most taking minutes, catches them early. Adjust timings to your area; leaf fall and frosts vary across the country. A one-page checklist is in section 14.1.

10.1 Spring (March to May)

10.2 Summer (June to August)

Walls are driest, paint and mortar cure properly, and lime work is safe from frost.

10.3 Autumn (September to November)

The most important season for damp prevention.

10.4 Winter (December to February)

10.5 Safety Checks All Year Round

• Smoke alarms: the government's Fire Kills campaign and fire services advise testing at least once a month. Have at least one on every storey.
• Carbon monoxide alarms: fit one in every room with a fuel-burning appliance, test monthly, and replace at the end of the life printed on it. Since 1 October 2022, Approved Document J has expected a CO alarm meeting BS EN 50291 whenever a new or replacement fixed combustion appliance (other than a gas cooker) is installed in a home in England.
• Rented homes in England: under the Smoke and Carbon Monoxide Alarm (England) Regulations 2015, amended in 2022, landlords must provide a smoke alarm on each storey with living accommodation and a CO alarm in any room with a fixed combustion appliance (excluding gas cookers), check them on the first day of a tenancy, and repair or replace faulty alarms once told.
• Gas: if you smell gas, call the National Gas Emergency Service on 0800 111 999, turn off the gas at the meter if safe, open windows and do not use electrical switches.

Call the right person for the stage you are at: someone to diagnose, then someone to repair. Mixing the two, especially for damp, is where most money is wasted.

11.1 Who to Call for What

11.2 Surveys

The RICS Home Survey Standard has three levels. Level 1 is a basic condition report; Level 2 suits conventional houses in reasonable condition; Level 3, a full building survey, is the right choice for older, altered or run-down houses and wherever you suspect damp. A mortgage valuation is not a survey. A survey that recommends a "specialist damp and timber report" is prompting investigation, not diagnosing; ask what evidence was found and consider an independent damp investigation (section 6.5).

11.3 Getting and Comparing Quotes

1. Write a short specification so every contractor prices the same job.
2. Get at least three written quotes (a fixed price, not an estimate) for significant work.
3. Check what is included: access and scaffolding, waste removal, making good, VAT, and any Building Regulations notification.
4. Ask why the problem happened, not just how to fix it.
5. Check registrations, insurance and recent references. TrustMark is the government-endorsed quality scheme for work in the home.
6. Avoid large up-front payments; stage payments against completed work. Keep invoices, certificates and guarantees with the deeds.

11.4 Buildings Insurance

Buildings insurance usually covers sudden events such as storm, flood, fire, escape of water and often subsidence, but generally not gradual deterioration, wear and tear or damage from poor maintenance. Read your policy, but expect that damp from condensation, slow leaks or failed pointing and gutters will usually be excluded, and that storm claims may be refused if the roof was already in poor repair, so keep dated maintenance records. Burst pipes are usually covered, often with "trace and access" cover for hidden leaks; report promptly and limit the damage. Report suspected subsidence before commissioning repairs.

11.5 Red Flags

• Cold callers who "noticed" a problem with your roof or chimney and want to start today, or want cash.
• Free damp surveys that end in an injection quote after a few minutes with a meter and no look outside.
• Pressure to sign now, or claims the work cannot wait, without evidence.
• Sealers, cement renders or tanking proposed for an old solid wall without a clear diagnosis.
• Guarantees with nothing behind them; no written quote, address or landline.
• Spray foam insulation under a roof: some lenders and surveyors are concerned it can hide timber condition and trap moisture, so take advice first.

For contracts made in your home, you usually have 14 days to cancel under the Consumer Contracts Regulations. Report doorstep traders to Trading Standards through the Citizens Advice consumer helpline.

These three cases are composites of typical situations, not single real houses, but the readings are realistic and the physics checked. Each shows the method of chapter 3: pattern, outside, season, measurement, then a remedy aimed at the cause.

12.1 Black Mould in a Bedroom of a 1930s Semi

12.2 A Damp Patch Below a Window: the Blocked Sill Drip

12.3 Cement Render on a Victorian Terrace

Terms used in this handbook and in surveyors' reports.

Quick-reference material for use around the house. Figures in 14.3 are from the documents named; check the current versions on GOV.UK before relying on them for building work.

14.1 Annual Maintenance Checklist

14.2 Diagnostic Flowchart (in Text)

Start at step 1 and follow the first answer that fits.

1. Near pipes, tanks, radiators or sanitary ware, and growing regardless of weather? Yes: suspect a leak (2.5). No: go to 2.
2. High on a wall, on a ceiling, or round a window or chimney, and worse after rain? Yes: suspect penetrating damp; inspect directly outside (chapter 5). No: go to 3.
3. Black spotty mould on cold external surfaces, worst in winter, no salts or tide marks? Yes: suspect condensation; log humidity and surface temperatures (chapter 4). No: go to 4.
4. Is it a band at the base of a wall, with a tide mark and salts? Yes: go to 5. No: go to 7.
5. Ground, paving or render at or above the DPC (or the internal floor, where there is no DPC), or a leaking downpipe there? Yes: suspect bridging or surface water; fix it and let the wall dry before deciding anything else (5.6, 6.3). No: go to 6.
6. Solid wall with cement render or cement or gypsum plaster? Yes: the wall may be unable to dry (6.3). Either way, get an independent survey with drilled samples before any injected DPC (6.5).
7. Still unexplained, or several causes? Commission an independent surveyor or building pathologist, with your diary and photographs.

14.3 Ventilation and Regulatory Quick Reference

Extract rates (Approved Document F, Volume 1, 2021, Tables 1.1 and 1.2)

Other Approved Document F points: extract fans as high as practicable and within 400mm of the ceiling; in a room with no openable window, intermittent fans should run on for at least 15 minutes after the room is vacated; a recirculating cooker hood alone does not comply; a wet room with no external wall needs an intermittent fan extracting at four air changes per hour. The 2026 edition, taking effect from 24 March 2027 for most work, keeps the same intermittent rates.

Humidity benchmarks (Approved Document F, Appendix B)

Fabric and moisture

Landlord timescales (England, social housing, from 27 October 2025)

Useful documents

• Understanding and addressing the health risks of damp and mould in the home, GOV.UK (OHID, UKHSA and MHCLG), 2023, updated 2026.
• Awaab's Law: guidance for social landlords, GOV.UK.
• Approved Documents C, F, L and J, GOV.UK.
• Investigation of moisture and its effects on traditional buildings: principles and competencies, joint position statement, RICS, Historic England and PCA, 2022.
• BRE Digest 245, Rising damp in walls: diagnosis and treatment.
• Historic England, Repointing Brick and Stone Walls.
• Guidance on the permeable surfacing of front gardens, GOV.UK.
• BS 5250, management of moisture in buildings; BS 6576, diagnosis of rising damp and installation of chemical damp-proof courses.