What this estimator works out
Once the cause of damp has been found and fixed, the wall is still wet. How long it takes to dry is the question that decides when replastering and redecorating can happen, and getting it wrong is expensive: new plaster on a wall that is still drying blows, salts and has to come off again.
This estimator applies the rule the building trade uses — roughly one month of drying for every 25 millimetres of wall thickness — and adjusts it for what the wall is made of, how well the room is heated and ventilated, whether both faces can dry, and how wet it got in the first place.
The answer is usually longer than people expect. A 225 mm solid brick wall, which is the standard construction of most British houses built before the 1920s, takes the better part of a year in ordinary conditions. That is not a fault in the wall and there is no product that changes it much.
It is a rule of thumb rather than a calculation, and the page gives a range rather than a single figure for that reason. What it is good for is knowing roughly when to start testing, and recognising a proposed schedule that cannot possibly work.
The wall and its conditions
Start the clock from the day the water stopped arriving, not from the day the damp appeared.
How the estimate works
There is no equation for this in the way there is for dew point or extract rates. What there is instead is a rule of thumb used across the building trade and quoted in guidance on drying out buildings:
Roughly one month of drying per 25 mm of wall thickness,
once the source of water has stopped,
in reasonable conditionsA 225 mm solid brick wall — one brick thick, the standard construction of most British houses built before the 1920s — comes out at about nine months on that rule. That figure surprises people, and it is the single most useful thing on this page.
Why it takes so long
Water leaves a wall by travelling to the surface and evaporating there. The travelling is the slow part. Liquid water moves through the fine pore structure of masonry by capillary action, at a rate set by the pore sizes rather than by how much heat you apply. Once the outer few millimetres have dried, water from deeper in has to migrate outwards through material that is already dry, which is slower still.
This is why the last stage takes disproportionately long, and why a wall that feels bone dry to the hand can still hold enough moisture at 100 mm depth to ruin new plaster.
What the multipliers do
The rule assumes brickwork of ordinary porosity, drying from both faces, in an occupied room. Each of the other inputs adjusts it.
- Material. Lime-based construction breathes noticeably better than cement-based. Dense stone and cement render dry slowly. A plasterboard partition dries in a fraction of the time because there is very little material to dry.
- Conditions. Gentle background heat with ventilation to carry the moisture away is the ideal. Heat without ventilation just moves water from the wall into the room air and then onto the windows.
- Faces exposed. A wall drying from both sides has half the distance to move water. One-sided drying roughly doubles the time.
- How wet it got. A saturated wall holds several times the water of a damp one, and it all has to leave by the same slow route.
The spread on this rule is genuinely wide — the realistic range given here runs from about 60% of the estimate to about 170% of it. That is not padding. Drying rate depends on the specific pore structure of the specific masonry, on how much salt is in it, on the weather over the period, and on how the building is used. Anyone quoting a confident single figure for how long your wall will take is guessing with more confidence than the evidence supports.
Worked example: after a burst pipe
A pipe in an upstairs floor void failed and ran for several hours before anyone noticed. The water tracked down inside a 225 mm solid brick wall in cement mortar. The leak was fixed in March. The room is heated normally and the windows get opened. Both faces of the wall are exposed — it is an internal wall.
| Input | Value |
|---|---|
| Thickness | 225 mm |
| Material | Solid brick, cement mortar or cement render |
| Conditions | Ordinary occupied room |
| Faces | Both faces exposed to air |
| How wet | Soaked |
The base figure is 225 ÷ 25 = 9 months. The cement multiplier of 1.35 takes it to about 12 months, with a realistic range of roughly 7 months to 21 months.
A year. The insurance work is scheduled for June.
What that means in practice
It does not mean nothing can happen until next March. It means the sequence matters:
- Improve the drying conditions first. Moving from "ordinary" to "heated and well ventilated" takes the estimate from about 12 months to about 9. Steady background heat and open trickle vents, not a fan heater blasting at a wall with the windows shut.
- Strip anything sealing the surface. If that wall has vinyl paper on it, the paper is doing more damage than the leak now. This is the difference between 12 months and something closer to three years.
- Test before replastering, do not schedule it. Deep readings from drilled samples, not a surface meter, and taken twice a few weeks apart so you can see the trend rather than a single number.
- Expect the loss adjuster's timetable to be shorter than the wall's. This is the most common conflict after an escape of water, and the wall does not negotiate. Replastering at six months on a wall that needs twelve produces blown plaster and salting within a year, and a second claim.
How to tell when it is actually dry
The estimate tells you roughly when to start testing. It does not tell you the wall is dry, and nothing except a measurement does.
The problem with hand-held moisture meters
The two-pronged meters sold in DIY shops, and used in most free damp surveys, measure electrical conductance between the pins. That is a reasonable proxy for moisture in timber, which is what they were designed for. In masonry it is not, for one specific reason: salts conduct. Masonry that has been wet contains dissolved salts left behind as water evaporated, and those salts give a high reading on a wall that is bone dry. They also read only the surface few millimetres, which is the part that dries first.
A high reading from a pin meter on plaster means "something here conducts electricity". It does not mean the wall is wet, and a great many chemical damp proof courses have been sold on exactly that ambiguity.
What actually measures moisture
- Carbide meter test. Drilled samples from depth are reacted with calcium carbide in a sealed vessel and the pressure gives the moisture content. Destructive, quick, and it measures the material rather than its surface.
- Oven-dry, or gravimetric, testing. Samples are weighed, dried in a laboratory oven and weighed again. The definitive method, and the one used where the answer has to stand up.
- Depth samples, not surface ones. Whichever method, take samples from several depths. A wall dries from the outside in, and the reading at 10 mm says nothing about the reading at 100 mm.
- Repeat over time. One reading is a number. Three readings a month apart are a trend, and the trend is what tells you whether drying has stalled.
A single moisture figure is hard to interpret, because what counts as dry varies with the material. A falling trend across repeated tests at the same points is unambiguous, and a flat trend tells you something is still supplying water — which usually means the original cause was not fully fixed.
What speeds drying up, and what does not
Genuinely helps
- Steady background heat. Gentle and constant beats hot and intermittent. The aim is to keep the fabric warm, not the air.
- Ventilation. Heat moves water out of the wall into the air; ventilation removes that air. Without it the moisture condenses on the coldest surface in the room and you have converted one damp problem into another.
- A dehumidifier, with the room closed. Useful precisely because it does what ventilation does without losing the heat. Empty it and note how much comes out — a falling daily volume is a good drying trend.
- Removing impermeable finishes. Vinyl wallpaper, waterproof render, gloss paint, tanking. Taking these off can be worth more than everything else combined.
- Air movement across the surface. A fan aimed along a wall rather than at it helps evaporation at the face.
Does not help, or makes it worse
- Intense heat at one spot. A heat gun or a fan heater aimed at the wall dries the surface, which then resists the passage of water from behind it. It can also drive salts to the surface and damage the plaster.
- Replastering early. New plaster on a wet wall traps the water and fails. It is the most expensive mistake on this page.
- Sealing the surface to "keep the damp in". Waterproof coatings on a damp wall push the moisture sideways and upwards to emerge somewhere else, usually higher up and in a room that was previously fine.
- A chemical damp proof course, if the cause was a leak or a flood. It addresses rising damp. It does nothing for a wall that got wet from a burst pipe, and it does not speed drying.
- Waiting without ventilating. A closed, unheated room can hold a wall damp almost indefinitely.
Common mistakes
- Starting the clock too early. Drying begins when the water stops arriving, not when the damp was first noticed or when the builder first attended.
- Trusting a pin meter on masonry. Salts read as moisture. This single confusion underlies a large share of unnecessary damp treatment in Britain.
- Judging by the surface. The face dries first and dries fastest. It tells you almost nothing about the middle.
- Replastering to the insurer's timetable. The wall dries at its own rate. Push back, in writing, with test results.
- Heating hard with the windows shut. This moves water from the wall to the air to the windows, and the room streams with condensation while the wall stays wet.
- Leaving vinyl paper or waterproof render in place. The estimate roughly triples, and no amount of heat compensates.
- Assuming both faces are drying. A wall against an earth bank, a neighbouring property or a tanked basement wall dries from one side only, and takes roughly twice as long.
- Treating the estimate as a promise. It is a rule of thumb with a wide range. Test before you commit money.
When not to use this estimator
Not while water is still arriving. If the cause has not been found and fixed, drying has not started and no estimate applies. Find the cause first — the damp guide sets out how.
Not for an insurance claim on its own. Insurers and loss adjusters work from measured readings by a drying contractor, not from a website. Use this to know what to expect and to judge whether a proposed schedule is realistic, then get the measurements.
Not for timber. Structural timber that has been wet raises questions about decay and beetle attack that a drying estimate does not answer, and wet timber in contact with masonry is a situation for a surveyor.
Not for interstitial moisture in an insulated build-up. Water inside a modern layered wall, roof or floor construction behaves quite differently from water in solid masonry, and the drying path may be blocked entirely by a vapour control layer.
Not for a listed or traditionally built building without advice. Solid-walled and historic buildings are designed to manage moisture by breathing rather than by resisting it, and the wrong intervention causes lasting damage. Historic England publishes guidance, and a conservation-accredited surveyor is worth the fee.
Not for deciding whether a building is safe to occupy after a flood. That involves contamination, electrical safety and structural questions that have nothing to do with drying rate. Follow the guidance from your local authority and the Environment Agency.
Frequently asked questions
How long does a wall really take to dry?
Longer than almost everyone expects. The working rule is about a month per 25 mm of thickness once the water has stopped arriving, so a 225 mm solid brick wall takes most of a year in ordinary conditions. Cement-based construction, one-sided drying or a sealed surface can extend that considerably.
Can I speed it up?
To a degree. Steady background heat with ventilation, or a dehumidifier in a closed room, is the effective combination, and stripping any impermeable finish off the surface is often worth more than both. What does not work is intense heat at one spot, which dries the face and slows the passage of water from behind it.
When can I replaster?
When testing says the wall is dry at depth, not when the estimate runs out. Use carbide or oven-dry testing on drilled samples from several depths, repeated a few weeks apart so you can see a trend. Replastering early is the most expensive mistake associated with damp, because the work fails and has to be done twice.
Why does my moisture meter still show high readings?
Possibly because the wall is wet, and possibly because it is not. Pin-type meters measure electrical conductance, and the salts left behind in masonry after water evaporates conduct very well. A high reading on dry, salt-contaminated plaster is entirely normal and is a well-known source of misdiagnosis. Depth samples tested properly settle it.
Does a dehumidifier work better than opening the windows?
It depends on the weather. A dehumidifier in a closed room is reliable year-round and keeps the heat in. Open windows are free and work well when the outside air is drier than the inside air, which in Britain is often not the case on a damp winter day. Many people use both, at different times.
The wall has been drying for a year and readings are not falling. What now?
A flat trend means water is still arriving from somewhere. The usual causes are an unfixed or second leak, a bridged damp proof course, ground level raised above the damp proof course outside, or a sealed surface holding moisture in. This is the point to pay for an independent survey from someone with no interest in selling a treatment.
Is a plasterboard partition really that much quicker?
Yes, by a wide margin, because there is so little material. Board that has been wet often needs replacing rather than drying, though, and the timber and insulation behind it need inspecting. Quick to dry is not the same as undamaged.
Does this apply to a flooded house?
The drying rule does, once the standing water has gone and the building has been cleaned. Everything else about flood recovery — contamination, electrics, insulation that has to be removed, structural checks — is outside what a drying estimate covers. Follow local authority and Environment Agency guidance for those.
Related tools
References
These organisations publish the guidance behind this page.
- Building Research Establishment — guidance on drying out buildings, moisture measurement and the limits of hand-held meters
- Property Care Association — trade guidance on moisture measurement, carbide testing and drying after escape of water
- Historic England — guidance on moisture in traditionally built and solid-walled buildings, and why breathability matters
- Royal Institution of Chartered Surveyors — professional standards for investigating damp and reporting on moisture in buildings
- Environment Agency — guidance on recovering a property after flooding
- Approved Documents, GOV.UK — Approved Document C on site preparation and resistance to moisture
External guidance changes. Check the current position at the source before relying on it for a decision that matters.