Damp

Damp is the most common defect in British housing and the most profitably misdiagnosed. Almost every damp problem is one of three things, two of them are usually cheap to fix, and much of what is sold as rising damp treatment is neither necessary nor a treatment. This page explains the three types, how to tell which you have, what a surveyor actually measures, and when the standard remedy is the wrong one.

What causes damp in houses?

Three mechanisms cause nearly all damp in UK homes: condensation, where moisture generated inside the house condenses on cold surfaces; penetrating damp, where rainwater gets in through a defect in the building fabric; and rising damp, where ground moisture climbs the base of a wall. Condensation is by far the most common, penetrating damp is the most common building fault, and rising damp is the rarest of the three despite receiving most of the attention and most of the invoices.

Telling them apart is most of the diagnosis, because the remedies have nothing in common: ventilating a bedroom, repointing a chimney and injecting a damp-proof course fix three different problems, and the wrong one costs money while the real cause carries on.

Rising, penetrating or condensation: which do you have?

The pattern usually tells you.

Where it is. Rising damp appears only at the base of ground-floor walls, as a band rising from the skirting, rarely much above a metre. Penetrating damp appears wherever the defect is: below a window, on a chimney breast, in a ceiling corner under a failed gutter. Condensation favours cold spots: external corners, window reveals, walls behind furniture, unheated bedrooms.

What it looks like. Rising damp leaves a horizontal tide mark with white salt deposits. Penetrating damp makes defined patches that darken during and after rain. Condensation produces misted windows and black spot mould, which the other two rarely cause.

When it happens. Penetrating damp follows the weather. Condensation follows living patterns and peaks in winter. Rising damp persists year round.

A wall can suffer more than one at once, and a cold wall base attracts condensation that mimics rising damp. That is where measurement beats guesswork, which is the damp meter section below.

What is rising damp?

Rising damp is ground moisture drawn up through the pores of brick, stone and mortar by capillary action, the way a sponge stood in a saucer slowly wets upward. It climbs until evaporation balances the rise, which is why genuine rising damp forms a low band rather than patches at height.

Most walls built since the late nineteenth century contain a damp-proof course, a waterproof layer near the base that interrupts the rise. Genuine rising damp therefore means a wall with no DPC, a failed one, or, far more often, a bridged one: ground raised above it, render carried down over it, a path or flower bed piled against the wall.

Rising damp is real, but it is over-diagnosed, because it is the diagnosis that sells the most treatment. A diagnosis without measurements, and without an explanation of why the DPC is not doing its job, deserves a second opinion before it earns an invoice.

What is penetrating damp?

Penetrating damp is rainwater breaching the building through a defect: slipped tiles, leaking or blocked gutters and downpipes, eroded pointing, cracked render, porous brickwork, failed seals and flashings. Water can track along timbers before it shows, so the stain is not always under the defect; a surveyor traces the path rather than trusting the patch. Solid-walled buildings, which is most housing built before roughly the 1930s, are more susceptible than cavity construction, and exposed elevations take far more driving rain than sheltered ones.

One modern cause deserves its own mention: cavity wall insulation, which in unsuitable properties can bridge the cavity and carry moisture to the inner leaf. Damp appearing some time after insulation was installed is a recognised pattern and worth investigating.

What is condensation?

Condensation is moisture generated by living: cooking, showering, laundry dried on radiators, breathing. Warm air holds it as vapour until it meets a surface cold enough, then releases it as water on the cold spots: window reveals, external corners, walls behind furniture, anywhere insulation is missing or bridged.

It is the most common form of damp in this country and the one most often mistaken for something more sinister. Its signature is black spot mould in those cold locations, plus water on the windows in the morning. The NHS links mould in the home to respiratory problems, which makes persistent condensation a health matter as well as a building one, particularly for children.

A variant forms inside the construction itself: interstitial condensation, where vapour cools to its dew point within a wall or roof, a particular risk in well-sealed modern homes with inadequate ventilation.

What are the signs of damp when viewing a house?

• A musty smell on walking in, especially in a house otherwise presented well.
• Tide marks or a band of staining at the base of ground-floor walls.
• Defined patches on walls or ceilings, around chimney breasts and under windows.
• Black spot mould in corners, on window reveals, or behind furniture.
• Blistering paint, lifting wallpaper, plaster that sounds hollow when tapped, white salt deposits.
• Freshly painted single walls or new plaster patches in an otherwise tired room; redecoration is cheaper than repair and sellers know it.
• Dehumidifiers in use, moisture traps on sills, furniture held away from walls.
• Outside: raised ground against the walls, stained brickwork below leaking gutters, render carried down to the ground.

None of these alone condemns a house. Together, and in pattern, they tell a story worth having read properly before you commit.

What do damp meter readings actually mean?

The instrument most people picture is an electrical moisture meter, and it is genuinely useful and widely misused. The meter reads true moisture content only in timber. Pressed against plaster or masonry it reads relative values, because salts and conductive materials inflate the numbers, so a single high reading proves very little. What means something is the profile: readings taken methodically up and across a wall, compared against dry reference areas of the same construction.

The profile is the diagnosis. Genuine rising damp shows high readings at the base falling away with height, dropping to normal above the tide line. A bridged DPC shows the same band with the cause in plain sight outside. Penetrating damp shows a halo of high readings around a patch, unrelated to height. Condensation shows a damp surface on a wall that reads dry underneath, because the moisture arrived from the room, not through the wall.

We record moisture readings in our survey reports, stating where they were taken and what they indicate, so the conclusion can be checked rather than taken on faith. A diagnosis made by waving a meter at one spot and announcing a number is theatre; ask for the profile.

Is damp a dealbreaker when buying a house?

Usually not; damp is one of the most common findings in survey reports, especially on older stock. What decides it is cause and cost, not the word. Condensation is a ventilation and management matter, often resolved for very little. Penetrating damp is a repair item, priced like any other maintenance and a fair subject for renegotiation. A rising damp claim deserves scrutiny before it deserves money, and long-standing damp of any type earns a check for secondary damage, because persistently wet timber is how wet rot and dry rot begin.

The test is the one we apply to every defect: is the cause identified, is the remedy priced, and does the price still work? A damp patch with a known cause and a quoted repair is a negotiating position; unexplained moisture is a reason to find out more before exchange, not after.

Why do older houses get damp?

Because they manage moisture on a different principle. A house built before around 1919 typically has solid walls in lime mortar, no cavity and, originally, permeable finishes: lime plaster inside, lime render or bare masonry outside. Such walls admit some moisture and release it again; they breathe, and in their original state the balance broadly works.

Problems arrive when the balance is sealed shut. Cement render and repointing, plastic paints, gypsum patches and sealed floors trap moisture in the wall, which escapes at the one face left open, usually the inside. A great deal of “rising damp” in Victorian terraces is a solid wall wrapped in impermeable modern materials, doing what physics says it must. The sympathetic remedy is often to restore breathability with lime-based finishes and breathable paints, not to add another layer of waterproofing.

What does damp treatment actually involve?

Whatever the type, the order is the same, and it starts outside the wall rather than inside it.

1. Remove the cause. Repair rainwater goods, roof and flashings, lower ground levels below the DPC, restore subfloor ventilation, ventilate and heat the rooms that condense. In a large share of cases this is the whole treatment.
2. Let the wall dry. Masonry gives up water slowly, and redecorating too early stains the new finish.
3. Repair the damage. Replace salt-contaminated or blown plaster in materials that suit the wall, deal with any decayed timber, redecorate breathably on solid walls.
4. Keep it fixed. Extractor fans that vent outside and get used, trickle vents, air behind furniture on cold walls, ground kept at least 150mm below the DPC.

What the sequence does not begin with is a chemical product. Chemicals do not fix gutters or ventilate bedrooms, and no treatment applied inside a wall survives an unresolved cause outside it.

When is a damp proof course not the answer?

More often than the damp-proofing industry would have you believe. The standard retrofit remedy, an injected chemical DPC with waterproof re-plastering, is sold in volume on the back of free surveys carried out by the companies that sell the treatment: a diagnosis from someone paid by the remedy.

An injected DPC cannot help a wall whose problem is condensation, a leaking gutter, raised ground bridging a sound DPC, or a solid wall sealed in cement and plastic paint, and those account for a large share of walls carrying the rising damp label. In such cases the injection fails politely: the waterproof re-plastering hides the symptom for a few years while the moisture carries on behind it. A retrofit DPC has a legitimate place where a DPC has genuinely failed or never existed, but it is the last remedy on the list, not the first. The parallel with how we approach underpinning in subsidence cases is exact.

How a survey identifies damp

A surveyor reads the building as a system: the outside defects that let water in, the inside evidence of where it goes, and the measurements that connect the two. That means moisture readings recorded in the report, inspection of roof voids and subfloor spaces where access allows, checks on ground levels, rainwater goods, pointing and render, and an assessment of ventilation and condensation risk.

A Level 2 Homebuyers Survey identifies damp, rates the affected elements and tells you what needs further investigation. A Level 3 Building Survey goes further into cause and remedy: which type you are dealing with, why, and what the repair should involve, which is the information a negotiation stands on. Either way the answer comes from someone with nothing to sell you afterwards, which in the damp business is rarer than it should be.

If you are buying a house and something about its walls bothers you, tell us the address and we will send a firm price in writing within the hour, during office hours.

Frequently asked questions

How much does a damp survey cost?

If you are buying, you may not need a separate one: damp inspection with recorded moisture readings is part of every Level 2 and Level 3 survey we carry out. Standalone reports from independent damp and timber specialists are priced by the job, and we would not put a figure on someone else’s work. A free damp survey from a company that sells damp treatment is not a survey; it is a sales visit.

What does rising damp look like?

A band of staining rising from the skirting on a ground-floor wall, usually with a tide mark, white salt deposits, and peeling paint or blown plaster. It does not appear upstairs, in patches at height, or as black mould in corners; those point to penetrating damp or condensation instead.

Can damp make you ill?

The damp itself is not the hazard; the mould it enables can be. NHS guidance links mould in the home to respiratory problems, allergies and asthma, with children, older people and those with existing conditions most affected. It is a good argument for treating condensation seriously rather than wiping it away each winter.

How long does a damp wall take to dry out?

A common rule of thumb is around a month for each 25mm of wall thickness once the source is fixed, so a 225mm solid brick wall can take the best part of a year to dry fully. Heating, ventilation and the season all move the figure. The practical point is not to rush it: replastering a wall that is still drying wastes the plaster.

Does home insurance cover damp?

Generally not. Damp arising gradually, which is most damp, is treated as a maintenance matter and excluded. Damage from a sudden insured event, such as a burst pipe or a storm stripping the roof, is usually covered, and the resulting moisture with it. The dividing line is sudden versus gradual, and insurers police it closely.

Does a damp proof course stop condensation?

No. A DPC, physical or injected, addresses moisture rising from the ground. Condensation forms from moisture already in the air of the house, and no barrier in the wall base affects it. If mould in cold corners is your problem, the remedies are ventilation, heating and insulation, not injection.