All houses, whatever their age, contain moisture. It will be present in the building materials used and the air within the home. The natural tendency of moisture is to spread out from wet to dry areas, and also to move downwards under the influence of gravity, and your house construction should allow for this.

Simply put, dampness is just a localised excess of naturally occurring moisture. The excess is always caused by one of two things:

  • either a sudden increase in the amount of moisture entering the building;
  • or a gradual local build-up of moisture caused by its inability to escape.

Dampness caused by a local build-up of moisture is often given a variety of names depending on where it shows up, such as ‘penetrating damp’ and ‘rising damp’. These names are both misleading and largely meaningless — the problem is almost always an excess of trapped moisture.

Old Homes

Unlike modern buildings, the construction of an older house is designed to prevent damp problems by preventing the excess build-up of moisture.

They were built almost entirely of permeable materials, with virtually no attempt at waterproofing in the modern sense, and so moisture will pass freely through the walls, floors and ceilings. Instead of being sealed into rooms as condensation, moisture inside the house is able to pass from room to room, and also pass through the external walls to the outside air.

Historically, most dampness was caused by the lack of maintenance leading to leaks. Nowadays, the majority of cases of damp in period houses are caused by inappropriate building work that stops moisture escaping.

With older solid walls it’s advisable to remove incompatible modern materials such as cement render or mortar pointing that trap damp. Replace with traditional lime-based breathable materials that allow moisture to evaporate. Allow time for walls to dry out and keep the house heated.

Obvious Causes of Damp

Dampness which is caused by any sudden increase in the amount of moisture entering the building usually appears without warning — and in a matter of hours. It can also disappear just as quickly in the early days.

The dampness associated with this type of problem is usually very well defined, as it is concentrated and intense. The cause of the problem is often fairly obvious too.


Any tiny drips from plumbing can be the cause of a large and intense damp patch — the escape of water may be small, but is often regular and concentrated on the same spot. This focused, repeated wetting quickly introduces more moisture than can escape, and the overload then spills into neighbouring areas. Hairline cracks in the grouting to bathroom wall tiles, when combined with ‘power showers’ can also be a recipe for damp in homes.

Check for:

  • corroded water pipes concealed in the walls
  • new plumbing for central heating, kitchens and bathrooms which drips when first used
  • failing waste pipes in older bathrooms
  • damaged seals around baths and showers

Leaking Gutters

When gutters at the eaves leak, water will either run directly down the outside of the wall for a short distance, or will splatter on the ground and window cills below. Leaks in other gutters often feed water directly into the roof space, and these can be extremely difficult to find.

Check for:

  • splattering from gutters splashing water onto walls
  • tracks of water in the roof which may be following roof timbers for several feet

Holes in Roofs

This normally means missing or slipped tiles or slates, but could also mean a worn out thatch, or a broken tile or slate.

Undersized Gutters

Changes in the climate or the roof area since your house was first built may mean that the gutters are too small to cope with current rainfall. The only cure is to replace them for larger-capacity versions or increase the number of downpipes.

Check for:

  • water spilling over the edges of the gutters on a regular basis

Grooves Under Window Cills

The majority of buildings from the mid-17th century onwards rely on windows with outside cills that project beyond the face of the wall. The underside of the window cill incorporates a groove that sheds surplus rainwater clear of the wall. If the groove is blocked, all of the rainwater running down the outside of the window will be directed straight into the wall immediately below the window.

Check for:

  • grooves blocked with paint which cause large volumes of water to track down the wall

Chimney stacks and roofs

These are also common weak points where water can seep into the walls.

Check that:

  • roofs and chimney stacks have watertight flashings
  • chimney pots have been fitted with rain caps


Old cast-iron downpipes can easily rust through at the back without anyone being aware. Also, downpipes that disappear straight into the ground at the bottom, or that contain any sharp bends, are very prone to blockage from debris washed in from the gutters.

Check for:

  • holes in the back of a downpipe
  • water spewing back up out of down pipe joints
  • water tracking from the downpipe along brackets and to the face of the wall

Humid air

Get rid of humid air before it condenses into water and forms patches of black speckled mould. Install extractor fans on kitchens and bathrooms. Reduce emissions of water vapour, for example, by venting tumble driers to outdoors. Fit trickle vents to windows. It also helps if the wall surfaces are well insulated.


Clear any climbing vegetation, as it has a tendency to harbour damp and inhibit walls from drying out.

Less Obvious Causes of Damp

The cause of the damp is not always obvious, and there may be multiple defects that are all contributing to the cause. Although modern building work is often the primary cause of damp problems, there can be a delay of years before the dampness becomes noticeable, so the link isn’t always obvious.

Injected Damp-Proof Course

Designed for use in dry walls, this type of treatment is incapable of working when installed in a wall that is already damp. They use either waxy creams that just form individual ‘fingers’ when injected, or chemicals dissolved in a water-based solvent. The solvents must completely evaporate away after injection to allow the chemicals to cure and form a water-repellent coating.

If a wall is damp, the solvents never completely dry out and the chemicals never cure; the process will just saturate the wall with even more water.

The walls may be replastered with a waterproof cement plaster to try and hide any damp for the duration of the guarantee. This stops the natural movement of moisture — making any damp problems worse.

Cement Rendering

Used on the exterior of walls, this sort of coating develops cracks all over its surface. The size of the cracks depends on the quality of the render and skill of the builder, but they’re impossible to prevent. The cracks encourage water into the wall, but the water cannot escape because the render is impermeable.

Even if cement render is not yet visibly cracked, it encourages condensation to form as moisture from inside the house can no longer escape — this process can even result in clay lump walls collapsing.

Replacement of Solid Floors

It is important that any excess moisture from below the floors can pass up through the floor and harmlessly into the air. If a floor is replaced with a modern solid design that prevents this from happening, moisture is forced to travel under the floor into the adjoining walls to escape. This will increase the overall amount of moisture in the base of the walls and potentially make those walls damp.

If the outside ground levels are correct and nothing on the outside of the walls prevents the moisture from escaping into the air, you may find only the internal walls are badly affected by the modern floor.

Modern Plaster

This too is relatively impermeable and so prevents moisture passing through. It may just form a thin layer over the original plaster, but in the worst cases, all of the original plaster will have been removed and replaced with waterproof cement render covered with a thin layer of pink plaster.

Modern Paint or Wallpaper

Used directly on solid walls (inside or outside in the case of paint), these too are mostly impermeable and prevent moisture moving through the walls and escaping.

Cement-Based Mortar

Cement mortars not only stop moisture moving freely through the wall and escaping into the air, but they also encourage water into the wall through cracks that always form in cement. These mortars also force natural salts to crystallise in brickwork, causing the bricks to break up.

Converted Cellar Rooms

Most below-ground rooms were cellars and cold stores which were an important part of the moisture control system. True cellar stores have their walls in contact with the ground and allow moisture to saturate them — ensuring they remain cool.

They are often built to intercept underground watercourses, and may have regularly flooded to a few inches. As nothing was stored on the floor or in direct contact with the walls, water and damp did not matter, and the flood water was used to save going outside to a frozen well or pump in winter.

Good ventilation allowed excess moisture to escape. If the cellars ever get converted into habitable rooms, the ventilation is stopped up — forcing moisture up into the rooms above.

High Ground Levels Outside

This is almost guaranteed to be a problem. As a general rule, houses were always raised up above their surroundings. If the house was built with a brick paved ground floor, at the very least, this was laid on top of the existing ground level with a layer of fine ash or sand beneath to give a level finish.

Nowadays, you can often see old steps that once led up to doors, partially buried beneath later pavings. It is also common to see door thresholds that were originally above a top step, now below the adjoining ground so you step down to them.

External ground levels should be at least 200mm below floor level, so it is important to ensure that earth is not banked up against outside walls. Patios are often laid too high, causing bridging of DPSc or rain splashing on hard surfaces saturating the walls. Installing a shallow gravel-filled trench around the base of the wall assists evaporation of moisture.

Damp plaster

Formerly damp plaster can retain a residue of salts deposited by the water. Sometimes when rooms are heated after being left empty, dampness can reappear on the surface of the paster because these salts absorb moisture from the atmosphere of the room. Salts should be brushed or vacuumed off.

Read more to find out how to deal with damp

Articles like this Comments
  • Richard Bracek

    Interesting article. We have a solid floor in an old property that needs replacing and we wish to have solid wood flooring. I’ve been advised it will need digging down, but is there a safe way to build it back up such that the wood can be laid without causing potential problems with damp moving to the walls?

  • Kathy Eyre

    I live in a stone cottage built in 1826 and have several issues with damp. I am convinced its related to condensation as the house has both chimneys closed up, although ventilated, has double glazing fitted with no additional ventilation and the floors are concrete. In places the plaster has fallen of and there is damp around the base of one chimney. On the outside of the building there is a black mark where the chimney is appearing from the roof down. I have removed the plaster on the internal wall where the plaster has started to fall off, can you advise
    – do I need to remove the render to take it back to the stone in order to address the damp issue that appears to be going on?
    – is the cement flooring pushing the damp to the walls as I also have evidence of damp on internal walls.
    – is the black mark on the outside of the walls where the chimney is a result of rain getting in through the chimney pot thus causing the damp in the chimney inside the house?

  • Caz Thomas

    HI I had cavity wall insulation (rock wool) removed due to damp on internal walls – all along base of the hall -which worsened when it rained . My surveyor advised the retro fittred cavity wall insultation was responsible as outside wall was sited to suffer wind driven rain. I bought the house with the problem and contacted the installers to remove under the CIGA guarantee. They agreed to remove as a good will gesture stating the insulation was not cause of damp but the wind driven rain and porous render would have been too much for the insulation and broke it down. Though inspector later advised me the wool that had been used was no longer used as it holds water, I was just happy just to get it removed. A month ago the team took it out. After a couple of dry weeks damp got much better we had rain and it all came back. I rang them they came up and looked with a camera advised me some wool was still remaining and they come to take it out. . Though the areas of damp did not relate to areas of wool left when i watched them removing it. I thought my damp problems were over and watched the lower walls all dry out again.i usea little water gadget though i know they are not considered very accurate I thought just by the pattern of readings decreasing show it is drying out. All good till we had raing again and walls looking damp again and all readings back up high. Has anyone had insulation removed and faced these issues ? does the damp go i am trying to understand if I should be contacting the insulation company again or exploring other options and considering my surveyor/home buyers report and the supporing damp survey i also obtained had the wrong cause of the damp ? Help anyone ?

  • Lindsey Davis

    Hi Caz,

    As the problem seems to be exacerbated by rain, and is persisting after you’ve had the insulation removed, I would suggest you get someone round to check the external wall and the damp-proof course.


  • Sparkly One

    Cavity wall insulation can and does cause damp. Extractions are often botched by the same cowboys who put it in.

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