What is damp?

All houses, whatever their age, contain moisture. Moisture is present in the materials of the building itself, and the air within the building. In this country, moisture also totally surrounds the building - it's in the air, in the ground and regularly falls out of the sky.

If your house was built any time in the thousand years prior to the second world war, it is likely to have been designed to actively manage moisture in order stop damp problems arising. If your house has been built since the second world war, it will be incapable of managing moisture and will rely on numerous static moisture barriers to stop moisture entering or leaving the building.

For a relatively short period from the early 1800s until the second world war, hybrid houses were increasingly constructed. These still actively managed moisture, but supplemented the natural moisture controls with experimental moisture barriers. These barriers were intended to reduce the need for high quality building materials and good building practice. Unfortunately, the moisture barriers in these hybrid houses have turned out to be a long term failure. It turns out they interfere with the correct operation of the natural moisture controls, and often increase dampness and decay.

The natural tendency of moisture, is to spread from wet to dry areas, and for water to move downwards under the influence of gravity. Moisture is not restricted to one type of material as it moves. It will just as happily move to and from air - the air is just another dry or wet area.

Dampness is just a localised excess of naturally occurring moisture. The excess is always caused by one of two things:

  • a sudden increase in the amount of moisture entering the building, or
  • a gradual local build-up of moisture caused by an imbalance between its rate of entry and its rate of escape.

Common causes of damp

If your house was built during the thousand years preceding the second world war, it will consist almost entirely of permeable materials with graded pores that attract water, and virtually no attempt at waterproofing in the modern sense. This allows moisture to pass freely through the walls, floors and ceilings. Instead of being sealed into rooms as condensation, airborne moisture inside the house is not only able to pass from room to room, but is also drawn from the rooms into the outside walls (the outside walls being drier than the room air). During periods of rain, some water is absorbed by surface of the outside walls, but like with a tent or umbrella, it generally doesn’t penetrate beyond the surface. If any water does enter into the walls, because they are very permeable, it rapidly spreads out as it moves from wet to dry. This water is then drawn back to the surface when the rain stops, as the water at the surface evaporates back into the air again (outside air being drier than the walls even immediately after rain).

For houses built since the second world war, a whole array of moisture barriers stop all moisture movement in or out of the house. These barriers seal airborne moisture into rooms as condensation, and cause all rain to run straight off the outside of the building.

Any dampness in your house is due to the failure of moisture to escape efficiently. This failure could be due to:

  • the normal escape route being overwhelmed by a sudden increase in the amount of moisture;
  • changes to the building that stop moisture escaping properly;
  • the lack of an escape route when moisture barriers fail to keep all moisture out;
  • inadequate escape routes for internal airborne moisture in buildings reliant upon moisture barriers.

Historically, most dampness was caused by lack of maintenance leading to leaks. Nowadays, the majority of dampness in period houses is caused by inappropriate building work that stops moisture escaping. Dampness caused by local build-ups of moisture is often erroneously given a variety of common 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 always excess trapped moisture.

Dampness caused by a sudden increase in the amount of moisture entering the building, usually appears without warning and very quickly – in a matter of hours or minutes. In houses built before the second world war, it can also disappear just as quickly in the early days. The dampness associated with this type of problem is also usually very well defined – it is concentrated and intense. The cause is often fairly obvious and is typically of the 'leak' variety. Typical examples of this type of problem are:

  • Plumbing – water pipes are often buried in the walls and these eventually corrode and leak. New plumbing for central heating, kitchens or bathrooms often leaks when first used. Old bathrooms may have leaking waste pipes or seals around baths, and showers are notorious for leaks. Any tiny leaks from plumbing can be the cause of a large intense damp patch – the leaks may be small, but they are often very regular and are concentrated on the same spot. This very intense, regular wetting, quickly introduces more moisture than can escape, this moisture overload then spills into neighbouring areas creating a snowball effect.
  • Missing/blocked grooves under outside window sills – these are often the cause of damp patches under windows. The majority of buildings from the mid 17th century onwards rely on windows with outside sills that project beyond the face of the wall. The underneath of the window sill incorporates a groove that sheds surplus rainwater clear of the wall. It is very common for these grooves to get blocked with paint. If the groove is blocked or covered by external wall insulation, all the rainwater running down the outside of the window will be directed straight into the wall immediately below the window. The quantities can be substantial and as it is concentrated in a single line, the moisture simply cannot escape quickly enough.
  • Downpipes – damp areas often coincide with a downpipe on the other side of the wall. 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 leaves and other debris washed in from the gutters. A blockage will cause water to back up and spew out of all the joints so it runs down the outside of the pipe. Any water running down the outside of a downpipe, will use the pipe brackets to cross to the wall and then be directed straight into the centre of the wall by the fixing screws.
  • Leaking gutters – when eaves gutters leak, water will either run directly down the outside of the wall for a short distance, or will spatter on the ground and window sills below. Any spattering will often cause considerable volumes of water to splash onto and into the nearby walls. Leaking roof valley gutters and leaking gutters behind chimneys, often feed water directly into the roof space. These latter leaks can be extremely difficult to find. The holes are often too small to be seen and the moisture usually follows sloping roof timbers for several feet or more, before affecting ceilings and walls.
  • Undersized gutters – these do not leak directly, but instead allow water to regularly spill over their edges. If your house was built prior to 2000, recent changes in the climate mean that the gutters are likely to be too small to cope with modern cloudbursts. The effect is the same as for leaking gutters but rather than repairing the gutters, often the only cure is to replace them for larger capacity versions or increase the number of downpipes.
  • Holes in roofs – this normally means missing or slipped tiles or slates, but could also be a worn out thatch, or broken tile or slate.

Dampness caused by trapped moisture gradually building up, generally comes on slowly, getting gradually more pronounced with time, and is very persistent – it doesn't come and go. It's often not particularly well defined or intense for the first few years, appearing just as a general area of damp that fades away round it's edges, or as mould growth in corners of rooms for trapped airborne moisture.

This type of damp is due to a build-up of moisture that can't escape, so the cause of the damp is not always obvious, and there may be multiple defects all contributing to the cause. Although modern building work is often the primary cause, there can be a delay of years before the dampness becomes noticeable, so the link between the building work and the subsequent dampness, isn't usually obvious.

Identifying the causes of this sort of damp is often a time consuming job requiring the use of specialist equipment – it cannot be achieved with a few prods of a so called 'damp meter'. This means you are unlikely to be able to accurately track down the problem yourself, but the following pointers will certainly help, and may even enable you to resolve the problem if it is straightforward.

Typical causes of this sort of problem are:

  • The use of modern cement based mortar in external wall repairs and pointing – this is especially bad for walls built during the thousand years preceding the second world war, but is no longer recommended even for modern walls. Cement mortars hold onto moisture to stop it readily escaping into the air, but more importantly, they also draw extra water into the wall through micro cracks that always form in cement. Cement mortars also force natural salts to crystallise in bricks adjacent to the mortar, causing the bricks to break up.
  • Modern cement rendering on the outside of the walls – this sort of rendering 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. All cracks draw extra water into the wall, but hairline and micro cracks are especially bad in this respect. As with cement mortars, cement renders hold onto water so it cannot readily escape back into the air. Another problem with cement render is it is relatively impermeable. This encourages condensation to form on the back of the render as airborne moisture within the wall builds up beside the less permeable render – this can even result in clay lump walls collapsing. Cement rendering an old wall can be one of the quickest ways there is of ensuring a wall becomes very damp.
  • Modern plaster (usually pink in colour) on the walls – the pink (gypsum) plaster itself isn’t the problem, it is the grey backing plaster it is usually applied onto, which is a type of cement render. In the worst cases the render backing contains a waterproofing agent so it can trap as much water as possible within the wall – this can cause the wall to fill with water like a fish tank.
  • Modern paint or vinyl wallpaper, used direct on solid walls (inside or outside in the case of paint). These are mostly impermeable and stop moisture entering or leaving the walls, while simultaneously encouraging condensation inside the wall against their backs.
  • An injected damp proof course – these are designed for use in dry walls that require a moisture barrier to prevent occasional moisture entry. As such they are incapable of successful installation in damp walls, and are incompatible with buildings constructed prior to the second world war. Unfortunately, most injected damp proof courses are inserted into the very walls they are incompatible with, destroying the ability of water to percolate down into the ground or escape into the air. To hide the subsequent moisture build-up, the walls are always re-plastered with a waterproof cement render to hide any damp for the duration of the guarantee.
  • High ground levels or paving – this is extremely common. As a general rule, houses were never built with their ground floors at or below the outside ground level. They were always raised up above their surroundings. If a house was built several hundred years ago with a brick paved ground floor, at worst these were laid on top of the existing ground level with a layer of fine ash or sand beneath to help get a level finish – thus achieving at least a modest increase over the outside ground level. Usually, the floor was raised by much more than this. 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. True basements containing rooms other than cold stores are quite rare in traditional East Anglian houses. What appear to be basements are often the true ground floors of the houses – again raised above the natural outside ground level at the time. The basement effect was produced by artificially raising the ground level a short distance back from the house to make entry at first floor level easier. (If a true basement room was required for a kitchen, the ground was excavated, drained and held back from the kitchen walls.)
  • Converted cellars – most historic below ground rooms were cellars and cold stores which are an important part of the moisture control system. Unlike basement rooms where the ground is kept away from the walls and either drained or artificially raised, all cellar stores have their walls in contact with the ground and deliberately allow the moisture to saturate them. This ensures they remain cool. They are also often built so as to intercept underground watercourses ensuring they regularly flood to a few inches. As nothing was ever stored directly on the floor or stored in direct contact with the walls, water and damp did not matter – the water from the flooding was actually used as a water source for the house to save going outside to a frozen well or pump in winter. Importantly, these cellars actively encourage moisture to pass through the walls and floor where it evaporates into the air. The cellars were always ventilated to prevent the air becoming too humid, so all the excess moisture simply escapes outside and stops any damp problems occurring in the rooms above. If they ever get converted into habitable rooms, the ventilation is stopped up and the moisture is sealed back into the walls and floor, forcing it up into the rooms above as dampness.
  • Replacement solid floors – it is important that water vapour from below the floors can pass up through the floor and harmlessly pass into the air. If a floor is replaced with a modern solid floor that stops the vapour from escaping, it will condense under the floor to make the ground damp. The damp ground will increase the overall amounts of moisture in the base of the walls and can make them damp near the replacement floor. 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 affected by the modern floor.

How to deal with damp

You can only start to put things right to eliminate the damp problem when you know the causes.

If you have identified the main cause as an increase in the amount of moisture entering the building, such as a leak, then this needs to be dealt with first. The required action is usually self-evident once you have identified the defect. Typical requirements are replacing defective plumbing, cleaning grooves under window sills of paint, rodding through or renewing downpipes, clearing gutters, renewing lead, fitting larger capacity gutters, and replacing tiles.

To allow trapped moisture to escape often involves redoing at least some of the building work causing the problems. The amount of work to strip away and redo, depends on the severity of the problems created, extent of the defective work, and what the alternatives are. The most important thing is to make sure moisture can easily escape from the outer face of your external walls. In general, this means:

  • removing any vegetation growing over them;
  • replacing any cement based mortar and render with lime based mortar and render;
  • using coloured limewashes instead of masonry paint when redecorating painted walls;
  • lowering high ground levels.

If your walls have cement pointing, over time this will naturally break up and fall away – in fact this could happen in as little as a year if it was done very poorly. If this has already started to happen, all you need to do is pull away the loose pointing and if required, replace it with lime mortar. Note the 'if required' – most old walls are repointed unnecessarily. Unlike walls built with cement mortar, a wall built using lime mortar does not need repointing until the face of the mortar has weathered back to a depth equal to the width of the joint. If the cement pointing is still in good condition, what the wall is built of becomes a factor in how to deal with it. Removing sound cement pointing from walls inevitably damages them, so unless the walling material is already decaying as a result of the pointing, other methods of allowing the moisture to escape should be considered first.

With cement rendered walls, as with cement pointing, the render will usually detach itself given time. Any loose or hollow sounding cement render should be removed and replaced with a lime render. Any minor damage done when removing the render can be repaired and covered by the new lime render. If the cement render is impossible to remove without causing serious damage, just remove and replace as much as you can. As long as moisture can escape from the wall, some small areas of cement render shouldn't create major damp problems and will gradually work its way loose over subsequent years.

Painted outside walls should ideally have the paint removed before either leaving the walls bare, or redecorating with a limewash. This is often not a practical option without calling in specialist companies to advise on safely removing paint from outside walls without damage. Given time though, the paint will inevitably become loose and weather off naturally, resolving the problem. To stop it looking too unsightly while the paint is weathering off, all blistering and loose paint should be removed along with the paint on the mortar joints. Any deeply recessed mortar joints can then be repointed in lime mortar, and the bare parts of the walls redecorated with a thick gloopy coat of coloured limewash.

High ground levels are not only the commonest fault found in all houses, they are also often the easiest to cure. Any soil or paving against the house should be lowered so that it is at least 6 inches (15 centimetres) below the adjacent floor level. Ideally, the ground level should be lowered even more than this provided the foundation or base of the wall is not exposed. If the lowering of the ground level next to the house, results in that level being considerably lower than the adjoining garden, the area next to the house can take the form of a trench. The edge of the trench away from the house can then either be sloped, or fully supported to prevent it falling in. The base of any trench should have a vapour permeable finish which allows water to slowly drain away naturally – the best finish is ordinary soil/turf. Note, filling the trench with pea shingle to create 'French drains' so beloved of builders is not suitable as, instead of allowing moisture to escape from the walls, the shingle allows water to drain straight to the bottom of the wall then holds the moisture against the walls.

Modern plaster can be a real problem in houses built before the second world war – no one wants to strip off all their plaster to reinstate the original lime plaster if they can avoid it. In East Anglia, if your walls are not too thick (up to about a foot/30 centimetres) and you still have open fireplaces, you can usually get away with leaving the plaster – provided there is nothing to stop moisture escaping from the outside of the wall. If your wall is much thicker than normal, or has become completely saturated from a leak or flooding, the plaster will need changing. There are essentially two choices:

  • If you are happy to use period decorative finishes (such as limewash, distemper or wallpaper) or modern ‘eco’ finishes (such as clay paints) you can simply replace all of the modern plaster in the vicinity of the damp area with lime plaster. This is also suitable for modern emulsions providing you use a lining paper on the plaster.
  • If you want the option of using modern washable wallpapers or ceramic wall tiles, then after stripping the old plaster off, you will need to line the walls with plasterboard or a similar material, leaving a small gap between the lining and the wall.

Modern decorative finishes can be viewed in much the same way as modern plaster – if the wall is not too thick, you can usually get away with them on the inside, provided the moisture can easily escape from the outside of the wall. Ideally though, a new lining paper should be used on the plaster prior to redecorating. The lining paper should be completely stripped off and renewed every time the walls are redecorated to ensure the low permeability modern decorations do not build up over time.

To eliminate the damp caused by a converted cellar without changing it back again, you need to try and recreate a basement in the original period building style. This means lowering the outside ground levels so they are below the level of the adjacent new floor – exactly the same as for any other high ground level. This is unlikely to be possible on all sides of a former cellar, so you will also need to to strip the plaster from any walls still in contact with soil, and create new walls inside them. There should be a gap between the old and the new walls. Sometimes the gap between the old and new walls also needs a drain at the bottom to pipe seeping water outside. These changes will allow the surplus moisture to harmlessly find its way out before it causes problems.

If you still have a damp problem

The above suggestions should cure, or at least improve, your damp problem. There are inevitably going to be situations where the cause of the problem is difficult or impossible for you to diagnose yourself. If this is the case, you are going to have to call in a surveyor with the expertise and equipment to find out why it is such a problem. A proper investigation will take several hours even if straightforward – a day or more is easily possible. The surveyor will then be able to provide you with the detailed advice and guidance to cure your particular damp problem.


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