How to deal with damp in walls

Traditional buildings can be prone to damp problems if not properly maintained. ‘Damp’ is the catch-all term for the various forms of decay that can result if moisture levels are too high.

Signs of damp include:

• surface staining
• mould
• stale air

Later modifications are another of the common causes of damp in traditional buildings. Modern homes rely on waterproof barriers to protect against moisture, but a traditional building must be able to breathe to let moisture out.

You can stop serious problems from getting worse by:

• looking out for any signs of water entering your property
• carrying out regular maintenance to keep your property watertight

It's important to inspect your roof and check your rainwater goods regularly. Checking your roofing leadwork, chimney, drainage and walls should also be on your list. If a damp problem doesn't stem from any of these areas, the problem may be internal.

To properly fix a damp problem, it’s vital to find its source. If the source isn’t treated, or the problem is incorrectly diagnosed, the original problem will continue to develop.

You must let affected areas dry out thoroughly before remedial work is carried out, but this doesn't always require the removal of lining material and plasterwork.

Most damp problems arise through:

• the failure of materials
• the failure of a building’s features
• inappropriate interventions – i.e. where work has been carried out with materials unsuitable for use in traditional buildings

If you spot a new problem, think carefully if any recent work you’ve had done might be the cause.

Many damp problems, however, go unnoticed for some time.

A small leak can cause a big damp problem if not detected for some time. It may lead to an increasing build up of moisture in walls, timber and plaster.

Your roof should be checked regularly for:

• dislodged slates or tiles
• defects in the surface of any areas of flat roof
• pointing missing from skews, copes, chimneys and other upper level masonry

Unless the problem is really serious, a leak from one of these areas will rarely cause actual drips inside. But it will create the ideal conditions for rot to flourish, leading to the decay of structural timber and decorative finishes.

You should also check that moisture levels in the roof space are under control. Felt and roofing papers are a modern addition to roofs, and may require vents to be added if used in a traditional roof.

Poorly working cast iron rain goods are often directly responsible for damp walls. Leaking gutters and overflowing downpipes can quickly saturate a wall, with moisture carried some distance from the source of the problem.

Check inside and outside your property for any signs of damage to gutters, drainpipes and so on.

Look out for:

• damp patches and staining, especially after rain, to see where the problem lies
• staining around the overflow of any parapet gutters

Appropriate repairs should be made straightaway. Wet and dry rot have the ideal conditions to flourish where structural timber comes into direct contact with saturated stonework. Insect and beetle attacks may also be encouraged.

Lead is often used on roofs in gullies, to line parapet gutters and as flashings for windows, roof lights and access hatches. Water can get in and soak a wall where this roofing leadwork is damaged.

Your roofing leadwork should be checked for:

• wind lift and distortion
• surface ripples and creep
• cuts, rips and tears
• restricted thermal movement
• corrosion

Common causes of problems

Failure of parapet gutters can occur due to wear and cracking of lead. This is particularly serious as significant amounts of water may enter the building fabric through the wall head.

Lead valleys can be worn through by the flow of water, by puncture damage from falling objects or from cracking. Staining often signals areas of high water flow, and these stained areas can develop small holes over time. Left unattended, the holes will let in enough water for the decay of structural timber to thrive.

Small cuts, rips and tears can be repaired temporarily by applying bituminous backed foil. Long-term repairs involve the careful welding of lead patches over damaged areas or the replacement of damaged lead.

Lead resists corrosion well, but it can be damaged by:

• alkalis from new cement
• acids from some timbers – oak in particular
• the run-off from lichen, moss and algae surface growths

Soil and other materials can gather around the base of walls and lead to dampness in lower levels. Blocked drains can clearly lead to wet walls.

You should regularly check your ground levels and below ground drains.

What to look for

Ensure that ground levels outside haven’t risen, compromising the original design and causing dampness in the walls (and adjacent timbers).

Rainwater from the roof should be piped away from the property – either to the main drain or to a soakaway placed well clear of the building. Check for blockages where the downpipe enters the ground, especially if it leads into an open gully, and that the drain is clear.

If below ground drainage fails, waste water can gradually saturate the ground beneath a property. Even a small crack or fracture in a waste water or sewage pipe can cause major problems. A specialist drain survey may be required to check for breaks or damage.

Vegetation close to walls may trap moisture while their roots could damage drains. It’s best to avoid having planted areas and shrubs too close to walls.

Keeping walls in good repair, using the right materials, is vital to avoid damp.

Pointing problems

The mortar that you can see between stones on walls is called pointing. It protects the joint and the bedding – the mortar that the stone blocks sit on. Damaged pointing can let water into walls.

Lime mortar should always be used for ashlar masonry pointing or rubble masonry pointing on traditional buildings. Check regularly for missing pointing, where water can enter joints and where moss, grasses, small bushes and saplings may begin to grow.

Render issues

Render or harling is a protective coating sometimes applied over rubble masonry. Using the wrong materials for render may mean that any water let in by damage to the coating is then trapped inside the stone wall beneath.

Traditional external finishes such as lime render, distemper or limewash allow moisture to pass through the structure. But their modern equivalents – e.g. damp resistant paint – tend to hold moisture in the building.

If you have mould or damp patches on the inside of walls and there is no apparent external damage, the cause could be internal.

Leaks

Plumbing could be the culprit. Leaks in concealed internal plumbing can be a major source of damp and associated decay. Even a small drip from an overflow pipe can cause considerable damage.

Heating and ventilation

Maintaining a steady level of heating inside a building (15 to 18ºC) is also important. This lets the walls warm up enough to prevent condensation on hidden surfaces. More moisture will be held in the fabric of a building left uninhabited or unheated for a period, but some warmth will soon drive this off.

Moisture will remain trapped indoors where there isn’t enough ventilation. Traditional buildings must be allowed to breathe. Extreme condensation on windows in warmer temperatures may be a sign of poor ventilation.