Damp in the walls of an old house: causes and solutions

Damp in the walls of an old house: where does it come from?

Damp patches, loose plaster, mould or white deposits on an old wall can have several causes. In older homes, first establish the source of the moisture. Treating only the visible patch often does not solve the problem.

Possible causes include penetrating rainwater, rising damp, condensation, a leak or problems with pointing, leadwork, gutters or drainage. The suitable solution depends entirely on the cause and the construction of the wall.

How do you recognise damp problems in an old wall?

Common signs include dark or discoloured patches, a musty smell, mould, flaking paint, loose plaster and salt-like white deposits. A wall that remains cold or clammy for a long time can also warrant further investigation.

The location of the damage sometimes offers a clue. Moisture near floor level may have a different cause from a patch around a window frame, chimney or roof edge. It remains sensible not to rely on the appearance of the patch alone.

Rising damp

Rising damp occurs when moisture from the ground is absorbed into a wall through porous materials. This can be particularly relevant in older buildings without a properly functioning damp-proof course. However, a damp patch at the bottom of a wall does not automatically mean rising damp.

Leaks, excessively high ground levels, poor drainage or salts in old masonry can cause similar damage. Have the cause properly assessed before choosing injection treatment or another damp-proofing method.

Penetrating damp through the facade

Penetrating damp occurs when rainwater enters the masonry from outside. Damaged or heavily weathered joints, cracks, porous masonry and prolonged wind-driven rain can all play a part.

Check the technical condition of the facade first. Poor joints or cracks need repair before considering additional treatment. Impregnation can reduce rainwater absorption in some situations, but it is not a universal solution to every damp problem.

Read our explanation of facade impregnation and when it is or is not advisable.

Condensation and insufficient ventilation

Not all moisture in an old home comes through the facade. Warm, humid indoor air can condense on cold surfaces. This happens more readily in poorly ventilated rooms or on cold sections of an exterior wall.

For condensation, the solution is more likely to involve ventilation, heating and sometimes insulation than making the outside wall water-resistant. Mould in corners or behind furniture can therefore require a very different approach from rainwater entering through damaged brickwork.

Leaks and construction defects

A localised damp patch may be caused by a leaking gutter, downpipe, water pipe, roof covering, chimney, window frame or junction. An incorrect slope or high ground level against the facade can also cause problems.

In old homes especially, it is sensible to rule out these relatively specific causes first. Otherwise, the wall may be treated while the actual leak remains.

What do white marks or deposits on old bricks mean?

White deposits can appear when soluble salts are carried to the surface by moisture and remain there after the water evaporates. Their presence alone does not establish exactly where the moisture comes from.

Aggressive cleaning is not always advisable. Old bricks in particular can have vulnerable surfaces. Before choosing an intensive cleaning method, read our information on sandblasting old facades.

Can poor pointing cause damp problems?

Worn, eroded or cracked joints can make a facade more vulnerable to rainwater. When repairing historical brickwork, the new pointing mortar must suit the brick and the existing construction.

Mortar that is too hard or otherwise unsuitable can be undesirable for softer old bricks. Our separate article on pointing mortar for old bricks explains this in more detail.

Should you impregnate a damp old facade?

Not without establishing the cause first. Impregnation is intended to reduce rainwater absorption through the face of the wall. It does not repair a leaking gutter, crack, defective joint or problem originating in the ground.

The facade must also be technically suitable for treatment. If moisture enters the structure from another source, impregnation alone cannot resolve the underlying problem.

Replacing old bricks damaged by damp

If individual bricks have become badly damaged or their faces have flaked away after prolonged exposure, local replacement may be necessary. In historical facades, find replacement bricks that match well in size, colour and texture.

Browse our reclaimed masonry bricks. In restoration, colour is not the only consideration: dimensions and surface also determine how unobtrusive the finished repair will be.

Steps to take when an old house has damp

Begin by establishing where and when moisture becomes visible. Then check rainwater drainage, the roof, frames, junctions, ground levels, cracks and pointing. Also consider ventilation and how the room is used.

Choose a treatment only once the likely source is clear. For unclear or serious problems, a building investigation is wiser than injecting, impregnating or replastering on guesswork.

Repairing historical masonry

Repairing an old facade is not only about resolving moisture problems but also preserving the character of the existing masonry. Appropriate bricks, pointing and workmanship can prevent repairs from remaining visible as mismatched patches.

Our projects show reclaimed materials used in restoration, renovation and new builds.

Frequently asked questions about damp in old walls

Is moisture at the bottom of a wall always rising damp?

No. Its position can provide a clue, but different causes can produce similar damage. Also investigate leaks, ground levels, drainage and other construction issues.

Does impregnation help against damp in a house?

Only when the moisture actually comes from rainwater passing through an exterior facade suitable for the treatment. Impregnation does not address every kind of damp and does not replace necessary repairs.

Can a poor joint let moisture through?

Damaged or heavily weathered pointing can contribute to rainwater absorption. Repairs must suit the brick type and existing masonry.

When do old bricks need replacing?

Primarily when they are so damaged that local repair is insufficient. For a historical facade, select reclaimed replacement bricks by size, colour and texture.