What do white salts and flaking plaster mean on a damp wall?

White salts and flaking plaster tell you that the wall needs investigating, but they do not identify one particular type of damp. Salts can be left where moisture has evaporated, including in old walls that are almost dry now. Before replastering, find out whether water is still entering and where it comes from. A new finish can hide a continuing problem without solving it.

What can the white deposits tell you?

Salt deposits are a sign to look at moisture movement through the wall. Rising damp can leave a horizontal tide-mark with discolouration below it, but salt on its own does not prove rising damp. The SPAB notes that old walls can give high electrical moisture meter readings because salts remain after evaporation even when the wall is virtually dry. A reading and a white mark therefore need context.

Notice where the deposits sit, whether there is a tide-mark and whether the area changes after rain. These observations help an assessor test possible routes. They cannot, by themselves, distinguish present moisture from residue left by an older problem. Flaking plaster also shows that the surface has been affected, but the damaged finish is not an explanation of the water's origin.

Could water be coming from somewhere else?

Rising damp is moisture moving up from below ground, and it is separate from rain penetration and condensation. Rain entry can affect old buildings at roofs, walls and chimneys. A chimney stack is worth checking where a stained wall meets an old breast: eroded joints in the stack, damaged flashings or a leaking back gutter can allow dampness. The white salt itself cannot confirm that this is the route.

Compare the indoor mark with what is outside and above it. If there is a chimney, check its exposed masonry and junction with the roof. If the issue is lower down, consider the surrounding wall and ground conditions. The aim is to follow evidence of water, rather than choosing a treatment because the deposit resembles a photograph of a particular damp type.

Why diagnose before replastering?

A fresh plaster surface cannot stop water entering through a defective stack or other route. Salt-resistant plasters can temporarily disguise a continuing damp problem. On an old wall, an inappropriate waterproof render or coating may make matters worse. Identify and address excess moisture before deciding which plaster repair is suitable for the building.

If rising damp has actually been established and remedial work is needed, the SPAB normally favours lime plaster for repairs to an old wall. That is guidance for a diagnosed situation, not a reason to apply lime plaster to every flaking patch. The choice of finish should come after the condition of the wall and the moisture route are understood.

Salts in Hebden Bridge stone terraces, Yorkshire

In a stone terrace around Hebden Bridge, an inside wall with salts may sit below an old chimney or against an exposed outer wall. Rain entry through weathered stack joints or roof flashings is one possibility to inspect; an earlier leak may also have left deposits behind. The salt line does not choose between those explanations. Ask for the outside details and the current moisture condition to be checked before any finish is replaced.

A practical next step

Record the pattern of deposits, flaking and any staining. Note whether the wall appears to change with rain and whether a chimney, roof junction or outside wall lies nearby. An independent, suitably qualified surveyor can assess the wall and seek the source of moisture if it is unclear. Ask what evidence supports the diagnosis before agreeing to substantial damp-proofing or plaster replacement.

Be wary of a single product offered as the answer to all salts. A retrospective damp-proof course is not automatically needed in an old building, and waterproof coatings can aggravate damp. The useful sequence is to identify the route, deal with any active defect, then repair only the affected finish with material suited to the wall. That leaves room for the possibility that the salts are a trace of an older issue rather than proof of a current one.

This page gives general information, not legal or medical advice. For your own situation, use the organisations listed under "Where this comes from" at the foot of the page.