Rising damp, honestly diagnosed

What rising damp actually is

Masonry is porous. Where a wall sits in wet ground with no effective barrier between the two, water is drawn upwards through the pores by capillary action, in the same way a sugar cube dipped in tea wets above the surface. It rises until the rate of evaporation from the wall face matches the rate at which it is drawn up, which in practice means it stops somewhere around a metre from the floor.

Two things follow from that, and both are useful for telling rising damp from its impersonators.

The evidence that points to it

• A horizontal upper limit. A reasonably level tide line, usually between 300mm and 1m above floor or external ground level. Damp that runs down from a window, spreads outwards from one point, or reaches head height is telling you something else.
• Salt bands. Groundwater carries dissolved nitrates and chlorides. As it evaporates at the wall face it leaves them behind, so you get a band of salting, blown plaster and paint failure at the top of the damp. Condensation leaves no salts.
• It is worse low down and constant. Rising damp does not come and go with the weather in the way penetrating damp does, and it does not concentrate on the coldest surfaces the way condensation does.
• Ground conditions that allow it. No damp-proof course, a failed or bridged one, or external ground raised above the internal floor.

What is usually happening instead

In most of the houses where we are called out expecting rising damp, we find one of these:

A bridged damp-proof course

The barrier is there and working, but something is carrying water past it — a raised path or patio, a flower bed banked against the wall, render carried down over the DPC, mortar droppings left in the cavity, or a build-up of debris in the cavity base. Lowering ground levels or clearing the bridge fixes it, and costs a fraction of a chemical injection.

Condensation

Cold external walls, especially behind furniture and in corners and unheated rooms, collect moisture from the air inside the house. It produces black spot mould, which rising damp does not, and no salt band.

Penetrating damp and plumbing

A blocked gully splashing a wall, a downpipe discharging against brickwork, a cracked sill, or a slow leak under a floor. All can produce damp low on a wall that looks convincingly like rising damp until you look outside.

If it is genuinely rising damp

Then the options are a new physical or chemical damp-proof course, along with dealing with the salt-contaminated plaster, which will keep drawing moisture from the air even after the wall is dry. That plaster removal and replacement is often the larger part of the work, and it is worth understanding that before you agree to anything.

We would rather find you a bridged DPC and a gutter to clear than sell you an injection you do not need. That is the whole point of surveying before treating — see damp surveys in Derby.