Rising damp

Groundwater moving upward through masonry by capillary action, because there is no effective barrier stopping it. Almost every building from the 1875 Public Health Act onward was built with a damp proof course precisely to prevent this, so genuine rising damp generally means one of a small number of specific failures rather than a general condition of old houses.

The usual causes: a bridged damp proof course, where a raised flowerbed, a new patio, a render coat or an internal floor screed has created a path for moisture around the barrier; an absent DPC in a genuinely early building; or a physically damaged one. Note that two of those three are external problems with external fixes, and neither needs the walls injecting.

Capillary rise is also self-limiting. Water will not climb indefinitely — roughly a metre is the practical ceiling in most masonry. Damp appearing at 1.8 metres, or on a first floor, is not rising damp whatever a meter says.

A conductance moisture meter measures electrical resistance. Hygroscopic salts, which are left behind in plaster by any historic moisture and pull water out of the air indefinitely, read as extremely wet on such a meter — even when the wall behind is bone dry and the original cause was resolved years ago. Condensation on a cold wall reads wet too. So does a slow plumbing leak.

Treating salt contamination with a chemical damp proof course achieves nothing, because the injected barrier addresses a route the water is not taking. The replastering that accompanies it often does appear to work, because it removes the contaminated plaster — which means the customer paid for two things and only needed one.

This is why the diagnosis comes first here, and why an honest assessment quite often ends with 'you do not need this'.

Where the evidence genuinely supports it: a chemical damp proof course, injected as a cream or fluid into the mortar bed at the correct height, forming a water-repellent barrier through the wall thickness. Then replastering with an appropriate salt-resistant specification, because the existing plaster is contaminated and will keep reading and behaving damp otherwise.

Where the cause is a bridge rather than a failure, the fix is to remove the bridge — lower the ground level, cut back the render, clear the cavity, deal with the patio. That is cheaper and more durable than injecting a wall whose DPC was working perfectly well all along.

Work is quoted fixed price, agreed in writing before it starts, and most of it carries a ten-year written guarantee, insurance-backed if you want that.