Plastering guide

Damp in old Devon houses: what it usually really is

Most damp in an old Devon house is not rising damp, whatever the chemical damp proofing salesman said. It is nearly always a solid wall that has been sealed up with cement render or gypsum plaster and can no longer dry out. Re-plastering over it does not fix it, and injecting it with chemicals does not either.

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Old Devon houses were built wet and dry out

A cob, stone or solid brick wall has no damp proof course, no cavity, and it was never supposed to have either.

The wall gets damp. That is not a fault. What matters is that it can dry out again, and it does that by letting moisture evaporate out through a soft, breathable lime plaster on the inside and a lime render or bare stone on the outside.

That is the entire system. Water comes in, water goes out, the wall stays sound. It has worked for centuries.

What breaks it

Cement render on the outside. Gypsum plaster on the inside. Modern impermeable paint on either. A concrete path laid up against the wall, bridging the plinth. A ground level that has crept up over the years.

Any of those stops the wall drying, and once it cannot dry, the moisture goes looking for somewhere else. It ends up in the timber, in the plaster, and at the base of the wall where it looks exactly like rising damp.

So is rising damp real?

It exists, but it is diagnosed far more often than it occurs, and it is a very profitable diagnosis for anyone selling a chemical damp proof course.

In an old Devon house, damp at the bottom of a wall is more usually one of: a sealed wall that cannot dry, ground levels built up too high, a leaking gutter or downpipe, a failed gulley, or salt contamination in the plaster holding moisture out of the air.

Injecting a chemical DPC into a cob or rubble stone wall is close to useless, because the wall is not a neat regular masonry structure with a clear line to inject into. It also does nothing about the actual cause.

Why the plaster keeps blowing

Because you are treating the symptom. Hack off the blown plaster, skim it again with gypsum, and you have sealed the wall up again. The moisture that had nowhere to go still has nowhere to go, and in eighteen months to two years it will push the new plaster off exactly as it pushed the old one off.

Salts make it worse. Groundwater carries salts into the wall, and when the water evaporates the salts stay behind. Those salts pull moisture out of the air, so the wall stays damp even in a dry spell, and they crystallise behind the plaster and force it off the wall.

You cannot skim over that. It has to come off, and the wall needs to be given a way to dry.

What actually fixes it

Find the water first. Gutters, downpipes, gullies, ground levels, a leaking pipe, a failed roof. It is remarkable how often the answer is a blocked gutter and not the wall at all.

Then let the wall breathe. Take the cement render off. Take the gypsum off. Put lime back on, inside and out. If the wall is salt-contaminated it may need to dry out for months before it can be finished, and anyone promising to sort it in a week is not being straight with you.

It is slower and it is less profitable than injecting chemicals into a wall. It is also the thing that works.

Good to know

Common questions

Is rising damp real?

It exists, but in old Devon houses it is diagnosed far more often than it actually occurs. Damp at the base of a solid wall is more usually caused by cement render or gypsum plaster sealing the wall, by ground levels being too high, or by a leaking gutter or downpipe.

Why does my plaster keep blowing?

Because the wall cannot dry out. If a solid wall has been sealed with gypsum or cement, moisture has nowhere to go and it pushes the plaster off. Salt contamination makes it worse. Re-skimming with gypsum just restarts the same cycle.

Will a chemical damp proof course fix it?

Usually not, in an old Devon house. Injected DPCs are close to useless in cob or rubble stone, because there is no regular masonry course to inject into, and they do nothing about the actual cause of the damp.

Should I re-plaster over damp?

No. It will blow again, usually within two years. The wall has to be able to dry, which means finding where the water is coming from and using breathable lime rather than gypsum.

How long does a damp wall take to dry out?

Longer than anyone wants to hear. A badly saturated solid wall can take months, sometimes a full drying season. Anyone promising to fix a genuinely wet wall in a week is telling you what you want to hear.