Most damp we find in Cotswold stone homes isn’t caused by a missing damp-proof course. It’s caused by the wrong materials, usually cement, used on walls built for lime.
How old stone walls work
Traditional Cotswold walls are solid stone, bedded and pointed in lime mortar. They have no cavity and no damp-proof course. They get a little wet, then dry out again.
Lime is softer and more porous than the stone around it. Moisture leaves the wall through the lime joints, not through the stone. When something has to give, it’s the mortar, which is cheap to replace, rather than the stone.
What goes wrong with cement
Cement is harder and far less porous than lime. Used on an old wall, it traps moisture inside. The water then has to escape through the stone faces instead. That leads to:
Damp inside: stained plaster, peeling paint and musty smells, often low down or around chimneys.
Crumbling stone: stone faces flaking or “blowing”, especially after frost.
Cracked render: hard cement renders crack, let rain in, then stop it drying out.
Rot in timbers: joist ends and lintels sitting in damp walls can decay.
Other common causes
Raised ground levels or patios built up against the wall
Blocked or leaking gutters and downpipes
Gypsum plaster and plastic (vinyl) paints on the inside face
Poorly ventilated rooms, especially after windows have been sealed up
Be wary of the quick fix
Injected chemical damp-proof courses and waterproof tanking are often the wrong answer for a solid stone wall. They can make things worse by trapping more moisture. Be cautious about damp reports from firms that also sell the treatment.
The right fix
Usually it’s a mix of the following:
Lower ground levels and clear drainage away from the walls.
Fix gutters, downpipes and leadwork.
Carefully rake out cement pointing by hand and repoint in lime.
Replace cement render with lime render where needed.
Use breathable plasters and paints inside.
Give the wall time to dry, often a few months.
Work should be done by a tradesperson experienced in lime. On listed homes, check whether consent is needed first.
How we help
We recommend a RICS Level 3 Building Survey for Cotswold stone homes. We explain what’s normal for an old wall, what needs watching and what needs fixing, and we call you after the inspection to talk it through. After your report, our helpful guides explain your next steps.
Buying a stone home in the Cotswolds? Use the Get My Quote button above, or call our Cotswolds line on 01451 828107.
Max Daniel AssocRICS, Director, MJD Residential Surveying