Every year the first proper run of wet weather brings the same phone call. A brown patch has appeared on a bedroom wall, or the paint is bubbling above a skirting board, or there’s black mould behind the wardrobe that wasn’t there in summer. The question is always the same: is this damp, and how bad is it?
“Damp” is really three different problems with three different causes. Get the diagnosis right and the fix is usually simpler and cheaper than people fear. Get it wrong and you can spend a lot of money making the wall wetter. This is how to tell them apart in a Bath home.
The Three Kinds of Damp
Condensation: the most common, by a long way
Condensation is moisture from inside the house settling on cold surfaces. Cooking, showers, drying washing and simply breathing all put water into the air. When that air meets a cold wall, the water comes out of it.
Bath’s terraces and villas are mostly built with solid stone walls, with no cavity. Solid walls stay colder than modern cavity walls, so they attract condensation more readily, especially on outside walls, in corners and around window reveals.
Signs it’s condensation:
- Black mould spots in corners, behind furniture or around windows
- Water running down the inside of the glass in the morning
- Worse in bedrooms and bathrooms, worse in cold snaps
- Spread over a wide area rather than one defined patch
The fix is ventilation and steady, low heat, not sealing the house up. Run the extractor fans, open trickle vents, don’t dry washing on radiators in a closed room, and pull furniture a few inches off outside walls. It’s not a builder’s job unless the extraction itself needs fitting or upgrading.
Penetrating damp: rain getting in from outside
Penetrating damp is rainwater getting through the wall, roof or a joint. It’s the one we see most on older Bath properties, and it almost always has a clear source you can find from outside.
Signs it’s penetrating damp:
- One defined patch, often with a stained or tide-like edge
- Worse after heavy or wind-driven rain, better after a dry spell
- Lines up with something outside: a gutter, downpipe, window sill, chimney or a bay roof
Common causes on Bath properties:
- Blocked or leaking gutters and downpipes soaking one section of wall (a routine property maintenance job)
- Failed pointing, or a wall that’s been repointed in hard cement instead of lime mortar
- Cracked or blown render that needs stripping and re-rendering
- Worn lead flashing around chimneys, dormers and bay windows
- Window sills without a working drip, or failed seals around windows and doors, so water runs back into the wall
Rising damp: real, but rarer than you’ve been told
True rising damp is ground water being drawn up into the base of a wall. It does happen, but it’s diagnosed far more often than it actually exists, especially in old houses.
Signs it might be rising damp:
- A tide mark at the base of the wall, usually below about a metre
- White, fluffy salt deposits on the plaster
- Rotting skirting boards or flaking paint at low level
- Present all year round, not just after rain
In a Bath stone house, “rising damp” is very often something else. Earth or a patio has been built up against the outside wall over the years, so the ground is now higher than the floor inside. Or a modern cement render, gypsum plaster or plastic paint has trapped moisture that the wall used to let out.
The Quick Test You Can Do Tonight
Tape a square of kitchen foil, about 30cm across, tightly to the damp area and leave it for 24 to 48 hours.
- Moisture on the room side of the foil: it’s condensation.
- Moisture underneath, against the wall: the water is coming through the wall, so it’s penetrating or rising damp.
It’s not a lab test, but it’s surprisingly reliable at pointing you in the right direction.
Why Old Bath Walls Need to Breathe
This is the part most people never get told.
Bath’s Georgian and Victorian houses were built from Bath stone bedded in lime mortar. Both are porous. When it rains the wall takes on some water, and when it stops the water evaporates back out. That’s how the building was designed to stay dry. There was never meant to be a perfect seal.
Over the last fifty years, a lot of these walls have had that balance wrecked. Lime pointing has been raked out and replaced with hard cement. Lime plaster inside has been replaced with gypsum. Outside walls have been coated in cement render or masonry paint. All of these stop the wall drying out.
The water still gets in through cracks and joints, but now it can’t get out. So it stays in the wall, comes through on the inside, and in a frost it freezes and starts pushing the face off the stone. Spalling stone next to cement pointing is one of the most common things we see on terraces in Oldfield Park, Larkhall and Widcombe.
The Fixes That Make It Worse
- Chemical damp-proof injection on a stone wall. It’s often sold after a free “damp survey”. Injecting a solid rubble stone wall is rarely effective, and if the real cause is a blocked gutter or raised ground level, it treats a problem you don’t have.
- Waterproof paints and tanking on the inside. They hide the patch for a season while the wall behind gets wetter.
- Cement repointing or render “to keep the water out”. On Bath stone this is usually the cause, not the cure.
- Damp meter readings on their own. Standard meters are designed for timber. Salts in old plaster make them read high even on a dry wall, so a reading alone proves very little.
If someone offers you a damp survey and the same company sells the treatment, get a second opinion before you sign anything.
What Actually Fixes It
The order matters.
- Stop the water at the source. Clear and repair gutters and downpipes, fix flashing, lower the ground level against the wall, sort out sills. Our property maintenance team handles this kind of work, and often it solves the problem on its own.
- Let the wall dry. A thick stone wall can take months to dry out properly. Patience here saves money later.
- Put back breathable materials. Rake out cement pointing and repoint in lime. Replace failed cement render or gypsum plaster with lime render and plaster. This lets the wall manage moisture the way it was built to.
On listed buildings and in conservation areas, step three is also what the conservation officer will expect. Our heritage restoration team works to those standards as a matter of course.
Why Winter Is the Time to Look, Not the Time to Plaster
Damp is easiest to diagnose in winter, because that’s when it shows. So now is the right time to find the cause, and the quick outside fixes like gutters, downpipes and flashing can be done straight away.
Lime work is different. Lime mortar and lime plaster don’t cure properly below about five degrees, and a hard frost can ruin fresh work. So any repointing or replastering we diagnose now is usually booked for spring. Getting the diagnosis and the quote done in winter means you’re at the front of the queue when the weather turns.
Common Questions
Is damp worse in winter?
Condensation is, because walls are colder and windows stay shut. Penetrating damp is worse in wet, windy months. True rising damp stays fairly constant all year.
Can I just paint over a damp patch?
Not until you’ve found and fixed the cause. Stain-block paint will hide it for a while, but the damp will come back, often with flaking paint and salts on top.
Does my house need a damp-proof course?
Many older Bath houses were built without one and stayed dry for 200 years because the walls could breathe. Before paying for a new damp-proof course, rule out gutters, ground levels and cement pointing or render.
How long does a damp wall take to dry out?
It depends on the wall, but a thick stone wall can take several months once the water source is fixed. Replastering too early just traps the moisture again.
Get a Proper Diagnosis
If you’ve got a damp patch and you’re not sure what’s causing it, we’ll come and look. James or one of the team will check outside first, find where the water is actually coming from, and tell you honestly what needs doing now and what can wait until spring. If it’s condensation and you just need better ventilation, we’ll tell you that too.
See our plastering and rendering service for lime plaster and render work, or our property maintenance service for gutters, downpipes and flashing. Or just get in touch and we’ll book a visit.
