Damp, mould & air
Penetrating damp: find the way in
Rain is patient. Give it a blocked gutter, a cracked cill or a strip of tired pointing and it will find the same way in every time it falls. Penetrating damp is diagnosed outside, in the rain if you can manage it, and the inside repair always waits its turn.
By the Tickover team · Published 20 April 2026 · 3 min read
How to recognise it
Of the three damp families, penetrating damp is the one that follows the weather. Patches darken during or after rain, sometimes hours later (a thick solid wall can lag a storm by a day or more), then fade through dry spells, often leaving a brownish stain. Compare that rhythm with condensation, which tracks the seasons and your own routines instead.
Is rain the culprit?
Walk the outside
Pick a wet day and walk the wall the patch lives on. You are matching height and position: damp inside at picture-rail height means the fault outside sits at that height or above it, because water travels down and sideways, almost never up. Binoculars beat ladders for everything above the ground floor.
The walk-round, top to bottom
- Gutters in heavy rain: overflow, drips at joints, water behind the run
- Downpipes: cracks, loose brackets, water tracking down the wall
- Flashings round the chimney and roof junctions (binoculars)
- Pointing: crumbly, cracked or missing mortar at the patch height
- Render: cracks, hollow-sounding patches, green staining
- Window cills: cracks, or the drip groove underneath clogged with paint
- Ground level: soil or paving piled above the damp-proof course
- Air bricks: blocked, painted over or buried
Start with the gutters: they are the most common culprit and the cheapest fix. A joint that drips onto the same brick for a season will soak a wall from the outside in. Then look at pointing, render and cills at the patch height, and only then at tiles and flashings above. Green algae stripes on brickwork are a map of where water already runs.
Fix outside, then dry inside
The order is fixed: stop the water, let the wall dry, then repair the inside. The old rule of thumb is a month of drying for every inch of wall thickness, so a solid brick wall can spend most of a year drying out. Redecorate too early and the fresh paint blisters off a wall that was still exhaling.
While it dries, ventilate the room, heat it gently, and resist sealing the wall under impermeable paint. If plaster has blown or salts have crusted through, a plasterer will say what needs to come off. And if the way in looks like a widening crack rather than a worn joint, read cracks in walls before anything else.
Common ways in, put right
Gutter clean (2-storey)
Gutter section repair
Replace 1–5 slipped tiles
Chimney flashing repair
Typical UK ranges, mid-2026, as a guide only. Your home, your area and the job itself all move the number — always compare real quotes.
Repointing and render repairs vary too much with area and access for one fair range; for those, get two or three quotes and ask each trade what they think let the water in. The answers are a diagnosis in themselves.
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Quick answers
How do I know if damp is penetrating or condensation?
Watch the weather. Penetrating damp darkens during or after rain, sits in a defined patch and often stains brown. Condensation tracks cold weather and daily routines, favours corners and window reveals, and grows black mould rather than stains. A week of noting patch against forecast usually separates them without an expert.
Why does the damp patch appear a day after it rains?
Walls take time to saturate. In a solid brick wall the moisture front can take hours or days to cross, so the inside patch lags the storm that caused it, and the thicker the wall, the longer the lag. That delay is itself a clue that water is crossing the wall rather than condensing on it.
Can I fix penetrating damp myself?
The ground-level causes, often: clearing a gutter you can reach safely from a sturdy ladder, unblocking a downpipe, cutting back soil bridging the damp-proof course. Anything at height, from pointing to flashings, is for someone with proper access equipment. The diagnosis walk-round, though, is all yours, and it is the valuable part.
Should I use damp-proof paint inside?
Not until the way in is fixed and the wall has dried. Sealing a wet wall traps moisture in the masonry, pushes it out somewhere else and tends to peel in time. Once the outside is sound and the wall has dried through, normal breathable decoration is usually all you need; sealers are a last resort.
This guide is general information for UK homes, not advice about your specific property. Anything involving gas must be done by a Gas Safe registered engineer, and notifiable electrical work by a qualified electrician. If you smell gas, call the National Gas Emergency line on 0800 111 999. If you’re ever unsure, stop and get a professional in — that’s exactly what Tickover is for.
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