A damp patch that grows after heavy rain, on a wall with cracked render or crumbling mortar, is usually water coming in from outside. Here is how to check, and what to fix first.
Damp that appears or spreads after heavy or wind-driven rain, mostly on outside walls, and sits near cracked render, open mortar joints, a leaking gutter or a faulty damp-proof course is usually penetrating damp. GOV.UK guidance lists cracks in render among the external defects to check. Repair the way the water gets in, give the wall time to dry, then see whether any condensation is left. A dehumidifier does not repair a wall that is letting rain in.
A patch of damp on an inside wall that darkens after a wet spell and fades in dry weather. Sometimes it is a tide-mark stain, sometimes bubbling paint or lifting wallpaper, sometimes a patch of mould that keeps coming back however often it is cleaned. On the outside the same wall may show hairline cracks in the render, a section that sounds hollow when tapped, mortar joints that have crumbled back from the brick face, or a bulge near a window or door.
GOV.UK guidance describes penetrating damp as water that gets into the building from outside through defects in walls, roofs, windows or floors. It describes condensation damp as moisture made inside the home that cools and condenses on colder surfaces such as window frames, corners and the low points of walls behind sofas or wardrobes. The two can sit side by side in the same room, which is why the pattern matters more than the single patch.
Render and pointing are the weather skin of a wall. When they are sound, they shed most of the rain that hits the wall. Once they crack, rain can be drawn in behind the surface. The wall then takes longer to dry out between showers, particularly in winter when the air is already humid and there is little wind or sun to help. Wet masonry also conducts cold more readily, so the inside face of the wall gets colder, and that invites condensation on top of the leak. A small defect can therefore produce a damp patch much larger than the crack itself.
The same guidance lists the external signs to look for: damaged or blocked pipes, gutters or downpipes, broken seals around windows, visible structural or facade defects such as cracks in render or the foundation, missing or broken roof finishes, and a bridged damp-proof course. It also lists the internal signs, including peeling wallpaper, visible damp or staining, defective plaster and low internal wall temperature.
Work down this list in order. Each step is cheap to check and tells you whether you need the next one.
Surface mould will come back unless the cause is fixed, and GOV.UK guidance is clear that simply removing surface mould will not stop damp and mould from reappearing. Clean the surface sensibly in the meantime, protect your lungs and skin as the packaging advises, and photograph the area before you start so you can see whether it returns. If you have asthma, a weakened immune system or young children at home, or if the mould covers a large area, take advice before cleaning it yourself.
Winters around Ashford are mild and wet. That keeps walls damp for long spells and gives them little chance to dry out, so a small weather defect can matter more here than a quick glance at the wall suggests. The area mixes older village houses with newer estates, and the failure points differ. Older brick and rendered walls tend to show cracked render, eroded pointing and gutters that have silted up. Newer homes are more likely to show gaps at window reveals, sealant joints and cladding junctions. Our area pages for Wye and Tenterden cover the older-building side, where it is worth checking with the council before changing the outside of a building.
If rain is getting into a wall, a dehumidifier will not stop it. It can lower the humidity of the air in the home, but it cannot dry out a wall that is being re-wetted from outside, and it should not be sold to you as a way of avoiding a repair. The same goes for damp that rises from the ground, a leaking pipe, or a blocked gutter. Get the building fixed first. A Dryzen unit also needs a loft to mount it, and it is not suitable for rentals unless the landlord authorises the changes. If you rent, report the problem to your landlord in writing, with your dated photographs.
A Dryzen system is a loft-mounted dehumidifier with insulated ductwork to the central area or stairway of the home. Each unit is rated for 150 square metres and has an extraction rate of about 12 litres a day, and it uses about 20 pounds a year in energy. It has an activated carbon filter, and the smart app, which shows temperature and humidity, is included. Parts and labour are covered for 10 years, with 1 year on the compressor. It suits owner-occupied homes with a loft. It is not suitable for rental properties unless the landlord authorises the changes. If you have already repaired the outside, the wall has dried and you still see condensation, mould in cold corners or a clammy feel in winter, a free survey can measure humidity through the day and tell you honestly whether a system would help.
Penetrating damp follows the weather and sits on outside walls near a defect such as cracked render, open pointing, a leaking gutter or a bad window seal. Condensation follows cold mornings and shows on glass, in corners and behind furniture. Many homes have both.
Paint or filler can hide a crack for a while, but it does not tell you why the render cracked or whether water is getting in behind it. Find the cause first, and ask a builder how the repair suits the wall.
GOV.UK guidance suggests checking at least six weeks after remedial work. Thick walls can take longer to dry, so judge the result across a full wet spell.
Sources: GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Property Care Association: Condensation help and advice for homeowners
Last reviewed 2026-10-18. General guidance only; this is not a survey of your home.