A damp patch that follows the rain usually starts outside. Here is how to confirm it and what to fix first.
A dark, damp patch on an outside wall is easy to blame on the weather inside the house. Often the real cause is much simpler and sits outside: a gutter that has stopped doing its job. When a gutter overflows, a joint drips or a downpipe blocks, rainwater runs down the face of the wall instead of away from it. Brickwork soaks it up, and the wet eventually shows on the inside as stains, flaking paint or a musty smell. This guide shows how to confirm that a gutter is the cause, what to fix, in what order, and when a dehumidifier is not the answer.
Damp that appears or grows after rain, usually below a gutter joint, downpipe or roof edge, is typically penetrating damp from a leaking or blocked gutter, not condensation. Confirm it with a free walk-round in the rain, clear and repair the gutter, and let the wall dry before redecorating. A dehumidifier does not fix water coming in from outside, so repair first.
Gutters and downpipes carry rain from the roof to the drains. When they work, the walls stay out of the way of most of the water. When they fail, a large roof area can send a lot of water down one spot on a wall. GOV.UK guidance on damp and mould describes penetrating damp as water that gets into a building from outside through defects, and it names damaged gutters and blocked downpipes as examples, alongside broken window seals, cracks and worn roof finishes.
The tell-tale feature is the link with rain. A leak patch tends to appear or darken after a wet spell, can have a tide-mark around its edge, and often sits directly beneath the fault. Condensation behaves differently: the Property Care Association describes it as appearing on cold surfaces, more so in winter, and linked to everyday moisture from cooking, showers, washing and drying and even breathing. Getting that distinction right saves money, because the fixes are completely different.
Pick a wet day and look at the outside of the house before you touch anything. Standing safely on the ground, check these places:
Then go inside and compare. Photograph the damp patch with the date, and mark its edge in pencil. Check again after the next dry spell and the next wet one. If the patch grows in the wet and fades in the dry, water coming in is the likely cause. If it follows cold nights and ignores the rain, think condensation instead and read our guides to condensation on windows and black mould in bedroom corners.
Blocked gutters are the commonest cause and the cheapest fix. Leaves, moss and silt build up and rainwater has nowhere to go but over the edge. Clearing them is a simple job, but ladder falls are a real source of serious injury, so use a stable, correctly set-up ladder with someone to foot it, or hire a gutter-clearing or roofing tradesperson if you are not confident or the gutter is high. Do not lean out sideways.
If the gutter still drips after clearing, the fault is in the gutter itself. Common repairs include re-sealing or replacing a joint, re-fixing a bracket so the gutter falls gently towards the outlet again, and replacing a cracked section or a split downpipe. Old cast-iron gutters can rust through, and plastic ones can crack with age. A roofer or a general builder can usually deal with this in a short visit.
Also look at the places where the roof meets the wall. Worn flashings, slipped tiles and damaged roof finishes can send water to the same spot, and GOV.UK guidance counts compromised roof finishes as a cause of penetrating damp. If the patch is high on the wall or on a ceiling, the roof is worth checking too. A bay window roof or a lean-to is a frequent weak point.
Once the fault is repaired, give the wall time to dry. This can take a good while with solid brick. Resist the urge to repaint or fit new boards over a wall that is still wet, because the moisture stays trapped and the damage comes back. Where mould has formed, GOV.UK guidance says it can be removed from hard surfaces with an appropriate cleaning product and the surface left to dry completely. It also warns that simply removing surface mould will not stop it coming back unless the damp is dealt with.
A dehumidifier does nothing about water entering from outside. If a gutter is still leaking, the wall will keep getting wet and no amount of drying the air will change that. That is why this rung comes last.
Once the leak is repaired and the wall has dried, you may still have condensation on the colder walls, because a gutter fault and condensation often turn up together. In an owner-occupied home with a loft, a loft-mounted Dryzen unit can then help keep humidity near 50-55%, which helps stop the conditions mould and dust mites need. One unit covers up to 150 m2 and extracts around 12 litres a day, with a running cost of about GBP 20 a year. It is not suitable for rentals unless the landlord authorises the changes, it needs a loft to mount in, and the insulated ductwork only serves the central area or stairway. It does not remove mould that is already growing, so clean that off first.
Optional free remote monitoring of your home and your Dryzen unit, included with yearly Gold membership: we manage the humidity, monitor the unit's performance and alert you if there are any issues.
Homes across Chatham and the Medway Towns range from older brick terraces to later estates, and in all of them a failed gutter or downpipe sends water to the same spot on the wall again and again. Rain-driven moisture is a particular worry on exposed gable ends and on terraces where one shared gutter or downpipe serves more than one house, so it is worth knowing where your own run starts and ends. Our pages for Rochester, Gillingham and Upnor explain how different housing ages behave, and what to check first.
A leak follows the rain. The patch appears or grows after wet weather, often directly below a gutter joint, downpipe or roof edge, and it can leave a tide-mark. Condensation follows the cold instead, showing up in corners and behind furniture in winter whatever the weather outside. GOV.UK guidance lists damaged gutters and blocked downpipes among the causes of penetrating damp.
Light clearing from a stable, correctly set-up ladder is within many people's reach, but falls from ladders cause serious injuries. If you are not confident, or the gutter is high, ask a gutter-cleaning or roofing tradesperson. Never lean out sideways or overreach.
No. A dehumidifier lowers moisture in the indoor air. It does not stop water getting into the wall from outside, so the leak has to be repaired first. Once the wall has been given time to dry, a dehumidifier is only worth considering if condensation is still a problem.
It varies with the wall, the weather and how long it was wet, and it can take weeks or longer for thick solid brick. Do not repaint or cover the area until it has dried and stopped changing between checks, or the moisture stays trapped.
Sources: GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Property Care Association: Condensation in homes.
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