The short answer
A gutter that leaks or overflows sends water down the face of the wall, where brick, stone and render soak it up. The damp shows as dark patches, green growth, flaking paint or stains inside the room. The cure is in the right order: repair the gutter and its outlets first, then let the wall dry before redecorating or judging the damage.
How a gutter wets a wall
A gutter collects rain from the roof and carries it to a downpipe. When it works, the wall below stays dry. When it fails, the water has nowhere to go but down the wall, and a wall that is wetted repeatedly takes in more water than it can lose.
There are several routes. A joint that has opened, or a seal that has perished, drips onto the wall at one point, leaving a vertical streak. A blocked gutter overflows over the front edge, soaking the face below in a broad band. A gutter that has sagged or has too little fall holds water that spills at the low point.
Overflow at the back is worse. If a gutter is pitched the wrong way, or the fascia has rotted so the gutter leans back, water runs behind it into the eaves and into the wall top, where it can reach the roof timbers and the inside plaster.
A cracked or disconnected downpipe is another source. Water escapes at the crack and runs down the wall, and at ground level a blocked drain can overflow against the base.
The gutter fall has to be right along its whole length: Brett Martin's guide is 1 mm per 600 mm, and BS EN 12056-3 governs roof drainage design.
Reading the signs
Outside, the signs include dark or green staining below the gutter, damp patches that darken in rain, crumbling or spalling brick faces, efflorescence (white salt deposits) and algae or moss growth. Mortar joints may soften, and render can blow or crack.
Inside, look at the wall and ceiling beneath the affected area. Damp patches, peeling paint, blistered wallpaper, a musty smell and mould in corners suggest that moisture has passed through the wall. A patch that grows after heavy rain, and sits below a gutter fault, is a likely gutter problem.
Watch the pattern. Damp that follows rainfall and sits under a known defect points to the gutter. Damp that does not change with the weather, or appears at ground level, may have a different cause, such as rising damp, a plumbing leak or condensation.
A simple test helps. On a dry day, run water through the gutter from a hose and watch for drips, overflow or water running down the wall. Do this from the ground or a safe position, and do not climb up in the wet.
Our page on leaking gutter joints covers the commonest point of failure, and overflowing gutters explains the reasons for spill.
Fix the gutter first, then dry the wall
The order matters. Treating the wall while the gutter still leaks is wasted effort, because the water keeps arriving. Repair or replace the faulty gutter, clear blockages, reseat sagging sections to the right fall and renew any cracked downpipe first.
That may mean new seals at joints, replacing a union or a section, adding brackets where the gutter has sagged, or replacing the run if it is worn out. If the fascia behind has rotted, it needs repair or replacement so the gutter has something firm to hang on. A gutter fixed to rotten timber will fail again.
Once the water supply has stopped, the wall can dry. Brick and stone dry slowly, often over weeks or months, and the rate depends on weather, wall thickness and ventilation. Do not seal the surface with waterproof paint or render while it is still wet, since this traps moisture.
Indoors, ventilate the room and use a dehumidifier to help, and wait before redecorating. Plaster that stayed wet for months may need replacing, and a damp specialist can advise if the wall does not dry.
Approved Document C expects roofs to resist the penetration of precipitation, and keeping roof water in the gutter and away from the wall is part of that aim. We do not give diagnosis of damp inside the building, only of the roof and its drainage.
What we check and report
On the survey we run our eye along the whole gutter and downpipe, noting joints, brackets, falls, outlets and the state of the fascia. We look at the wall below for the pattern of staining, and photograph both.
You see the pictures with an explanation of what is wrong and where. If the gutter has simply failed at a joint, a repair may be enough. If a long run has sagged, or the fascia is rotten, we explain what replacement involves, and why.
We tell you plainly what the roofer can fix and what belongs to someone else. A damp wall that does not improve after the gutter is mended may need a builder or damp specialist.
Access is a factor. Gutters are often high, so a tower or scaffold may be needed, licensed by the highway authority where it stands on the road. The quote is one total in writing, access included.
The gutters, soffits and fascias hub covers the full range of roofline work, and a leak caught early is nearly always cheaper to fix than the wall damage that follows.
