Why is rain staining the wall below my gutter?

A gutter that works sends every drop to the downpipe. A stain on the wall means some water is taking another route, and its shape is a clue. A dark patch directly under a joint or a corner points to a failed seal or a crack. A long, even tide mark along the whole wall usually means overflow, because the run is full of debris or is too small for the roof above.

Green or black streaks that run down from one point are algae feeding on a steady trickle. Pale, chalky tracks can be salts brought to the surface as the brick dries out. A stain only at the downpipe suggests the outlet or the pipe is blocked and the water is backing up. Look at the stain in dry weather and again in rain to learn which one you have. Take a phone picture of it each time, because a stain that grows after a storm is easier to prove than one you remember.

Four faults cover most cases. First, a joint where the seal has perished lets water seep out at the union. Second, a blocked gutter or outlet fills and spills over the front lip. Third, a run that sags or has lost its fall holds water in a pool and then overflows at the low point. Fourth, a crack from age or from the sun makes a slow leak that wets the same piece of brick every time it rains.

Our answers on leaking gutter joints and a sagging gutter deal with two of them in detail. A roofer looks at all four before recommending the smallest repair, and photographs the fault so you can see it. Often the cure is a new seal, a short length of replacement gutter or a cleared outlet, not a whole new run.

A wall that is soaked repeatedly in the same place stays damp for longer, and the damp is not just cosmetic. Brick and render hold water, and the surface can go green with algae, shed paint or render, and push damp through to the inside of the house, where a patch of wall may peel or turn mouldy. Where a stain is on the outside wall of a bedroom or a hallway, check indoors for a damp patch at the same height.

Fixing the gutter stops the supply, and a wall that is dry through the next spell of settled weather has had the chance to recover. The wall then needs time to dry before you clean or repaint it, otherwise you seal the water in. Our answer on blocked gutters and damp describes how a spilling gutter can make itself felt indoors, and what to look at first if you can see it. Also check the ground below: a splash zone full of mud and a green base to the wall tell the same story.

The roofer works from the stain upwards. Running water through the gutter, or waiting for rain, shows exactly where it escapes. A failed seal is renewed, and a crack in a plastic run can sometimes be replaced as a single section. A blocked gutter is cleaned, and a run that has settled out of line is lifted back to the right slope, as our answer on what fall a gutter needs explains.

When the whole run is brittle, cracked in several places and past patching, a replacement is cheaper than chasing leaks. You get a clear verdict on the gutter, whether to repair, patch or replace, with survey photographs, and a single total price on paper. If the fascia behind it is also wet and soft, that needs attention at the same visit, since fixing the run on a rotten board would not last.

Once the gutter is mended and the wall has dried, the stain can often be lifted. Algae and dirt usually respond to a soft brush and mild cleaner made for the surface, whereas pressure washing can strip paint or force water into old brickwork and render, so use it with care. Efflorescence, the white salt bloom, is best brushed off dry, and it often fades by itself once the wall stops being wet. Painted render or masonry paint that has flaked will need patching after it dries.

If the stain returns after cleaning, the gutter is still failing or the cause lies elsewhere, such as a roof edge or flashing that sheds water onto the wall. Our guttering, soffits and fascias page lists what we can look at, and a survey will show whether the fault is in the gutter or the roof above it.