Why does damp come through my wall only after heavy rain?

Penetrating damp behaves completely differently from the other two causes: it comes and goes with the weather, it has no level top edge, and it appears at whatever height the fault is. That makes it the easiest of the three to prove and often the cheapest to fix, because the answer is usually a length of gutter or a bag of mortar rather than a treatment.

What you'll need

• Waterproof coat and a torch
• Binoculars
• Ladder for gutter level only, used safely (optional)
• Hose (optional)

Step by step

1. Mark the patch in pencil on a dry week.Draw round it and date it. If the outline grows after rain and shrinks a week later, the diagnosis is settled before you have looked at anything.
2. Work out which way the weather comes from.In most of the UK that is the south-west. Damp on the south-west and west elevations, and not on the sheltered ones, is a strong indication of driving rain rather than a plumbing leak.
3. Go outside in the rain and watch.This is the single most informative half hour available and it can only be done in the wet. Look for gutters overtopping, water running behind a gutter at the fascia, a downpipe joint spraying, water sheeting down a wall from a sill instead of dripping clear, and puddles standing against the brickwork.
4. Look directly above the patch.Water travels down and sideways along mortar beds, so the entry point is usually higher than the stain and often a little to one side, but rarely below it. Scan the elevation above with binoculars.
5. Check the rainwater goods carefully.Blocked or sagging gutters, a joint dripping onto the wall, a downpipe discharging onto a wall rather than into a gully, and a hopper overflowing behind a bay. These cause more penetrating damp than the fabric itself.
6. Check the sills and the heads.A window sill should project past the wall and have a drip groove on its underside. A sill with a filled or missing groove sheds water straight down the wall below it, which is the classic damp patch under a window.
7. Check the render, the pointing and the flashings.Cracks in render let water in and the render then stops it leaving. Failed pointing lets it in at the joints. Flashings at abutments, chimneys and porch roofs leak more often than tiles do.
8. Test a suspect area with a hose, one element at a time.Run water gently on one specific area for twenty minutes while somebody watches inside, then move on. Testing everything at once tells you nothing. Never point a hose up under tiles or into a flashing.
9. Fix the cheapest and most likely thing first.Clear the gutter, reseat the downpipe, reinstate the drip groove, repoint the failed joints. Then wait for the next storm and see whether the patch grows.

Tips

• A wall that has been wetted for years will stay damp inside for months after the leak is fixed. Do not conclude the repair failed based on the first fortnight.
• If the patch is on a party wall or a shared elevation, take photographs during rain before you raise it with a neighbour or a management company.
• Weep holes in the bottom of a cavity wall and above openings are there to let water out of the cavity. Blocked or painted-over weep holes are a real and commonly missed cause.

Common mistakes

• Looking for the fault level with the damp patch — Water enters high and tracks down, often along a mortar bed or the back of a render coat, so the entry point can be a metre or more above the stain and offset sideways.
• Rendering or painting over the wall to stop it — Any coating over an unfixed entry point traps the water instead of excluding it, and the patch reappears larger.

If it doesn't work

Questions people ask