Why is water coming into my house when it rains?

Water appearing inside during rain has three completely different causes — an external leak, a plumbing leak and condensation — and they look identical on a wall. Telling them apart before ringing anyone saves the most money.

Difficulty: beginner
Time: 1 hr
Read: 6 min
Safety: caution

Short answer

Ask when it appears. Only during or just after rain, and it dries between: it is coming in from outside — roof, gutter, wall or ground. Any time regardless of weather: it is plumbing, and shutting the stopcock for an hour proves it. Worst on cold mornings, spread evenly on cold surfaces and wipes off: it is condensation, and no repair to the building will fix that.

The single most expensive mistake with water inside a house is calling the wrong trade. A roofer cannot fix condensation, a plasterer cannot fix an active leak, and a damp-proofing company will often propose an injected course for something that is neither. The diagnosis is not difficult and it is almost entirely a matter of observation — when it appears, where it appears, whether it dries, and what the weather was doing.

Safety

Water and electricity together are the immediate risk. If water is running near a light fitting, a socket or the consumer unit, switch that circuit off at the consumer unit before investigating, and get an electrician if anything has been wet. Never go onto a roof to look, and never lean out of a window or use a ladder in wind or rain — the diagnosis on this page is all done from the ground and from inside.

What you'll need

A notebook or phone note to record dates, weather and appearance
A torch
Binoculars, for looking at gutters and the roof from the ground (optional)
A cheap hygrometer or damp meter (optional)
A camera

Where is the water actually coming from?

Step by step

Record when it appears and what the weather was doing.Date, rainfall, wind direction, and whether the patch grew, stayed or dried. Three or four entries usually decide the diagnosis on their own, and no other single thing is as useful.
Check whether it is only during rain.If it appears in rain and dries in between, it is entering from outside. If it is there in a dry fortnight, it is plumbing. If it is worst on cold still mornings and there has been no rain, it is condensation.
For rain-related water, watch the wind direction.Water that only appears when rain is driven onto one particular wall points to wind-driven rain through failed pointing, cracked render or a defective seal, rather than the roof.
Look at the gutters and downpipes during the rain, from the ground.This is the highest-value five minutes in the whole diagnosis. An overflowing gutter, a leaking joint or a downpipe discharging against the wall is the cause of a large proportion of damp walls, and it is cheap to fix.
Locate the stain relative to the roof and the chimney.Top-floor ceiling stains point to the roof covering, a slipped tile or the flashing. A stain following a chimney breast points to the stack, its flashing or its pot. A stain at floor level after heavy rain points to ground and surface water, not the roof.
Test for plumbing by turning the water off.Shut the internal stopcock, do not use any water, and check the wet patch in an hour and again in the morning. If it stops growing, it is the incoming supply. If it continues, it is waste, heating or something external.
Check the heating system pressure.A combi boiler losing pressure repeatedly means the heating system is leaking somewhere, often under a floor. It is a common cause of a wet patch with no obvious source.
Test the condensation theory properly.Condensation is spread evenly over cold surfaces — corners, window reveals, behind wardrobes, north walls — appears on cold mornings, wipes away, and is accompanied by black spot mould. A leak makes a defined patch with a tide mark and a stain.
Check the ground outside for the level and the fall.Soil, paving or a drive built up above the damp proof course lets water bridge it, and ground falling towards the house sends everything at the wall. Look for a course of bricks visible above the ground level all the way round — if it is buried, that is your answer.
Photograph and mark the edge of the patch with a pencil.Date the pencil line. Whether the patch grows, shrinks or stays exactly the same over a fortnight tells any surveyor more than a single visit ever will.

Tips

Water travels. A stain on a ceiling can be metres from where the water gets in, because it runs along a rafter or a service pipe before it drips. Never assume the entry point is directly above the stain.
Blocked gullies at the base of a downpipe are the most common cause of water against a wall in heavy rain, and clearing one takes two minutes with a gloved hand.
Condensation is by far the most common cause of damp in UK homes and by far the most misdiagnosed. If the walls are wet in winter and dry in summer, and the mould is black spot rather than a brown stain, start there.
A damp meter reading on a plastered wall reads salt content as much as moisture. Treat readings as an indication, not a verdict.
If a builder offers a chemical damp-proof course before having explained where the water is coming from, get a second opinion. An independent damp and timber surveyor with no interest in selling the remedy is worth the fee.

Common mistakes

Redecorating over the stain — It reappears, usually worse, and the record of how far it spread is lost. Fix the cause, let it dry fully, then decorate.
Calling a roofer for water at floor level — Water at the bottom of a wall after heavy rain is almost never the roof. It is ground level, surface water, a blocked gully or a bridged damp course.
Treating condensation as a leak — No repair to the fabric changes it. Condensation is moisture produced inside meeting cold surfaces, and it is fixed by ventilation, heating and insulation — nothing else.
Going up on the roof in the rain to find it — It is the most dangerous thing anyone does about a leak, and it usually fails because the entry point is not where the water lands. Look from the ground and from inside the loft with a torch during rain instead.

If it doesn't work

Stain on a top-floor ceiling after heavy rain

Cause: Slipped tile, failed flashing or a blocked valley — Fix: Go into the loft during the rain with a torch and find the wet timber, which tells the roofer where to look. Do not go outside or onto the roof.

Damp patch on a wall only in driving rain from one direction

Cause: Wind-driven rain through failed pointing, cracked render or a defective window seal — Fix: Have the pointing and render inspected on that elevation and the window seals replaced. A breathable water-repellent treatment can help solid walls, but only after the defects are fixed.

Water at the base of a wall in heavy rain

Cause: Blocked gully, ground level above the damp proof course, or paving falling towards the house — Fix: Clear the gully first, then lower the ground or fit a drainage channel so the damp course is at least 150mm above the finished ground level.

Wet patch that never dries, in any weather

Cause: A plumbing or heating leak — Fix: Shut the stopcock for an hour and watch. Check boiler pressure. If neither test isolates it, a leak detection company using acoustic or thermal equipment is far cheaper than lifting floors speculatively.

Water running down inside the chimney breast

Cause: Missing pot cover, failed flashing or an unlined flue taking rain — Fix: Have the stack, flashing and pot inspected. A rain cowl on a disused flue solves a surprising number of these, but the flue still needs ventilation or it will grow damp instead.

Questions people ask

How do I tell condensation from rising damp?

Condensation is worst on cold surfaces, in cold weather, in rooms with moisture — kitchen, bathroom, bedrooms — and produces black spot mould. Rising damp is confined to the bottom metre or so of a ground floor wall, has a tide mark with salts on it, and is present all year. Genuine rising damp is much rarer than the number of treatments sold for it.

Does insurance cover it?

Sudden escape of water from plumbing is covered by most policies. Damage from a storm may be covered, but gradual damp, wear and tear and lack of maintenance are excluded everywhere, which is why a leaking gutter left for two years is not paid.

Should I get a damp survey?

Yes, but from an independent surveyor who does not sell the remedial work. A free survey from a damp-proofing contractor almost always concludes that their product is needed.