The short answer
Yes. Water that gets through a roof covering does not drop straight down. It runs along the underlay, down the face of a rafter, across a batten or along a pipe or beam, and appears wherever it finally finds a way through the ceiling. That spot can be several metres from the entry point, which is why tracing a leak means working back up the slope.
Why water does not drop straight down
Beneath the visible covering, a pitched roof is built up from several layers, and each gives water a path. The covering, whether slate, clay or stone, sheds most of the rain. Beneath it the underlay, the membrane fixed over the rafters, catches what gets past. Battens, the laths nailed across the rafters, carry the covering. The rafters are the sloping timbers under all of it, and below them is the loft floor and the ceiling.
When a slate slips or a tile cracks, rain gets through the covering and lands on the underlay. The underlay is laid on a slope, so the water runs down it, not through it. It follows the pitch until it meets a join in the sheet, a tear, a nail hole or a batten, and then it gets through at that point. Water can also run along the underside of a rafter, because of surface tension, and drip from a point well down the slope.
It may be a metre from the fault, or the water can cross to another part of the house. This is the main reason that the ceiling stain and the roof fault do not line up.
The routes water takes inside the roof
Once past the covering, water uses whatever surface is nearest. These are the common routes.
- Along the underlay. It runs down the slope and gathers at laps, tears and fixings, or follows a sag in the sheet to its lowest point.
- Down and along timbers. A rafter, purlin or ceiling joist gives water a path. It can run along a joist for some distance before it drips.
- Along pipes and cables. Water follows a soil pipe, a vent or a cable through the ceiling, and it can appear at a light fitting far from the roof fault.
- Through insulation. Mineral wool soaks up water and passes it sideways, so a wet patch spreads below the original leak.
- Behind a chimney or wall. Water entering at a flashing, the lead strip dressed over a roof joint, can travel down inside the wall and show up in a bedroom or a stairwell.
On a flat roof the same thing happens along the deck. Water that gets under a membrane can travel under it, between the layers, and appear at a joint or an upstand some distance away.
Where leaks most often start
Because the stain is a poor guide, roofers look for the causes that are known to be common. The first places to check include:
- Flashings around chimneys, dormers and abutting walls, where a lifted or cracked edge lets water in.
- Valleys, the internal angles where two roof slopes meet, particularly if the lead is split or the channel is blocked.
- Slipped, cracked or missing tiles and slates.
- Ridge and hip tiles whose bedding has failed.
- Gutters that overflow and soak the eaves.
- Penetrations, such as soil pipes, vents and roof windows.
Several of these sit high on the roof, while the stain is low. A leak at a ridge, for example, runs a long way down before it reaches the ceiling. If the leak only appears in strong winds, our page on repairing your own roof safely and the wider guide to leaks that appear when it is windy cover the likely causes, since wind-driven rain gets in at places that stay dry in a normal shower.
How a roofer traces the source
Tracing a leak is a matter of working backwards. A roofer starts in the loft, since the roof's underside often tells the story better than the top. Wet or stained timber, dark patches on the underlay and damp insulation show the line the water has taken. The wetter end of that line is closer to the entry point, so you follow the staining up the slope.
Outside, the roof is checked at that position and above it. The usual sequence is:
- Locate the stain on the ceiling and mark its position.
- Check the loft above for wet timber, tracks and drips, and follow them upslope.
- Inspect the covering, flashings, valleys and ridge on the outside along that line.
- If nothing is obvious, test with water. A hose is run over small areas, starting low and working up, while someone inside watches.
- Confirm the cause before repairing, so the fix is made where it is needed.
Guessing at the source and patching the wrong place is the commonest reason a repair fails. A stain does not move because a patch was put elsewhere, and the next downpour gives the same result.
What to do while you wait
If water is coming through, protect the room first. Move furniture, lay down towels or sheeting, and put a bucket under the drip. If the ceiling is bulging with water, pierce it carefully at its lowest point to release the water into a bucket, rather than letting the plaster fall in one piece. Keep away from light fittings that are wet, and switch off the circuit if water is near the electrics.
Take photographs of the stain, the loft and any visible damage, with the date. They help the roofer, and they help if you make an insurance claim. Do not get onto the roof yourself.
A temporary cover can hold the water back until a proper repair is possible, and our page on temporary roof repairs explains how tarpaulins and patches are fixed, and how long they last. Where a leak is found, a single cracked tile may be the cause, and the page on a cracked roof tile covers how urgent that is. We prioritise leaks, and water coming in gets stopped before the permanent fix is planned. Surveying is free, and the price we then write down is a fixed one. The full list of repairs is on the roof repairs in Bath page, and any repair we carry out comes with a 10-year written workmanship guarantee.
