Water does not fall straight down through a roof. It runs along a rafter, soaks into insulation, follows a pipe or a wire, and appears on the ceiling a metre or more from where it got in. A roofer who simply covers the wet patch with a new tile is guessing, and the leak usually returns with the next heavy shower.
This guide sets out the order in which we trace a leak: what you can see from inside, what the loft tells us, which parts of a roof leak most often, and how a controlled water test proves the answer. It is a method, not a puzzle about when a leak shows. If yours appears only in certain weather, read why a roof leaks only in heavy rain. For the wider service and its typical prices, see roof repairs.
Why the drip is never where the hole is
Every roof is layered: covering, battens, underlay, rafters, insulation, ceiling. Water that gets past the outer layer lands on the underlay or the timber and then moves according to gravity and surface tension. It may run down the slope of a rafter for a metre or two before it finds a gap, a nail hole or a join in the ceiling board to drip through.
How water travels
Three habits explain most of the confusion. Water follows the underside of timber, clinging to it and running along it. It soaks into mineral wool and plasterboard and spreads sideways before it shows. And it follows pipes, cables and chimney breasts, which give it a route straight down through a floor. A stain beside a chimney breast, for example, may come from the flashing above, but just as easily from a tile three courses up that the water has tracked sideways from.
What a stain can tell you
The shape and age of a mark carry clues. A brown ring with a darker edge has dried and re-wetted several times, which points to a slow, repeated leak. A wet patch that grows within minutes of rain starting suggests a direct route in. A damp area on an outer wall, running down from the eaves, often means a gutter or the edge of the roof, not the main slope.
Mark the stain with a pencil line and date when you notice it spreading. That record helps later.
Reading the clues from inside the house
Before any ladder goes up, we ask where the water shows, and when. This takes ten minutes and often halves the search.
Questions that narrow the search
- Does the stain appear in rain from one direction only? That points to one slope, one wall or one gable.
- Does it appear after the rain stops, or during? A delay suggests water stored in insulation or a long run along timber.
- Is it near a chimney, a rooflight, a soil pipe, a bathroom extractor or an extension join? Each has its own junction to check.
- Has any work been done on the roof or loft recently? New insulation can hide a leak or push cold air into a place that never had condensation before.
Not every wet ceiling is a roof leak
A tank, a heating pipe, a shower tray or an overflow can all stain a ceiling. Condensation, which forms where warm moist air meets a cold surface, can drip from the underside of the roof covering in winter and look exactly like a leak. Its pattern is different: it tends to appear in cold spells, on the nail points or the underside of the roof timbers, and often covers a wide area at once rather than one spot. If a ceiling stain appears with no link to rain, check the plumbing first and tell us what you found.
Take photographs of the stain, a wide shot and a close one with something for scale. They sit alongside the roofer's own photographs from the survey, and they matter if you decide to claim on your insurance, as our guide to storm damage claims explains.
In the loft: what the timbers and underlay show
With a torch and a dry afternoon, the loft is the best place to look. We check it first, usually before the roof, because it shows where water has been, even when the roof is dry.
Signs to look for
Dark tide marks on rafters and battens, wet or flattened insulation, chinks of daylight, and stains on the back of the roof boards or on the underlay itself. Timber that is dark, soft or crumbling under a screwdriver has been damp for months. Fresh drips on a rafter show the line of the path. Following a wet mark along a rafter to its highest point, and then looking at what sits outside at that spot, locates most leaks.
The underlay
Underlay is the membrane laid under the battens as a second line of defence. Where it has torn, sagged into a gutter-like pocket, or lost its overlap at the eaves, water collects. A sagging underlay can carry water to a nail hole or batten join, producing a drip far from the cause. We note which underlay type is present, because breathable and non-breathable types are ventilated differently under BS 5250, as Marley's guidance explains.
Safety in the loft
Step on joists, not on ceiling boards, and keep insulation intact when you move it. If you are not sure the floor will bear your weight, leave the inspection to us.
Mark the loft position of anything you find. Measure from a fixed point, such as the gable wall or the chimney breast, so the spot can be matched to the outside.
Outside: the parts of a roof that leak most often
Looking at the roof from the ground comes first. Binoculars show a surprising amount: slipped tiles, a sagging ridge, a bare patch of felt, moss packed along a valley. Then we go up, using a ladder, a tower or full scaffold as the roof demands.
The usual suspects
Leaks tend to cluster at junctions and interruptions, where the covering stops and something else begins. In rough order of how often we start with them:
- Flashings around the chimney, against a wall or at a dormer, where the lead has lifted or the pointing has gone.
- Valleys, where two slopes drain into one line and blockage is common.
- The ridge and the verge, where mortar cracks and pieces loosen.
- Loose or broken tiles and slates, and the fixings behind them.
- Penetrations: soil pipes, vents, flues, aerials, solar panel brackets.
- Rooflights and roof windows, which have seals and flashings of their own.
- Gutters and the eaves, where water can overflow behind the fascia.
That order is how we approach the search, not a statistic. What fails on a Stroud roof depends on its age, covering and aspect.
The chimney in detail
A chimney stack has several ways to leak: the flashing round its base, the pointing between its bricks, the flaunching (the mortar bed that sets the pots in place), and the pots themselves if they are uncapped. Water that enters the top of an unused flue can come out in a room below, miles from any roof surface. Because the stack stands proud of the roof, it takes the weather from every direction and shows the first signs of wear.
Water testing with a hose: proving the source
When a visual check turns up several possible causes, or none, the next step is a controlled water test. It needs two people and some patience, and it is the way to separate a probable fault from a proved one.
How a hose test runs
One roofer sits in the loft with a torch, or stands in the room below, watching the stain. The other works on the roof with a hose, starting at the lowest suspect point and wetting one small area at a time, for a few minutes each. The hose moves up the roof only when that area has been given long enough to show a result. Starting at the top is the classic mistake: water poured on a high point runs down over everything below it, and the test proves nothing.
Isolating one junction at a time
If the flashing is suspected, water is run along it alone, not sprayed at the whole stack. If it is a valley, the hose runs down the valley line with the sides held back by a sheet. A test can take an hour or more, and a leak that needs wind to drive it may not show under a simple hose. When that happens we try a spray to imitate wind-driven rain, or we stop and take stock.
A hose test only proves a cause by reproducing the leak. When the leak does not return, the result is useful too: it eliminates a suspect and narrows what is left.
Other methods
Surveys by drone or thermal imaging are also used for hard-to-reach roofs. MyBuilder's inspection guide lists a standard roof inspection at £150 to £250, a drone survey at £200 to £400 and a thermal survey at £300 to £500, all national ranges updated 21 August 2026. We use photographs and a hands-on check for most domestic jobs.
From a found leak to a repair you can see
Once the source is proved, the repair is the easy part. What matters is that you can see what was found. Our survey produces photographs of your own roof with the fault marked, so the quote for the repair refers to something specific, not to a general description.
What the written quote should say
It should name the fault and its position, the repair proposed, and the cost, covering access, materials, labour, making good and waste. If the leak has soaked insulation or timber, the quote lists what needs replacing inside as well, or says what we are leaving to your judgement. We will say where a repair is a stopgap and where it is a lasting fix.
When the cause is not a single fault
Sometimes a roof leaks in several places because the covering has reached the limit of its working life. Slipped slates in three places, perished battens, a failing flashing and a blocked valley may share one cause: age. A string of repairs can then cost more than one proper job, and the survey spells that out with the evidence. If water is coming in while you read this, our guide on what to do first when a roof is leaking covers the first hour indoors.
Keeping the evidence
Photographs of the stain, the loft and the roof, dated, are what an insurer looks for if the leak follows a storm. Keep them, along with the quote. Our repairs come with a 10-year workmanship guarantee on paper, and we take the waste away.