Tracing roof leaks
Pitched roof repairs: tracing leaks and fixing damage properly
A leak in a tiled roof is traced from the stain back up the slope to the junction or layer that has failed. Valleys, abutments and underlay are the places a roofer looks hardest.
A damp ceiling tells you a roof has let go, not where. Rain that gets past a tiled covering is caught by the underlay and meant to drain out at the eaves. It only reaches the house when something has failed twice: the covering and the layer behind it, or a junction where metal, mortar and tile meet.
That is why a proper repair starts with finding the route, not with the stain. A patch over the wrong spot dries the ceiling for a fortnight and then fails again in the next heavy shower.
This page follows the order a roofer works in: how a leak is traced, then the three places on a tiled roof that give most trouble to the repair itself, namely valleys, walls and chimneys where the roof abuts them, and the underlay under the tiles. It ends with what a repair costs on published figures, and what points the job towards a bigger fix. Anything about the roof's structure or an insurance claim is flagged where it comes up.
01In order
Finding where the water gets in
Water travels. It can run down the underside of the underlay, down the side of a rafter and over the top of the ceiling lining before it shows as a brown ring in the corner of a bedroom, so the stain tells you roughly which side of the house to search and nothing more.
A roofer works up the slope from the stain in a set order:
- Check inside the loft during or just after rain, looking for drips, dark timber and wet insulation.
- Follow any wet rafter or batten upwards to the highest point where moisture shows.
- Look outside at that spot for a cracked tile, a lifted flashing, a gap at a wall or chimney, or a valley that has silted up.
- Where nothing shows, test the suspect section with a hose from the bottom upward, with a person in the loft, so the entry point is proved before any work.
The last step matters. A hose test starts low on the slope and moves up in stages, because wetting everything at once muddles the result. It is a job for a roofer and a helper, with the helper inside, not for one person on a ladder with a garden hose.
Guessing wastes money and leaves the fault in place. We would sooner spend an hour proving the route than quote for a repair that might not cure it, which keeps a repair a fair-priced repair and not a hopeful one. For slipped tiles specifically, our guide on slipped tiles: refix or replace covers the decision.
02Valleys
Valleys: where two slopes meet
A valley is the inside corner formed by two meeting slopes, and the rain from both runs down it in one channel. No other stretch of the roof carries so much water, so a small defect there shows up early.
Valleys on tiled roofs are lined in lead, in a manufactured valley trough or in a sheet material, and the tiles are cut to follow the line. The faults are of three kinds:
- The lining is split, cracked or lifting at a lap.
- The channel is blocked with moss, leaves or debris, so water backs up under the tile edges.
- The cut tiles beside the valley have slipped or cracked.
Where lead is used, the specification sets the thickness. The Royston Lead brochure shows Code 4 and Code 5 lead both listed for pitched valley gutters, with heavier Codes 6, 7 and 8 also listed, and says Code 3 is for soakers only. A thin lining where a heavier code was needed is a fault that shows up years in, as splits at the folds.
A repair can be as light as clearing the channel and refixing the edge tiles, or as heavy as stripping the nearby tiles, replacing the lining and relaying. The line between the two is the lining itself: a lining that is sound but dirty is cleaned, and one that has split is replaced, because a patch on a split may not hold.
Lead repairs of any size are paid for in the same way as the rest of the roof: one written total, after the survey. Our pitched roofing page sets out the wider work on a tiled roof.
Leak repairs: your questions
Can a roof leak be fixed from the inside?
Only temporarily. Catching drips and protecting what is below stops damage spreading, and the loft is where the route is traced. The cure sits on the roof, at the flashing, valley, tile or underlay that failed, and that work needs a roofer with proper access. Photos of the damp patches help the survey.
03Flashings · Chimneys
Abutments, flashings and chimney joints
An abutment is a joint between the slope and something that rises above it, such as the side wall of the house, an extension or a chimney stack. The gap is closed with flashing, a strip of metal tucked into the wall and dressed down over the tiles, and with soakers, small pieces of lead woven under tiles at the edge.
MyJobQuote says correctly installed lead flashing can last for over 80 years. That depends on the word correctly: the lead has to be cut, dressed and fixed with the right allowance for movement, and the top edge has to be properly tucked into the wall, not sealed with a bead of mastic.
A mortar fillet, a sloping bead of cement along the wall, is the cheap way to seal an abutment, and it cracks as the building moves. When it does, rain goes straight in at the joint.
For scale, MyJobQuote's figures for a chimney run at around £200 for a lead flashing patch, against an average of around £570 for replacing it in full. Elsewhere it puts a whole leadwork replacement at £350 to £920, a spread that depends largely on scaffolding. Checkatrade's repair rate is £50 to £70 per metre of lead flashing.
The failures are easy to describe:
| Fault | What it looks like | Repair |
|---|---|---|
| Lifted flashing | Edge stands off the wall | Dress down, refix |
| Mortar fillet instead of lead | Cracked cement line | Replace with lead |
| Open chimney joint | Daylight, damp in the stack | Rework the flashing |
| Torn soaker | Stain on the wall below | New soaker |
04Underlay · Tiles
Underlay failure beneath sound-looking tiles
Tiles can look fine from the garden while the underlay beneath is in poor shape. Underlay is not meant to be seen, and its failure shows first inside the loft: light through splits, damp patches around the nail holes, or sagging sections between the battens.
Underlay fails in a few ways. It can tear where it was folded badly at the ridge, perish with age, or be pierced by nails. In a cold roof it can also hold condensation, which then drips and looks exactly like a leak.
That last case needs its own mention. Marley's guidance is that HR underlay, short for high resistance, lets no vapour through. A cold roof laid with it needs open eaves: 25 mm at pitches of 15 degrees or less, and 10 mm where the roof is steeper. If insulation has been pushed over the eaves, the gap closes and moisture builds up under the underlay. The moisture code, BS 5250, is listed by BSI as current but under review.
Our answer on how long roofing felt lasts deals with the lifespan of the layer, and says why no one figure suits every roof.
Where underlay has failed across a slope, the fair answer is the larger job, and we show you the loft evidence before recommending it.
The repair is a matter of extent:
- A small local tear is patched under the tiles above it.
- A run of perished underlay needs the tiles lifted along that strip and new underlay laid, with the tiles refixed.
- Underlay perished across the whole roof means a strip and re-cover, because patches will not hold.
Why does my roof only leak in certain winds?
Wind-driven rain pushes water sideways and uphill, so a gap that sheds rain in still weather can pass it in a gale. A lifted flashing, a gap at a verge or a slipped tile on the exposed side are the likely places. The side the weather hits is a useful clue for tracing the leak.
05Repairs
Repair, re-cover or strip: choosing the scale
Once the route is found, the question is how much of the roof needs work. Four plain tests settle most of these jobs.
- Is the fault one spot? A lifted flashing or a split valley lining can be put right in a day without touching the rest of the covering.
- Does the same fault repeat along the roof? Then the cause is a method or material, and fixing one instance only moves the leak.
- Are the battens sound? Rotten battens under sound tiles mean the tiles come off to be refixed.
- Is the underlay failing across a whole slope? Then a re-cover is the repair.
For a sense of scale, Which? Trusted Traders priced a job of 1 to 6 broken tiles at £75 to £400 excluding VAT in a November 2025 survey, adding that Kent and East Sussex fall in the region where the top of the range is likely. MyJobQuote's figure for an average roof repair is around £650 in 2026. Those are separate sources measuring different jobs, so neither is a quote.
Access can change the price more than the fault. For scaffolding over public highway in Kent, the council fee is £52 per week for 2026 to 2027. East Sussex Highways lists £127 for 28 days on its fee table, a figure to confirm. The written quote says whether scaffold is in it, and we arrange the licence with our scaffolder.
If the covering is being replaced rather than repaired, the Planning Portal's test comes into play. The work has to stay under 25 per cent of the building envelope and under 50 per cent of the roof for no application to be expected. Separately, approval is needed for a covering that is heavier by 15 per cent or more, in any area.
06Key points
Keeping the repair from failing again
A leak that comes back is a repair that missed its cause. A few habits keep a roof dry between visits.
Is sealant a proper repair for a flashing?
A bead of sealant can buy a short time, but it is not a repair that lasts. Lead is meant to be dressed into the wall and over the tiles with room to move, and sealant cracks as the joint moves. A roofer reworks the flashing so the detail itself keeps the water out.
- Clear valleys and gutters of leaves and moss, from a safe position, because blocked channels push water under the tile edges.
- Look at the flashing at each chimney and wall after a gale, from the ground.
- Check the loft after heavy rain for new dark patches, and note them with the date.
- Keep insulation back from the eaves so the ventilation gap stays open.
- Photograph any repair, inside and out, so the next visit starts from a record of what was done.
Fixings matter on exposed slopes. Cupa states: "Buildings positioned on slopes, hills or coastal areas as well as tall buildings will be graded with higher exposure." BS 5534 provides the calculation behind a fixing specification. Marley adds that the standard is not legally binding, yet the 2015 amendments ruled out mortar as a roof's only fixing, so a ridge or verge that is bedded and not mechanically held is worth a second look.
Our answer on roof tiles blown off in wind explains what to do when a gale has lifted tiles.
Listed houses are handled differently: on a listed building, work touching its character needs listed building consent, so the council is asked first. Where a repair is only a tidy-up of like-for-like tiles on a house that is not listed, no application is normally involved, but the Planning Portal limits above still frame any wider re-covering.
Tell us what's wrong with the roof.
You'll get one written total price.