A brown ring on the ceiling, a drip in the loft after heavy rain, a damp patch that comes and goes. Roof leaks are frustrating, and the obvious fix often doesn’t work. Someone replaces a few slates above the stain, the next storm comes, and the stain grows again.
That’s usually because the water got in somewhere else. This guide explains why leaks are hard to find, where they most often start on pitched and flat roofs, what can look like a leak but isn’t, and how a leak is traced step by step.
Why the stain isn’t under the hole
Water rarely drops straight down. Once it’s past the slates or tiles, it runs along the underlay, down a rafter, along a joist or across the top of a ceiling until it finds a low point. Only then does it show. On a flat roof it can travel across the deck or through insulation before it reaches a gap in the ceiling below.
Wind makes it harder still. Rain blown sideways can get in at a flashing or under a ridge that stays dry in calmer weather. That’s why some leaks only appear in particular storms.
Timing adds another layer of confusion. Water can soak into timber, plaster or insulation for days before a stain appears, and a stain can keep spreading after the rain has stopped. Old stains also stay visible long after a leak has been fixed. So the mark you can see may be old, new, or a mix of both. Moisture readings help show which areas are wet now.
Where leaks most often start on pitched roofs
- Flashings. Lead or mortar where the roof meets a chimney, wall or dormer can split, lift or crack.
- Valleys. Where two slopes meet, water is concentrated. Worn lead or blocked valleys overflow.
- Chimneys. Cracked flaunching, open pointing and failed flashings let water into the stack and the room below.
- Slipped or cracked slates and tiles. On older slate roofs, corroded nails let slates slide out of place.
- Ridges and hips. Loose or cracked bedding mortar lets water in along the top.
- Gutters. Blocked parapet, valley and box gutters can overflow inside the building.
Where leaks start on flat roofs
On flat roofs the usual suspects are splits and blisters in the membrane, failed joints and laps, upstands where the roof meets a wall, blocked or badly sealed outlets, and areas where water ponds. Our guide to flat roof problems covers these in more detail.
On larger membrane roofs, electronic leak detection can help pinpoint holes in the membrane. It suits waterproofing membranes, and the method depends on how the roof is built. The guide to electronic leak detection explains how it works.
Want a roof surveyed? We cover London and the South East, with written reports and photos from £150 including VAT. Book a survey or call 07833 053749.
Things that look like roof leaks
Not every damp ceiling is a roof leak. Before anyone climbs a ladder, it’s worth ruling these out:
| What you see | Possible cause to check |
|---|---|
| Damp in the loft in cold weather, beads of water on nails | Condensation from warm air rising into the roof space |
| A stain that appears without rain | Plumbing, a water tank or a pipe in the roof space |
| Damp at the top of an outside wall | An overflowing or leaking gutter |
| Damp only in driving rain | Rain getting in through walls, windows or a flashing |
| Damp around a chimney breast | The stack, its flashings, or water entering an open pot |
Our guide on telling condensation from a roof leak has simple checks you can do first.
How a leak is traced properly
A roof leak investigation works through the evidence in order:
- The history. When does it leak? In every shower, or only in strong wind from one direction? Any recent work on the roof?
- Inside. The stain, the ceiling around it and the walls nearby.
- The loft. Where there’s safe access, we follow the staining on timbers and underlay back towards its source.
- Outside. The flashings, valleys, chimneys, gutters and coverings above and uphill of the stain.
- Extra methods. Moisture detection to see where materials are wet. Thermal imaging, where useful, to show temperature patterns.
Thermal imaging shows surface temperature, not water itself, so findings are checked with moisture readings. It works best after a sunny day, with a dry roof surface and little wind. Our thermal roof survey page explains more.
What to do while you wait
- Put a bucket or tray under any drip, and move electrics and belongings away.
- Take dated photos of the stain, so you can see if it grows.
- Note the weather each time it leaks: rain, wind direction, how heavy.
- Look at the gutters from the ground for overflowing water or plants.
- Don’t climb onto the roof yourself.
Those notes save time on the day and often point straight to the cause. If the leak only happens in one kind of weather, tell us when you get in touch. It may change what we look at first.
What our investigation gives you
You’ll get a written report with photographs, setting out where we believe water is getting in, why, and what we recommend. The investigation is the finding, not the repair. With a clear report, you can brief roofers and compare quotes on the same basis. That helps avoid paying for work in the wrong place, which is how many leaks end up being “fixed” more than once.
Sometimes a single visit can’t prove the source, especially when a leak only happens in rare conditions. When that’s the case, the report says so plainly and explains the most likely sources and what would confirm them.
We cover London and South East England, on weekdays, Saturdays and Sundays. Call 07833 053749 or contact us, and tell us when the leak appears and what you’ve noticed.
Quick answers
Why is my roof leaking only in heavy rain?
Wind-driven rain can get in at flashings, ridges or valleys that stay dry in lighter weather. Note the wind direction each time.
Can thermal imaging find a roof leak?
It shows temperature differences that can point to trapped moisture. It doesn’t see water, so findings are checked with moisture readings.
Is the repair part of a leak investigation?
No. A leak investigation is about finding where water gets in. You get a written report with recommendations to brief a roofer.
Could it be condensation instead?
Yes. Damp in the loft in cold weather, with no link to rain, often points to condensation rather than a leak.
Related services
More advice
- Condensation or a Roof Leak? How to Tell
- Flat Roof Problems: Ponding, Blisters and Splits
- How Electronic Leak Detection Works
- Lead Flashing Problems on Older Roofs
- Thermal Imaging on Roofs: What It Can Show
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.