The short answer
Flashing has failed when water gets past it. Outside, watch for lead standing off a wall or chimney, joints that have opened, sheet that has slipped or split, and loose or missing pointing where the lead tucks in. Inside, look for damp patches on a chimney breast or on a ceiling below a wall. Staining that appears after wind-driven rain points hard at the flashing.
Failed flashing shows itself in small ways, outside and in. Nearly every clue is visible from the lawn or from a bedroom ceiling.
Clues outside, seen from the lawn
Stand well back and look at every place where the roof meets something else: the chimney stack, a side wall, a dormer, a porch roof. Binoculars help a lot, and they keep your feet on the ground.
What does a failed flashing look like?
Lead that stands away from the brickwork, so you can see a dark gap behind it, has lifted. A strip that has dropped down the slope, leaving bare wall above it, has slipped. Sheet with a visible line across it, or a corner that has opened, has split. Compare both sides of a chimney: if one side sits tight and the other flaps, the loose one is your problem.
Also look at the mortar line where the lead is tucked into the wall. If it is crumbling, missing or hollow, the lead has lost its grip and rain can run in behind it. Our page on lead flashing chased into a wall explains why that joint matters so much.
Finally, check for blobs of sealant or bitumen smeared along an edge. They are a sign someone has patched the flashing before, and a patch usually means the fault has returned.
Clues inside the house
Water that gets past a flashing rarely drips in the place where it entered. It runs down a timber or along the back of a wall until it finds a way out, so the stain can be some distance from the cause.
Where do damp patches show up?
The classic spots are the breast of a chimney upstairs, the loft side of the stack, the ceiling beside it, and the top of a wall where a roof slope meets it. Paint that blisters or flakes, wallpaper that lifts, a tide mark with a brown edge and a musty smell in a cupboard near the roof all point the same way. A patch that grows after heavy rain, especially when the wind is blowing at that side of the house, suggests flashing, because the water is being driven sideways.
Compare it with a leak elsewhere. A stain in the middle of a ceiling away from walls is more likely a slipped tile, and our answers in the pitched roofing section cover those. A stain that sits right beside a stack or a wall most likely means the lead. Where water is coming in at this moment, we make the roof safe before anything else. Note which way the wind was blowing too, since water driven against one wall tells a roofer a good deal about where it got in.
Mistakes that look like failure, and ones that hide it
Not every mark on a roof is a leak, and not every dry day proves the roof is sound. Some faults are easy to misread.
Is white staining a sign of failure?
Not by itself. White streaks below new lead are a surface reaction and are usually harmless, as we explain in our answer on white staining under lead. A sagging line along a ridge, a gutter that overflows or a cracked chimney pot can all be mistaken for flashing trouble.
Can flashing fail without any stain?
Yes. Water can sit in the wall for a season before it shows. A rotten timber at the end of a wall plate, a patch of mould in a loft corner or a smell in a wardrobe can appear long before any ceiling mark. That is why a survey includes the loft as well as the outside. Lead that has simply cracked with age is another quiet failure, usually caused by a length cut too long or a fixing that holds too hard.
Photos, dates and who to call
Take photos of what you can see, outside and in, with the date. They help any roofer judge how long a fault has been present, and they help you if the damage ever becomes an insurance matter. Should the damage become an insurance matter, the order is simple: tell the insurer first, then arrange a temporary fix, and keep every photo and receipt. Complaints about how a claim was handled go to the Financial Ombudsman.
Should I go up and look myself?
No. Climbing up is a bad idea for an amateur, and a fall hurts far more than a leak. Anything above a ground-floor eave is a job for someone with the right access and training. Do not climb on lead either, because it creases and splits underfoot.
Our visit covers the lead, the wall around it and the loft beside the stack, with pictures for you to keep. If a local repair will cure it, that is the advice. Our answer on replacing flashing without a new roof explains how, and lead work covers the rest. One written total comes once the survey is done.
More on spotting flashing faults
How often should flashing be checked?
A look from the ground once a year is sensible, and again after a severe storm. Check chimneys, walls and dormers, and the ceilings beneath them. Catching a lifted edge early is a small job, whereas a flashing left to leak can damage timber and plaster.
Does damp by a chimney always point to the flashing?
No. Damp by a chimney can come from a failed flashing, a cracked stack, a missing cap, condensation or rain coming down an open flue. The position of the stain and the weather when it grows help narrow it down, and a roof and loft survey settles it.
The full picture: Lead work in Taunton ยท More short answers