What it looks like
The signs below are the ones homeowners most often notice first. Spotting one early makes the problem far easier and cheaper to put right.
Why it happens
Understanding the cause matters — the right fix depends on it. These are the usual culprits behind damaged flashing.
Thermal movement fatiguing the lead
Lead expands and contracts with every warm and cold cycle. If a run of lead is too long or was poorly clipped, that constant movement eventually cracks it — most often along the chimney back where it works hardest.
Failed mortar pointing
On many homes the flashing is bedded into a mortar joint. Mortar is porous and cracks with age and frost, letting water past the top edge of the lead and behind the flashing.
Poor original installation
Flashing that was undersized, not dressed into the courses correctly, or replaced with mastic as a shortcut will let water in. Sealant "repairs" over lead are a common temporary bodge that fails within a season or two.
Valley blockage and corrosion
Valleys carry a large volume of water off two roof planes. Leaves and debris cause water to back up over the edges, and older metal valleys corrode through at the wettest point.
What to do
- 1
Match the damp to the nearest junction
Internal damp near a chimney breast, a party wall or a valley strongly points to flashing rather than the tiles. Noting the exact position helps a roofer confirm the cause quickly.
- 2
Look for lifted lead or cracked mortar
From the ground, check whether the lead has pulled away from the wall or the mortar line above it has failed. Rust streaks below a detail are a clear sign the flashing is past its best.
- 3
Avoid DIY sealant fixes
Smearing mastic over failing lead traps moisture and rarely lasts. It also makes a proper repair messier later. Resist the temptation and get the junction re-dressed correctly.
- 4
Have the flashing re-dressed or replaced
A roofer will re-dress or renew the lead and re-point the joint so the junction sheds water as intended — the only durable fix for a failed flashing.
When to call a professional
Some situations are worth getting a roofer to look at straight away:
- Damp is appearing on a chimney breast or internal wall
- You can see lifted lead or a failed mortar line at a junction
- A valley is overflowing or shows signs of corrosion
- A previous sealant repair has failed and is leaking again
Safety first
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