Lead flashing is the unsung hero of your roof. It seals the joins where the roof meets a chimney, wall, dormer or roof light, places where tiles alone can't keep the water out. When flashing fails, the leak often appears some distance away, which is why it's one of the most commonly misdiagnosed roof problems. This guide explains what flashing does, how it fails and how it should be repaired.
Where You'll Find Lead on a Roof
- Chimney flashing: wrapped around the base of the chimney stack, with stepped pieces running up the sides and a back gutter behind it.
- Abutment flashing: where a roof meets a wall, such as on an extension or a semi-detached house with a higher neighbour.
- Valleys: the channels where two roof slopes meet, often lined with lead on older homes.
- Dormers and bays: lead cheeks, aprons and coverings on dormer windows and bay roofs.
- Parapet and box gutters: common behind the front walls of Georgian and Victorian terraces in Richmond, Twickenham and Kingston.
Signs Your Lead Flashing Has Failed
- Damp on a chimney breast inside the house, especially after rain
- Water stains on the ceiling next to a chimney or where an extension meets the house
- Lead that has lifted or come away from the brickwork, visible from the ground with binoculars
- Cracks or splits in the lead, often from pieces that are too long to cope with expansion and contraction
- Missing or crumbling mortar where the lead tucks into the brick joints
- Sealant or mastic smeared over the flashing, usually a sign of a previous quick fix
- Missing lead altogether: lead theft from roofs does happen, particularly on valleys and porches
Why Flashing Fails
Good lead work can last for many decades, so when it fails early there's usually a reason:
- Pieces too long: lead expands and contracts with temperature. Lengths that exceed recommended limits crack over time.
- Wrong thickness: lead comes in different codes (thicknesses). Using lead that's too thin for the job shortens its life.
- Poor fixing: flashing should be tucked well into the mortar joint and secured with lead wedges, then pointed. Relying on mortar or sealant alone doesn't last.
- Movement: chimneys and walls move slightly over time, and old mortar cracks and lets water behind the lead.
Why Sealant Isn't a Real Fix
We often find flashing that a previous roofer has "repaired" with a tube of silicone or bitumen mastic. It might stop a leak for a season, but sealant breaks down in sunlight, shrinks and pulls away, and it hides what's really going on. On one East Molesey job, damp on a chimney breast traced back to exactly this kind of bodge. We removed it, repointed the stack and fitted new Code 5 lead flashing. You can see the East Molesey chimney job here.
How Flashing Should Be Repaired
A proper repair usually means:
- Removing the failed lead and any old sealant
- Raking out the mortar joints where the new lead will tuck in
- Cutting and dressing new lead of the correct code, in lengths that allow for movement
- Securing it with lead wedges and pointing the joints
- Applying patination oil to protect the new lead and stop it staining the brickwork
On chimneys, it's sensible to check the flaunching (the mortar around the pots), the pointing and the cowls at the same time, so you only pay for access once.
Lead Alternatives
Where lead theft is a concern, or on some flat roof details, modern alternatives such as GRP or lead-substitute flashing can be used. They're not always appropriate on listed or conservation-area properties, where traditional lead is usually expected.
Think Your Flashing Has Failed?
We repair and renew lead flashing, valleys and chimney leadwork across West Molesey, Elmbridge and Surrey, always in proper lead rather than sealant. See our chimney repairs and roof repairs services, or our guide on what to do if your roof is leaking. Call 07470 744247 for a free inspection with no call-out fee.