Lead flashing seals the junctions where a roof meets chimneys, walls and dormers. Correctly dressed lead lasts 50 to 80 years; mastic 'repairs' fail within 3 to 5 years. In Southampton's coastal air, salt accelerates surface oxidation but not structural failure. Flashing replacement costs £250 to £1,000 depending on the junction.
Lead flashing is the material used at every junction on a roof where the tile covering meets a vertical surface — chimney stacks, dormer walls, abutments and extension junctions. Done correctly, a lead flashing lasts 50 to 80 years. Done with mastic as a shortcut, it fails within 3 to 5 years. This guide covers everything Southampton homeowners need to know.
What Lead Flashings Do
At every roof junction — where a chimney emerges through the tiling, where a dormer wall meets the main roof, where an extension roof abuts the house wall — there is a gap that the tile covering cannot seal. Lead is dressed into these gaps to form a continuous waterproof junction. It is the only material that is sufficiently flexible, durable and workable to do this job correctly for the long term.
The Lead Sheet Association specifies lead codes (Code 4 for step flashings, Code 5 for valleys and back gutters) for correct, durable installation.
Source: Lead Sheet Association
Why Lead Fails on Southampton Properties
The two main failure modes are thermal cracking and fixing failure. Lead expands and contracts significantly with temperature — if flashings are fixed too tightly without adequate movement provision, they crack at fixing points over years of thermal cycling. Southampton's coastal climate also means the salt air from the Solent oxidises lead surfaces faster than inland environments, though this affects appearance rather than performance.
Why Mastic is Not a Lead Repair
We see mastic used on failed lead flashings on perhaps 40 percent of Southampton properties we inspect. It looks like a repair. It is not. Mastic adheres temporarily and fails within 3 to 5 years as thermal movement works it loose. A correctly re-dressed lead flashing costs more for the initial repair but lasts 50 years. Mastic costs less initially but will need doing again within 5 years — repeatedly.
Lead Flashing Costs — Southampton 2025
| Job Type | Typical Cost |
|---|---|
| Chimney step flashing replacement | £250–£600 |
| Chimney back gutter replacement | £200–£450 |
| Lead valley replacement | £400–£1,000 |
| Dormer flashing replacement | £300–£700 |
| Bay roof lead replacement | £350–£800 |
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Call 0238 184 9628Last reviewed: 20 July 2026 by Southampton Roofing Experts