If you have a leak around your chimney, skylight, dormer or where your roof meets a wall, there is a very high chance that lead flashing is involved. Lead flashing is one of the most important — and most commonly misunderstood — components of a roof, and it is responsible for sealing the joints that tiles and slates simply cannot cover.
In this guide we explain exactly what lead flashing is, the different types used in UK roofing, the standard it should be installed to, how long it lasts, why it fails, and how to tell when yours needs attention.
What Is Lead Flashing?
Lead flashing is sheet lead — typically between 1.8 mm and 2.5 mm thick, in grades referred to as Code 3 through to Code 5 — that is shaped and fixed to seal the junction between a roof surface and a vertical structure. Chimneys, parapet walls, dormers, skylights, bay window roofs and roof-to-wall abutments all rely on lead flashing to keep water out.
Lead is used rather than other materials because of its unique combination of properties: it is malleable enough to be dressed tightly against any surface, it expands and contracts with temperature changes without cracking (when installed correctly), it is virtually immune to corrosion, and it has an extraordinarily long service life. No synthetic alternative has yet matched lead’s long-term performance in these applications, which is why it remains the professional’s preferred choice despite its cost.
Types of Lead Flashing
Chimney Flashing
A chimney typically requires several separate lead components. The front apron covers the junction between the bottom face of the chimney and the roof slope. The two sides use a combination of step flashings and soakers woven between the tile courses. The back gutter (or back flashing) collects any water running down the rear of the chimney stack and directs it onto the roof slope. Done correctly, chimney flashing is a multi-element system — not a single piece of lead slapped against the brickwork.
Step Flashings and Soakers
Where a roof meets a wall at an angle — a common detail on Staffordshire semi-detached and terraced houses — step flashings are required. Each step flashing is a small L-shaped piece of lead that steps up the wall alongside each tile course. Soakers are tucked behind the tiles and under the tiles above, providing a primary waterproof layer. Cover flashing (also called cap flashing) is then dressed over the soakers and pointed into a mortar joint in the wall to complete the system. This two-layer approach is the correct method; many older installations use only a mortar fillet, which invariably fails.
Valley Lead
Where two roof slopes meet at an internal angle — a valley — lead is used to form a waterproof gutter that collects and directs rainwater off the roof. Open valleys, where the lead is visible, allow water to run off efficiently. Closed valleys, where tiles are cut and mitred over the top, are more prone to debris build-up and blockage. Lead valley gutters should be formed in manageable lengths, typically no more than 1.5 m per piece, to allow for thermal movement.
Apron Flashing
An apron flashing is a single piece of lead that covers the junction between a horizontal or near-horizontal surface and a vertical face — for example, below a skylight or Velux window, or at the base of a chimney. It is dressed over the tiles or slates below and up against the structure above.
The Standard: BS 6915:2016
The correct installation of lead flashings in the UK is governed by BS 6915:2016, the British Standard for the design and construction of sheet lead flashings and roofings. This standard specifies the correct lead code (thickness) for each application, the maximum length of individual pieces (to prevent buckling and fatigue from thermal movement), and how lead should be fixed and dressed. Any competent roofer working with lead should be following this standard, even if they do not cite it by name. If a roofer is using single long runs of lead without expansion points, or using mortar fillets instead of proper lead soakers, they are not working to standard.
How Long Does Lead Flashing Last?
Properly installed lead flashing, using the correct code lead for the application and following BS 6915:2016, can genuinely last 60 to 100 years. The Lead Sheet Association quotes a typical lifespan of 60 years for most flashing applications. This longevity is one of the key reasons lead remains the preferred material despite its higher upfront cost compared with alternatives such as lead flashing tape, fibreglass upstands or zinc.
In practice, many lead flashings on older Staffordshire properties fail well before this because of poor original installation, ongoing maintenance issues, or because the lead was never installed at all — just a mortar fillet covering the joint.
Common Causes of Lead Flashing Failure
Mortar Fillets Instead of Lead
The most common cause of chimney and abutment leaks on older UK properties is that mortar fillets were used instead of proper lead soakers and step flashings. A mortar fillet is simply a wedge of sand and cement pointed against the roof surface. It is not a waterproofing solution — it is a very temporary fix that will crack within a few years as the roof structure and chimney move independently.
Thermal Fatigue
Lead expands significantly in warm weather and contracts in cold weather. If lead is fixed over too long a length without break points, or if it is nailed through in a way that prevents movement, it will crack over time. This is why BS 6915:2016 specifies maximum lengths and approved fixing methods.
Poor Installation
Lead flashing requires skill to dress correctly. Lead that is not dressed tightly against the surface, that has not been properly turned into a chase, or that lacks adequate overlap will allow water to penetrate. Many cheap roofing jobs use inappropriate materials — lead flashing tape, self-adhesive flashing products, or aluminium — as substitutes for proper lead work, and these invariably fail within a few years.
Signs Your Lead Flashing Needs Replacing
- Damp patches on ceilings near a chimney breast, skylight or external wall — particularly if they appear or worsen after rainfall.
- Visible cracks or splits in the lead — especially along the edges or where the lead meets the brickwork.
- Lead that has lifted away from the wall — if you can see a gap between the lead and the brickwork, water is getting in.
- Cracked or missing mortar fillets — a mortar fillet alone is not a substitute for proper lead, but if yours has failed, the gap is an open invitation for water.
- Lead that looks thin or has pinholes — very old lead can erode to the point where it is no longer watertight.
- Visible rust staining on the brickwork below the flashing line — suggests water has been running behind the lead for some time.
Repair or Replace?
Small areas of lifted or cracked lead can sometimes be repaired — cleaning the area, re-dressing the lead against the wall, and re-pointing the chase in the brickwork. Where the lead has genuinely cracked through, a lead repair patch or specialist lead bonding compound can buy additional life.
However, if the underlying issue is that the flashing was never installed correctly — mortar fillets, self-adhesive tape, or other substitutes — then repair is futile. The only reliable solution is to strip back the affected area and install proper lead flashing to the correct standard. This is the approach we take at Aether Roofing Solutions, and it is the reason our lead work carries a 10-year guarantee.
If you have a leak around a chimney, dormer or wall abutment anywhere in Staffordshire, visit our lead flashing repair page or general roof repairs page to find out how we can help.
