Lead is the material responsible for most of the waterproofing details that sit at the junctions of your roof — where the roof meets the chimney, where two roof slopes meet at a valley, where the roof meets a wall. On Northampton's Victorian, Edwardian and 1930s housing stock, this lead is now 80 to 130 years old. Understanding what it does and when it fails prevents the persistent damp problems that are among the most frustrating maintenance issues on these properties.
Lead flashings on Northampton properties seal chimney, wall and valley junctions, lasting 50-100 years when correctly dressed. Sealant is not a permanent fix - thermal movement tears it within one to three years. Correct repair uses the right lead code, proper laps and clips. Repairs cost £200-£900.
What Lead Does on Your Roof
Lead flashings are the shaped lead components that seal the junctions between the roof surface and vertical elements — chimneys, walls, and dormer cheeks. Without them, water runs down the vertical surface and finds the gap between it and the roof tiles, tracking inside.
Step flashings are the most common type — a series of lead pieces, each overlapping the next, that step up the chimney or wall in line with the tile courses. Each piece is fixed into the mortar joint of the brickwork and laps over the edge of the tile below. A correctly dressed step flashing is essentially invisible but essential.
Valley linings are the lead sheets that form the channel at the junction of two roof slopes. On Northampton's Victorian and 1930s housing, valleys are almost always lead-lined. Water from both slopes drains into the valley and is channelled to the gutter — the lead lining prevents the valley structure from getting wet.
Bay window leads are flat lead sheets that form the roof of a bay window projection. These are among the most complex lead applications on domestic properties, requiring the lead to be shaped, cut and dressed to create a weathertight surface with correct falls on a complex geometry.
Why Lead Fails: The East Midlands Thermal Cycle
Lead is not inert — it expands and contracts with temperature changes, and the East Midlands experiences a wider daily and seasonal temperature range than milder coastal climates. A lead flashing in Northampton moves more over the course of a year than the same flashing in Cornwall or coastal Wales.
The Lead Sheet Training Academy specifies correct lead codes and thermal movement provision - lead fixed too rigidly cracks along fatigue lines regardless of its thickness.
Source: Lead Sheet Training Academy guidance
The primary cause of lead failure on Northampton properties is fatigue — the lead has been flexing with the thermal cycle for 80 to 130 years and has developed micro-cracks that eventually open to become leak paths. This is not a defect — it is the natural end of a long service life.
The secondary cause is incorrect previous repairs. Hard cement applied over lead — a common 'fix' on Victorian properties — prevents the lead from moving with the thermal cycle and concentrates stress at the cement edge, accelerating fatigue. Sealant over failed joints traps moisture beneath it and corrodes the lead faster. Both make the eventual proper repair harder.
A third cause is incorrect original installation or replacement: lead fixed too rigidly with nails driven through the lead body rather than clips, lead of insufficient code weight for the application, or incorrect step and lap dimensions. We see this on properties where the lead was replaced in the 1970s or 1980s using practices that were then current but are now known to shorten lead life.
How to Tell if Your Lead is Failing
The most reliable indicator is internal: a damp patch on the ceiling or wall in a location that corresponds to a lead junction — below the chimney stack, below the valley line, on the wall adjacent to a bay window. Damp that appears or worsens during rain and dries out between rain events, rather than being consistently damp, is characteristic of a roof junction failure.
External indicators are less reliable but useful for early identification. Green staining (verdigris) on the tile surface below a lead junction indicates lead is oxidising and water is washing it down the tile. This is normal for aged lead but its concentration in a specific spot can indicate where the main leak path is.
Visible cracks in the lead surface, particularly along lines parallel to the edge of the flashing, indicate fatigue cracking. These are typically very fine initially but widen with each thermal cycle. Cement or sealant repairs on top of the lead are an indicator that someone has already identified a problem — the question is how adequately they addressed it.
The Correct Repair vs the Wrong One
The correct repair to a failed lead flashing is re-dressing: stripping out the old lead, cleaning the surface and mortar joint, and installing new lead to the correct specification. The new lead is cut to correct dimensions, stepped to the mortar joint courses, fixed with clips (not nails through the body), lapped correctly with each piece overlapping the one below by at least 65mm, and dressed to the tile surface.
The wrong repair is applying sealant or cement over the existing lead. This is the most common 'repair' applied to failed Northampton chimney flashings by contractors who are not specialist lead workers. It delays the problem by one to three years — until the sealant cracks, UV-degrades or is pushed open by the thermal movement of the lead beneath it.
A correctly specified and installed lead flashing in Northampton will last 50 to 100 years. The service life of a sealant repair is one to three years. The cost difference between a sealant repair and a correct re-dress is £200 to £400. The long-term value comparison is obvious.
Lead Code and Specification
Lead comes in code weights ranging from Code 3 to Code 8. For roofing applications in Northampton, the relevant codes are: Code 3 (1.32mm thick) for step flashings and cover flashings; Code 4 (1.80mm) for valley linings, flat sections over 150mm width, and exposed horizontal applications; Code 5 (2.24mm) for bay window flat roofs and large flat sections.
Underweight lead is a common cause of premature failure. A valley lining laid in Code 3 where Code 4 is required will fatigue in half the time — the thinner lead flexes more with each thermal cycle and reaches the end of its fatigue life faster. We specify the correct code for each application and do not cut costs by underspecifying.
For listed buildings and conservation area properties in Northampton and surrounding villages, Code 4 or Code 5 lead is typically required for all external applications. English Heritage guidance recommends Code 4 as a minimum for most historic building lead work. We work to these standards as a matter of course.
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Why Proper Leadwork Outlasts the Alternatives
Lead has been used on roofs for centuries for good reason: dressed correctly, it forms a durable, flexible weather seal at exactly the awkward junctions — chimneys, valleys, abutments — where roofs leak most. The key phrase is 'dressed correctly'. Proper leadwork uses the right code (thickness) of lead for the job, is dressed to shape rather than forced, is fixed and wedged so it can move with temperature without splitting, and is finished with appropriate pointing. Done this way it outlasts the tiles around it. The alternative you'll often see on Northampton roofs — sealant or mortar smeared over a failed joint — is a false economy: it looks fine briefly, then cracks as the roof moves and the leak returns, usually worse. If a roofer proposes to fix a chimney or valley leak with a tube of sealant rather than dressing new lead, that tells you what kind of job you're getting. Real leadwork costs more upfront and saves money over the life of the roof.
Frequently Asked Questions
Last reviewed: July 2026