Failed lead flashing is, in our experience, the single most common cause of roof leaks on Tonbridge period properties — and the most frequently misdiagnosed. We regularly attend properties where a previous roofer has patched the tile surface or re-pointed the ridge, leaving failed flashing untouched, and the leak has continued uninterrupted. This guide explains how lead flashings work, the most common failure modes on Tonbridge properties, and what correct installation looks like so you can assess any quote you receive.
How Lead Flashing Works
Lead flashing is the weatherproofing system at the junctions where your roof meets a vertical surface — most commonly a chimney stack, a parapet wall, a dormer cheek, or where a lean-to roof meets the main house wall. At these junctions, tile alone cannot provide a continuous, watertight seal — the gap between the tile surface and the vertical masonry is the vulnerability. Lead, being highly malleable, can be dressed (formed by hand using a wooden bossing tool) to create a continuous, tight-fitting seal at these junctions that moves slightly with thermal expansion without cracking or opening.
A properly installed chimney flashing system consists of multiple components:
- Apron flashing — covers the front (downslope) face of the chimney stack where it meets the tile surface
- Soakers — individual small pieces of lead that sit under each tile course at the sides of the chimney, turned up against the chimney face
- Step flashing — a continuous lead strip dressed in steps over the soakers up the chimney side, chased into the mortar joint and turned down over the soakers
- Back gutter — the flashing behind the chimney (the upslope side), where water running off the chimney face is collected and directed to the sides
Each component must be correctly overlapped, correctly chased, and correctly dressed to work as a system. Failure of any single element compromises the whole.
Suspected flashing failure? Free inspection — 0173 224 1138. We'll find the source and quote the fix.
📞 0173 224 1138Lead standard: The Lead Sheet Association specifies Code 5 (2.24mm, 25.4 kg/m²) as the minimum for chimney apron and step flashings on exposed UK positions, with welted cross-joints at maximum 150mm centres. Code 3, commonly installed on domestic properties, does not meet this standard for exposed chimney work. Source: Lead Sheet Association, Technical Guidance Note TGN 6.1
Lead Code Specification — Why It Matters
Lead sheet is sold in codes that correspond to its weight and thickness. The code number directly reflects the weight in pounds per square foot of the original imperial specification (now expressed in kg/m² in metric). For roofing applications, the relevant codes are:
| Code | Thickness | Weight | Application |
|---|---|---|---|
| Code 3 | 1.32mm | 15kg/m² | Not recommended for exposed roofing |
| Code 4 | 1.80mm | 20kg/m² | Soakers, valleys, lining (sheltered) |
| Code 5 | 2.24mm | 25kg/m² | Apron flashings, exposed elements |
| Code 6 | 2.65mm | 30kg/m² | Back gutters, heavily exposed positions |
The most common cause of premature flashing failure we see on Tonbridge properties is under-specification — Code 3 used where Code 4 or Code 5 is required. Code 3 lead is too thin for exposed chimney aprons: it fatigues through thermal cycling faster, develops splits at welds and joints sooner, and is more easily damaged by physical contact during maintenance. Any quote for chimney flashing work that doesn't specify the lead code in writing should be clarified before you proceed.
Joint Types — Welts vs Laps
Lead sheet can't be installed in a single continuous piece over any significant length — thermal expansion and contraction would cause buckling or cracking if it were. Instead, lead is installed in sections with joints between them. There are two joint types used in roofing:
Welt joints are formed by folding the two ends of adjacent pieces over each other and dressing them flat. A welted joint, done correctly, is fully watertight — the overlapping fold means water cannot track between the sheets. It also allows a small amount of thermal movement without opening.
Lapped joints — where one piece simply overlaps another without forming — are faster to install and are acceptable in some less-exposed positions, but they rely on gravity to keep water flowing in the correct direction and are not suitable for positions where water can track upward under the lap. On valley lead in Tonbridge where water flow rates are high during heavy rainfall, welted joints are the correct specification.
If a roofer describes their flashing installation as 'lapped' on a chimney apron or valley, ask whether they mean welted. The terminology is sometimes used loosely, but the distinction in durability is significant.
Why Tonbridge Victorian Chimney Flashings Fail Prematurely
The combination of factors specific to Tonbridge period properties accelerates flashing failure:
North-facing stacks. Victorian terrace chimney stacks on the north-facing slopes of Tonbridge streets — Pembury Road, Vale Road and the streets running off them — remain perpetually damp. A flashing that never fully dries out experiences more freeze-thaw cycling per winter than a south-facing equivalent, and the mortar in the chased joint above it erodes faster. Once the chase empties of mortar, the top edge of the flashing is exposed to wind movement and water ingress from above.
Sandstone and lime mortar joints. Some older Tonbridge properties have chimney stacks built in local sandstone rather than engineering brick. Sandstone is more porous than engineering brick and requires lime mortar for repointing — not OPC cement, which is harder than the masonry it sits in and causes spalling. A chased joint in sandstone repointed in OPC cement will crack the sandstone as the cement expands and contracts, destroying the seal at the top of the flashing and creating a direct water ingress point.