Lead sheet roofing: bays, valleys, dormers and flat lead roofs
Lead is the traditional metal for the awkward parts of period roofs. Most failures blamed on lead are really failures of how it was laid, which is why understanding the details helps you judge a repair or replacement quote.
Lead sheet is used on small flat and curved roofs such as bay windows, porches, dormer cheeks and tops, and to line valleys and gutters. It lasts for generations when laid in correctly sized bays with rolls, drips and proper fixings, and fails early when pieces are too large, nailed through or laid without allowance for movement.
Where lead is used on a roof
On houses, lead covers bay window roofs, porch canopies, dormer tops and cheeks, and small flat roofs over projections. It lines valley gutters between slopes and parapet or box gutters behind walls. It also forms flashings at chimneys and abutments, which have their own guide. Each use has its own detailing rules, but the principles of movement and fixing are the same.
Why bay sizes matter
Lead expands and contracts noticeably with temperature. A sheet that is too large, or fixed too rigidly, cannot move freely, so it cracks or splits over time. Flat and low-pitched lead roofs are therefore divided into bays joined by rolls running with the fall and drips across it. Industry guidance sets maximum bay sizes for each code (thickness) of lead; good leadwork follows it.
Rolls, drips and fixings
Wood-cored rolls join bays along the fall, with one sheet dressed over the other so water cannot enter. Drips are steps across the fall that let one bay lap over the next. Sheets are fixed along one edge and held at the others by clips so they can slide. Nailing through the middle of a sheet is a classic fault that leads to splits.
Substrate and underlay
Lead needs a smooth, sound deck, usually plywood or boards, with an appropriate underlay to allow movement and separate it from the timber. Ventilation beneath the deck matters: without it, condensation can form on the underside of the lead and cause corrosion that is invisible from above. This is one reason a survey looks under the roof as well as on top.
Lead valleys
Valleys carry a lot of water from two slopes, so failures show quickly. Lead valleys are laid in lengths with generous laps, on boards with a tilting fillet at each side, and the tiles or slates are cut to leave a clear channel. Valleys filled with debris or with mortar bedded onto the lead are common causes of leaks. The site offers valley relining and lead valley replacement as separate repair services.
Common lead faults
Watch for splits and cracks, often at the edges of oversized sheets; lifted or missing clips after storms; open laps at drips; patches of old bitumen or tape covering earlier failures; and white staining, which is a normal weathering product unless it runs onto other surfaces. Theft is also a risk on low, accessible roofs.
Lead or an alternative?
Lead's formability makes it ideal for complex shapes and for matching period work. Alternatives such as synthetic flashing products, GRP and single-ply membranes can suit some locations, particularly where theft is a concern or weight is a limitation. The lead alternatives guide explains where each works. On listed or conservation properties, lead is often expected.
Repair or relay?
A single split in otherwise healthy lead can sometimes be repaired, but patches with sealant rarely last. Where several bays are cracked, sheets are oversized or the deck has rotted, relaying the roof in correctly sized bays is the lasting fix. A good quote states the lead code, the bay layout and how the deck and ventilation will be dealt with.
FAQs
Correctly laid lead on a sound, ventilated deck is one of the longest-lasting roofing materials. Poorly detailed lead can fail much sooner.
Usually because the sheets were too large for their thickness, fixed too rigidly or nailed through, so they could not move with temperature.
Small splits can sometimes be repaired, but sealant and tape patches rarely last. Repeated splitting means the lead needs relaying.
Usually not. It is a natural weathering product. It only needs attention if it is washing onto other surfaces.
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