Waterproofing and leadwork: where dormers live or fail

Short answer

A dormer is waterproofed with several layers: a lead or rubber apron at the base, soakers up the sides, an upstand behind the cladding, and a flashing at the top. Each one has to be dressed into the tile profile, not just laid on top — that is the single rule that keeps the dormer dry for decades.

The waterproofing layers

• Roof underlay. A breathable membrane behind the cladding routes any incidental moisture down to the gutter.
• Insulation envelope. Sealed at the perimeter to stop condensation forming inside the structure.
• Lead apron. The visible metal band at the foot of the dormer, dressed into the tiles.
• Side soakers. Smaller lead pieces interleaved with the tiles up each side.
• Top flashing. Where the dormer roof meets the main roof — often the most leak-prone joint.

Lead apron, soakers and upstands

The apron should sit on top of the tiles for at least 150 mm and be dressed (bossed) into the tile profile so wind-driven rain runs off, not under it. The soakers up the sides should be one per tile course. Behind the cladding, an upstand of at least 150 mm prevents water creeping inwards.

Common waterproofing mistakes

• Lead apron laid flat on the tiles, not dressed in — wind drives rain underneath.
• Soakers missing on the sides, sealed with mastic instead — mastic ages out in 5–8 years.
• Upstand behind the cladding too short or absent.
• Wrong lead grade (code 3) where code 4 or 5 is needed.
• Top flashing relying on a single bead of sealant.

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