Dormer trim and lead flashings
The junction between dormer and existing roof is THE point where 80% of dormer leaks originate — not the frame or roof covering itself. Good connections demand skilled work in lead, zinc or EPDM strips, with correct overlap, upstand and sealant. In 2026 EPDM flashings increasingly replace lead — lighter, equally watertight, welstand accepts them in new build.
- Lead side flashing
- Code 18 (≥1.8 mm), 50+ years
- EPDM flashing (alternative)
- 30–40 years, lighter
- Min. upstand on roof
- 150 mm vertical
- Aluminium drip edge
- RAL coated, 30+ years
- Tile flashings
- Per-tile, sealant-free
- Leak risk 'weak point'
- 85% of defects
Where is lead used and why?
A dormer has 4 critical junctions: (1) behind dormer where roof continues (no lead needed, covering overlaps), (2) cheeks where dormer meets sloped roof (lead or EPDM), (3) front below frame where flat roof meets drip edge, (4) corners where 3 planes meet (most critical).
On the cheeks code 18 lead (1.8 mm thick), 30 cm wide is used: 15 cm vertically under the outer covering layer, 15 cm horizontally over the cheek. On tile roofs, individual 'tile soakers' are dressed per tile for a clean look.
Lead vs lead alternatives (EPDM, aluminium)
Lead: 50+ year lifespan, beautiful patina, fully shapeable by hand. Downside: heavy (11 kg/m at code 18), environmentally sensitive, €25–€40/kg in 2026. A 3-metre dormer needs 18–25 kg lead (€450–€1,000).
EPDM flashings (Ubiflex, Wakaflex): rubber lead alternatives with aluminium mesh. 30–40 year lifespan with proper install. Lighter (1 kg/m), sealant-free fusion joints, cheaper. Welstand approves in new build; monuments often still require lead.
Aluminium drip edge: used at the front of flat dormer to finish EPDM/bitumen cleanly. RAL coated (anthracite or black), 1.5–2 mm aluminium, 30+ year life.
EPDM and bitumen flat roofs
The flat roof of the dormer itself is finished with EPDM (single 1.2–1.5 mm rubber sheet) or bitumen (two layers torched on). EPDM gains share (60% in 2026 vs 40% bitumen) due to faster install and 40+ year maintenance-free life.
Flat roof to frame junction needs an 80 mm upstand against the frame (finished with aluminium upstand profile), with EPDM bonded by contact adhesive to the top of the upstand. Under the front drip edge 100 mm overlap.
Where leaks originate — top 5
From damage reports by VvE managers and roofers, by frequency:
- Cheek/flat-roof corners: 35% — three-plane junction, needs pre-formed EPDM corner or soldered lead corner.
- Sealant joints around frame after 8–12 years: 25% — sealant ages, joint cracks. Replace every 10 years.
- Drip edge blown loose or poorly fixed: 15% — screws or clips.
- Tile soakers with too little upstand: 15% — installer cuts lead too tight; rain driven under tiles.
- EPDM bond failure: 10% — often cold install or dirty substrate.
FAQ
- Is EPDM flashing as good as lead?
- Functionally yes — both deliver 30+ years waterproofing. Lead lasts 50+ years and is repairable in pieces. Welstand often requires lead in protected views.
- How often replace sealant around a dormer?
- High-grade polyurethane or MS polymer: inspect every 8–12 years, replace at cracks or detachment. Acrylic sealant lasts only 4–6 years — not recommended outdoors.
- Can I repair junction leaks myself?
- Small sealant joints yes (Bostik MS Polymer, ~€8/cartridge). Larger lead or EPDM issues no — needs skilled work and warranty. Wrong repairs often worsen the problem through trapped water.
- Cost to replace lead flashing?
- Replacing lead side flashings on a 3-metre dormer: €450–€800 incl. scaffold hire. Full re-check of all junctions + new flashings: €1,200–€2,000. Use a VEBIDAK-certified roofer.
- How to recognise good workmanship at handover?
- Ask for photos during execution (a good roofer takes them at each junction). At handover check: lead/EPDM at least 15 cm up and 15 cm over, no visible gaps, drip edge straight and square, sealant joints smooth. Take your own photos for warranty records.