Dormer leakage: 2026 guide

The spot where water appears inside is rarely the spot where it enters outside. Water runs along beams, insulation and plasterboard to the lowest point and only becomes visible there. For diagnosis work from inside out: note location (above window, beside wall, mid ceiling), timing (only in rain, only with wind, also dry weather), and size (drops, puddle, stain).

Leakage only in heavy rain usually points to sealant joints or small lead problem. Leakage in rain with wind points to gap action or a shifted tile. Leakage without rain points to condensation or a water/CH pipe leak in the attic — not a dormer issue.

For locating, a garden hose aimed at different dormer exterior parts helps: 5 minutes on lower lead, then on cheeks, then on top. Wait 15 minutes between tests and look inside where water appears. Not always successful but often gives a clear hint.

Lead connects the dormer to existing roof tiles at sides and bottom. Through temperature changes (expand summer, shrink winter) lead sags millimetre by millimetre after 10 to 20 years. At average 1-2 mm per year, after 15 years a 15-30 mm gap can form between lead and frame — wide enough for rainwater.

Visually: creases in the lead, visible shadow or line where lead was pressed, or simply the clearly open edge. Repair: re-dress lead (€150-€300 for one side), or for severe damage replace lead section (€450-€900). On replacement always ask for lead 25 (1.25 mm thick) or heavier, not lead 20 or thinner.

A good contractor treats lead with patination oil at install to prevent quick weathering. Plan 10 to 20 years lifespan for well-laid lead 25. Lead 30 or zinc alternatives last 30+ years but are 30-50% more at purchase.

Sealant connects frame to wall, wall to roof and corners. Acrylic sealant (budget) dries out after 8-12 years, shrinks and cracks; MS polymer (premium) lasts 25-30 years. Early sign: hair cracks in the joint, or a 1-2 mm opening where the joint was flat.

Local repair: carefully cut and scrape old sealant, clean joint with spirit, prime, then new sealant. DIY: €15-€25 materials, 1-2 hours work. By contractor: €150-€350 per metre including finish and warranty.

On replacements always ask for MS polymer (Soudaflex 40FC, Bostik Simson ISR70), not standard acrylic. Extra cost €5-€10 per tube but 3x lifespan. Many contractors still default to acrylic from habit; asking for premium pays off.

Blocked gutter or downpipe: a gutter full of leaves or moss can overflow after heavy rain and water runs behind gutter brackets or between dormer and roof into the inside. Yearly gutter inspection (€0 DIY, €75-€150 by roofer) solves this preventively.

Shifted or damaged tiles around the dormer: storm damage or poor underlayment can shift 1-2 tiles and let water through. Repair: realign or replace tiles (€100-€300 for a few by roofer). Important: tiles specifically cut to fit around the dormer cannot always be replaced 1-for-1 with the same model.

Condensation confused with leakage: a well-insulated dormer with poor ventilation can get moisture drops on cold surfaces (especially the window side). Looks like leakage but happens without rain and is spread over a large area rather than one spot. Solution: improve ventilation (see Ventilation page). No repair contractor needed, but a ventilation check.