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Common mistakes · Detail

Leaks at a dormer

Leaks are the most feared dormer problem, and the most expensive. The good news is that almost all leaks are caused by a few predictable mistakes.
6 min leestijd·Onafhankelijke informatie

Short answer

Most dormer leaks happen at the junction with the existing roof, not through the dormer itself. The cause is usually rushed leadwork, missing flashings, or an opening left unprotected in the rain.

Causes

Leaks are rarely mysterious. They come from a short list of installation errors that are visible to the trained eye.

  • Rushed leadwork — not enough overlaps, poor dressing, or wrong lead code.
  • Missing or poor flashings — step flashings not interleaved with tiles.
  • Open roof in rain — no temporary cover while the roof was open.
  • Sealant failure — windows or cladding joints not sealed properly.

Where leaks start

Water follows the path of least resistance. In a dormer, the weak points are almost always the same.

  • Top of the dormer where the lead apron meets the roof.
  • Sides where step flashings join the roof covering.
  • Window reveals and corners.
  • Any point where the roof covering was disturbed.

Prevention

A watertight dormer is the result of careful detailing, not expensive materials. Make sure the company plans the weatherproofing before opening the roof.

The full checklist

Checklist

Leak prevention checklist

  1. 01Lead apron is code 4 or 5 with correct overlaps.
  2. 02Step flashings are interleaved with roof tiles or slates.
  3. 03Roof covering matches the existing roof and is properly lapped.
  4. 04Window seals are intact and flexible.
  5. 05Cladding joints are sealed and drained.
  6. 06Roof was never left open overnight without cover.
  7. 07A water check or rainfall test was done after install.
  8. 08I know where to look for early signs of a leak.

FAQ

FAQ

Veelgestelde vragen

01What is the most common cause of dormer leaks?
Poor detailing where the dormer meets the existing roof, especially the lead apron and flashings. Most leaks are not caused by the dormer box itself.
02Can a new dormer leak immediately?
Yes, if the leadwork or flashings were rushed or if the roof was left open in the rain. A proper installer tests the work before leaving.
03How do I spot a dormer leak early?
Stains on the ceiling, damp patches near the window reveals, or water running down the wall after rain. Check the loft after heavy rainfall.
04Should I accept a small leak as normal?
No. A new dormer should be watertight. Small leaks usually get worse and can damage insulation and plaster.
Conclusie
Leaks are preventable. The detail that matters is the junction between the dormer and the roof. Ask about it, inspect it, and never accept a wet ceiling as normal.