Who this guide is for
- Homeowners replacing windows and planning the detailing
- People with leaks tracing to a window opening
- Renovators integrating windows with the barrier
- Anyone briefing an installer on window flashing
Why window openings leak
Water hits the wall and concentrates at the window: it pools at the head, runs down the jambs and collects at the sill. Without correct flashing, any of these can let water into the wall. The opening is a designed weak point that flashing exists to protect.
Sill pan, jambs and head
Three flashing zones protect the opening: a sill pan that catches and drains water at the bottom, jamb flashing down the sides, and head flashing that sheds water over the top. Each has a role, and they work as a set. Plan all three, not just the visible head.
- Sill pan catches and drains water at the base
- Jamb flashing protects the sides
- Head flashing sheds water over the top
- All must integrate with the weather barrier
Sequence and shingling
Flashing only works if it is layered so each piece sheds over the one below, like shingles, and integrated with the weather barrier in the right order. A reversed lap funnels water inward. The sequence is as important as the parts.
Integrating with barrier and trim
The flashing ties into the weather barrier so water always reaches the drainage plane and sheds out, and the trim then covers it. If the trim is detailed to let water behind, the flashing's work is undone. Plan the flashing, barrier and trim together.
Hidden, critical work and professionals
Because window flashing is hidden once the window and trim go in, mistakes are expensive to find, and the detail is weather-critical, so a qualified window installer or builder should handle it. A leak at a window should be assessed for its real path. Requirements vary by location and project, and this is not DIY work.
Window flashing planning checklist
- 1Recognise the opening concentrates water at head, jambs and sill
- 2Plan a sill pan to catch and drain water
- 3Plan jamb flashing down the sides
- 4Plan head flashing to shed over the top
- 5Sequence the flashing so each piece sheds over the one below
- 6Integrate the flashing with the weather barrier
- 7Coordinate the trim so it doesn't let water behind
- 8Use a qualified installer for the hidden, critical flashing
Common mistakes to avoid
- Flashing only the head and skipping the sill pan
- Reversing a lap so water funnels into the wall
- Not integrating the flashing with the weather barrier
- Detailing the trim so water gets behind it
- Treating the visible sealant as the only defence
- Assuming a window leak is right where the stain shows
When to involve a professional
- Use a qualified window installer or builder for the flashing
- Treat window flashing as weather-critical, hidden work
- Have a window leak's real path assessed by a professional
- Coordinate flashing, barrier and trim sequence
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why do window openings leak?
Because water concentrates at the opening, pooling at the head, running down the jambs and collecting at the sill. Without correct flashing at all three zones, any of these can let water into the wall, which is why the opening is a designed weak point that flashing protects.
What is a sill pan?
A sill pan is flashing at the bottom of the window opening that catches and drains any water reaching the sill, leading it back out rather than into the wall. It works with jamb and head flashing as a set to protect the whole opening.
Why does the flashing sequence matter?
Flashing only works if each piece sheds water over the one below, like shingles, and integrates with the weather barrier in the right order. A single reversed lap funnels water inward, so the sequence is as important as the individual parts.
Can I flash a window myself?
Window flashing is weather-critical and hidden once the window and trim go in, so mistakes are expensive to find. A qualified installer or builder should handle it, and a window leak should be assessed for its real path. Requirements vary by location and project.
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