Most window leaks happen at the installation flashing around the window, not through the glass. Water gets in at the bottom sill corners, at mull joints between ganged windows, or behind a head flashing that is missing or lapped under the weather barrier instead of over it. With no sloped sill pan, that water runs into the wall. Lasting fixes re-flash the opening with a sloped pan, correctly lapped jamb and head flashing and a cap above the trim; caulking the outside rarely solves it. This page is general information.
On this page: Signs · Causes · Ground check · Fixes · Basis · When to call
Signs of a window flashing leak
- Stains, swelling or peeling paint on the wall below a window corner, inside or out.
- Soft or rotted window sills, especially at the ends.
- Water on the interior sill or floor after wind-driven rain, with dry glass.
- Rot in the head trim above the window.
- Streaks down the siding from the bottom corners.
How water gets around a window
| Cause | What happens |
|---|---|
| No sloped sill pan or back dam | Water under the window flows inward instead of out |
| Weather barrier reverse-lapped at the head | Water running down the wall is directed behind the window |
| Head flashing tape applied on top of the barrier | Same result: water is let in at the top |
| Unsealed or undrained factory mull joint | Water runs down between ganged units |
| No cap flashing over head trim | Water sits on the trim, rots it and runs behind |
| Bottom flange sealed shut | Water inside the opening is trapped |
An outside and inside check
- Inside, look for stains or soft drywall below the window corners after a storm, and note the wind direction.
- Outside, look above the head trim for a metal drip cap tucked under the siding. Many older installs have none.
- Check the sill and lower trim for soft spots with a screwdriver, and look for staining below the corners.
- On ganged windows, look for streaking down the joint between units.
- Do not spray a hose at the window to test it yourself; uncontrolled water can push leaks into the wall. A contractor runs a controlled water test.
The correct fix, and the wrong ones
| Fix | When it fits |
|---|---|
| Repair frame corner seals and perimeter sealant per the window maker | Short-term stopgap while planning a proper repair. |
| Add cap flashing above the head trim, tucked under the weather barrier | Head trim rotting; no drip cap. |
| Remove trim and siding around the window, re-flash head and jambs, re-lap the barrier | Water entering at the head or sides. |
| Remove and reinstall the window over a sloped sill pan with a back dam | Water entering at the sill corners. |
| Seal mull joints per the manufacturer, with a continuous pan under the assembly | Leaks at ganged windows. |
Wrong fixes: caulking the bottom of the window to the sill or trim, relying on exterior caulk as the only barrier, or replacing interior drywall without finding the leak.
Code and building science basis
- Code. The 2021 IRC, section R703.4, requires flashing at exterior window and door openings, and section R703.4.1 requires it to extend to the surface of the wall finish or to the weather barrier. Where the window maker gives no flashing details, pan flashing is required at the sill.
- Building Science Corporation, Window Leakage project. Leaks began after the first year and concentrated at bottom sill corners and factory mull joints; sill flashing ran only about 6 inches up the opening with no sloped sill or back dam.
- BSI-085 Windows Can Be a Pain. Calls for a sloped sill with pan flashing, tape on the jambs first and then the head, and says sealant behind the flanges is not necessary for water control; its role is air sealing.
- BSD-013 Rain Control in Buildings. Sill and head flashing should extend at least 2 inches past the jambs, and flashing must be waterproof material, not building paper or housewrap.
Rot below a window is covered in rotted exterior trim, and wall-wide damage in water behind siding.
When to call
Call when you see staining below window corners or soft sills. Integrity installs siding, so when a wall is re-sided we can open the area around a leaking window, replace damaged sheathing and trim, and flash the siding, trim and drip cap back into the opening. Whether the window itself needs repair or reinstallation is a call for the window’s manufacturer or a window installer. See the exterior problems guide.
Book an exterior inspection
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. Colorado Springs area: call (719) 488-2800 or book a free inspection. Denver metro: call (303) 209-0152 or book a free Denver inspection. Austin: call (512) 610-5897 or book a free Austin inspection. We reply the same business day.
Sources: 2021 IRC Chapter 7, sections R703.4 and R703.4.1 (UpCodes); Building Science Corporation, Window Leakage project; Joseph Lstiburek, BSI-085 Windows Can Be a Pain; John Straube, BSD-013 Rain Control in Buildings.
