Water Coming In Under a Window Sill? Sill Pans, Flashing and the Real Fix
| # | Table of Contents |
|---|---|
| 1 | How Water Gets Under the Sill |
| 2 | What a Sill Pan Does |
| 3 | The Most Common Causes |
| 4 | How to Narrow It Down |
| 5 | What the Fix Involves |
| 6 | Why It Matters for the Wall |
| 7 | FAQ |
How Water Gets Under the Sill
Every window is a hole in the wall, and no window installation is perfectly watertight forever. Wind-driven rain gets through corner joints in the frame, through gaps between the window and the siding or trim, and through sealant that has cracked with age. Modern installation practice accepts this. Instead of trying to seal every joint permanently, it builds a drained opening: whatever water gets past the window is collected at the bottom of the opening and directed back out.
When that drainage is missing or blocked, the water collects on the framing under the window. From there it shows up indoors as a wet sill, stained drywall below the window corners, swollen trim or a damp carpet edge. It often appears only during storms with wind from one direction, which makes it easy to blame on something else.
What a Sill Pan Does
A sill pan (also called pan flashing) sits under the window at the bottom of the rough opening. A Building Science Corporation guide prepared for the U.S. Department of Energy's Building America program lists four things a working pan needs:
- A continuous waterproof surface with no holes, tears or wrinkles.
- A back dam or a slope so water runs to the outside, not toward the room.
- End dams at both sides so water cannot run off the ends into the wall.
- A lap over the water barrier below, so water leaving the pan lands on the outside of the house wrap, not behind it.
The same guide says the window should be sealed at the head and sides, but "the sill should be left free to drain to the exterior."
The code says the same thing in its own words. Section R703.4.1 of the 2024 IRC, the edition the City of Austin enforces, says window flashing follows the window manufacturer's instructions, and where those instructions give no flashing details, "pan flashing shall be installed at the sill." The pan must be "sealed or sloped" to send water to the outside of the wall finish or to the water-resistive barrier.
The Most Common Causes
| Cause | What happens |
|---|---|
| No sill pan at all | Water that gets past the frame lands on bare framing or sheathing and soaks in. |
| Pan with no back dam or slope | Water runs toward the room and comes out under the interior sill or stool. |
| Pan with no end dams | Water runs off the ends into the wall cavity beside the window. |
| Reverse-lapped house wrap | The pan drains behind the water barrier instead of in front of it. |
| Bottom flange or sill caulked shut | Water collected in the pan has no exit, so it overflows inward. |
| Frame or glazing failure | Water enters through the window itself, more than any pan can handle. |
The caulked bottom edge deserves special mention. The Building Science Corporation guide warns that a pan is "entirely subverted" if the window's bottom flange is sealed to the drainage plane. It looks tidy and it feels like a fix, but it traps water in the opening.
How to Narrow It Down
- Note when it leaks. Only in wind-driven storms from one direction points to the opening and flashing. Every rain points more toward the window unit or the wall above.
- Check the window's own drainage. If your window has weep openings on the outside of the bottom frame, make sure they are not clogged with dirt, debris or paint. A blocked weep lets water back up in the frame. Clearing them is a free first step.
- Look outside under the window. A continuous bead of caulk along the bottom of the window or trim, or siding sealed tight to the sill, may be blocking drainage.
- Look above the window. Missing head flashing or a gap at the top trim can send water down inside the wall to the sill, where it shows up. That is a head-of-window problem, not a sill problem.
- Check the inside corners. Stains spreading from the bottom corners of the window are typical of water running off a pan with no end dams.
A controlled water test, wetting one part of the window and wall at a time from the bottom up, can confirm which part of the opening is letting water in.
What the Fix Involves
If the window unit is sound and the weeps are clear, the lasting fix is to correct the opening. That usually means removing the window and the trim around it, installing a proper sill pan lapped over the water barrier below, re-integrating the house wrap and flashing in shingle fashion, and reinstalling the window with the bottom left free to drain. The wood under the sill is checked at the same time, and soft sheathing or framing is replaced before anything is closed up.
If the window is old, failing or due for replacement anyway, it rarely makes sense to pull it, flash the opening and put the same window back. Replacing the window with a correctly flashed opening solves both problems in one visit. A proper window replacement treats the opening, the flashing and the siding around it as one job.
What does not work long term is more caulk inside or a bead of sealant along the outside bottom edge. The water is already in the opening; sealing it in only moves the damage somewhere you cannot see.
Why It Matters for the Wall
Water under a sill does not stay under the sill. It wets the sheathing and framing below the window and can travel down inside the wall. On homes with siding, the first outside signs are often swollen or soft siding below the window corners and paint peeling at the bottom of the trim. If the siding around the window has already softened, our siding repair work can open the wall, replace damaged material and re-flash the opening in the same job.
The same principle applies anywhere water lands on a horizontal ledge in a wall. Our guide to the stone-to-siding transition explains how that ledge should be flashed.
Sources: 2024 International Residential Code sections R703.4 and R703.4.1; Building Science Corporation Information Sheet 302, "Pan Flashing for Exterior Wall Openings," prepared for the U.S. Department of Energy Building America program; ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights (scope).
Frequently Asked Questions
Why is water coming in under my window sill only when it rains hard?
Hard, wind-driven rain pushes water through small gaps in the window frame, trim and sealant. If the opening below the window has no working sill pan, or the bottom edge is caulked shut, that water has nowhere to drain except inward.
Is a sill pan required by code?
Under section R703.4.1 of the 2024 International Residential Code, window flashing follows the manufacturer’s instructions, and where those give no flashing details, pan flashing is required at the sill. Austin enforces the 2024 IRC.
Can I just caulk under the window to stop the leak?
Usually not. Sealing the bottom edge blocks the drainage path the opening relies on, which traps water inside the wall. Caulk belongs at the head and sides; the sill needs to drain.
Do I need a new window to fix a sill leak?
Not always. If the window is in good shape, it can be removed, the opening flashed with a proper sill pan, and the window reinstalled. If the window is old or failing, replacing it with a correctly flashed opening is often the better value.
How do I know if the wall under the window is damaged?
Soft or swollen siding below the window corners, peeling paint at the bottom of the trim, stained drywall and musty smells are common signs. The only way to know the extent is to open the wall at that spot.
