Water Stains at Sill Only After Driving Rain Post-Install: Decision Tree

Why this matters

Interior water staining at a window sill that only appears after wind-driven rain, and only since the new window went in, is a drainage and flashing failure, not a glass failure. Insulating glass and weatherstrip handle the sash; the sill pan, the flashing laps, and the weep system handle the water that gets past the sash, which is expected to happen in a driving rain. A correctly installed window leaks water at the sash all the time in a storm and drains it harmlessly to the exterior; a window with a missing or back-pitched sill pan, a reversed flashing lap, or blocked weeps collects that same water and dumps it onto the framing and into the room. This tree separates an exterior-envelope failure above the window from the window's own drainage, so the repair stops the water instead of re-caulking a symptom.

Symptom presentation

The homeowner reports a stain, swelling, or paint blister at the interior stool/apron or the wall just below the window, appearing only after a hard, wind-driven rain (often from a particular direction), with no leak in a calm rain. It is recent, following the replacement. The stain may be at one corner, across the full sill, or running down the wall below, and each pattern points to a different entry path.

Quick checks

Confirm where the water lands and where it comes from before opening anything.

Map the stain. Centered on the sill across the width suggests the sill pan/under-window drainage. Concentrated at one corner suggests a jamb-to-sill flashing lap at that corner. Below the window on the wall, not at the sill, suggests water entering above (head flashing, or a path higher up) and traveling down behind the wall to daylight below the window.

Check the weeps. On the exterior, confirm the weep slots in the sill are open and draining. A garden-hose trickle at the exterior sill should exit the weeps, not back up. Painted-over or caulked-shut weeps are a frequent install error that traps storm water inside the frame.

Check the exterior sealant and the flashing flange. Look for caulk applied across the bottom of the window or across the weep path, which dams water inside; and look at how the siding/trim laps the flange. The bottom flange should drain over the WRB/flashing, never be sealed solid.

Do a controlled water test. Per ASTM E2112/E1105 method, run water low to high: spray the sill first, then the jambs, then the head, watching the interior for the first appearance of water. Whatever zone produces the leak is the failure location; testing bottom-up prevents a head leak from masquerading as a sill leak.

Isolation tree

Branch on the water-test result and the stain map.

Leak appears when only the sill/exterior sill is sprayed. Sill-pan/drainage failure. Either there is no sill pan, the pan is back-pitched (slopes toward the interior), the pan corners/end dams are not sealed, or the weeps are blocked so water collects and overtops inward. Pull the interior stool/apron to inspect the pan and the rough-sill seal.

Leak appears when a jamb is sprayed, concentrated at the lower corner. Jamb-to-sill flashing lap failure. The jamb flashing must lap over the sill pan/pan flange (shingle-style); if it is tucked under or unsealed at the corner, wind-driven water at the jamb runs to the corner and inboard. Inspect the corner lap with the casing off.

Leak appears only when the head is sprayed, water shows at the sill below. Head-flashing failure feeding down behind the wall. Missing head flashing/drip cap, or a head flange sealed solid with no drainage, lets water in at the head; it runs down the back of the WRB and exits at the sill, staining there. The stain location lies; the entry is the head.

Leak tracks to the wall below the window, not the sill itself. Water entering above the window (head, or a building element higher up) traveling to daylight below. Verify by spraying progressively higher and watching for the leak to stop when you stop above the suspect element.

Weeps confirmed blocked, leak at sill center. Storm water that correctly entered the sash chamber cannot drain out, so it backs up and overtops the interior leg of the sill. Clearing the weeps and re-testing should resolve it.

Confirming diagnosis

Prove the path before you tear in.

Sill pan: with the stool removed, check the pan slope with a level (it must drain to the exterior) and inspect the back dam and sealed corners. Water poured into the pan should exit the weeps, not pool against the interior leg.

Jamb corner: with the casing off, the unsealed or under-lapped flashing corner is visible, and the localized water test reproduces the corner leak.

Head: spraying the head reproduces the leak while spraying the sill and jambs does not; pulling the head trim shows missing or sealed-solid head flashing. Blocked weeps confirm when clearing them and repeating the sill water test eliminates the leak.

Remediation

Sill pan/drainage: install or correct a sloped, end-dammed sill pan with sealed corners and a back dam, set the window in continuous sealant on the pan's back and side legs only (never seal the front/weep path), and confirm positive drainage to the exterior weeps. Re-seal the interior air seal at the rough opening per ASTM E2112.

Jamb flashing: re-lap the jamb flashing shingle-style over the sill pan flange and under the head flashing, seal the corners, and integrate with the water-resistive barrier so every lap sheds downhill and outboard.

Head flashing: install a proper head flashing/drip cap that laps over the window flange and behind the WRB above, leaving the head able to drain; remove any solid bead that dammed the head drainage.

Weeps: clear all weep slots and remove any caulk that bridges them; verify the chamber daylights to the exterior.

Re-test after any repair with the same bottom-up water test and confirm the interior stays dry before closing the wall. Replace water-damaged framing/finish only after the path is proven dry.

References

• ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights. Sill-pan, flashing-lap sequence, end-dam, and weep/drainage requirements.
• ASTM E1105, Field Determination of Water Penetration of Installed Windows, Skylights, Doors, and Curtain Walls by Uniform or Cyclic Static Air Pressure Difference (field water-test method).
• AAMA 511, Voluntary Guideline for Forensic Water Penetration Testing of Fenestration Products (diagnosing installed-window leaks).
• IRC R703 and R609, exterior wall water-resistive barrier, flashing integration, and window-installation requirements.
• Manufacturer installation instructions (Andersen, Marvin, Pella, Milgard, Simonton): required sill-pan/flashing method and which legs may be sealed without blocking drainage.