How to Fix a Window Leaking During Rain: Flashing, Caulk, and Drainage Diagnosis (2026)
A window that leaks during rain has failed caulk, missing or damaged flashing, or a blocked weep hole. This guide covers finding the leak entry point, re-caulking, and when the problem requires flashing repair.
Window leaking during rain: (1) First, look from inside during rain — where is the water appearing? Dripping from the top or sides of the frame = exterior seal failure. Pooling on the sill and seeping in = weep holes blocked. (2) For caulk failure: remove all old caulk from the exterior frame perimeter, apply fresh siliconized acrylic caulk (not 100% silicone), leave the sill corners open for drainage. (3) For blocked weep holes: clear the small slots at the bottom of the exterior window frame with a toothpick or compressed air. (4) If the leak comes from above the window (ceiling or top of the frame): the flashing above the window has failed — this is beyond caulk.
Frequently Asked Questions
How do I find exactly where a window is leaking?
Have someone outside spray the window with a garden hose while you watch inside. Start low (below the window) and spray upward. When water begins to appear inside, the spray is near the entry point. Common locations: (1) Corner of the window frame where the horizontal sill meets the vertical jamb — caulk at these corners cracks and separates. (2) Top of the window at the head — water running down the wall from above enters here if the head flashing or trim caulk has failed. (3) Weep holes at the sill — blocked weep holes trap water in the sill channel until it overflows inward. (4) Interior condensation vs leak: is the water on the glass or frame inside? If it only appears on the glass surface: that is condensation from humidity, not a leak.
What is window flashing and how do I know if it has failed?
Flashing is a thin metal or membrane material installed above and beside the window opening during construction to direct water away from the wall cavity. Head flashing (above the window) channels water out over the window trim. Side flashing (behind the siding at the jambs) prevents water from entering between the window frame and the rough opening. Signs of flashing failure: (1) Water entry at the top of the window despite fresh exterior caulk — the head flashing is missing or damaged. (2) Water staining on the interior wall below the window — water is running down the wall behind the siding. (3) Swollen or rotting interior drywall below the window — long-term water entry through failed flashing. Flashing repair requires partial siding removal and is a more involved DIY or contractor job.
Why should I leave the bottom corners of the window open when caulking?
Vinyl and aluminum window frames have weep holes — small slots or holes at the bottom exterior of the frame that let water drain out of the internal sill channel. If you caulk the bottom of the window frame, you block these weep holes. Water that enters the sill channel (from rain blowing under the sash) has nowhere to drain and eventually seeps into the house. Always leave the sill corners open — or leave small gaps in the caulk bead at the weep hole locations. The correct caulk application is: seal the top of the frame, seal the sides, leave the bottom corners open.
My window leaks only during wind-driven rain. Is that a different problem?
Wind-driven rain leaks are often different from gravity leaks: (1) Sash seal failure — the weatherstripping between the sash and the frame compresses under wind pressure, allowing water in. Inspect the weatherstripping around the sash perimeter — if it is cracked, flattened, or missing: replace it. (2) The weep holes are working in reverse — high wind pressure inside the sill channel (from air entering elsewhere) can push water back in through the weep holes. Install weep hole covers (small flap covers that allow outflow but prevent inflow under wind pressure). (3) The gap at the meeting rail — double-hung windows can flex slightly in high wind at the point where the upper and lower sash overlap.
There is rotted wood around my window frame from a long-term leak. How do I fix that?
Rotted window frame wood requires the same treatment as any wood rot: remove all soft material with a chisel, treat with liquid wood consolidant (Minwax Wood Hardener), let cure 24 hours, fill with two-part epoxy wood filler. Prime with oil-based primer and paint. Address the leak source first — there is no point repairing the rot until the water entry is stopped. After repair: seal all surfaces with primer and maintain paint and caulk to prevent recurrence. Extensive window frame rot (rot has spread to the structural rough opening framing, not just the window casing) requires removing the window, rebuilding the framing, and reinstalling — this is a contractor job.
Window leaking during rain: (1) First, look from inside during rain — where is the water appearing? Dripping from the top or sides of the frame = exterior seal failure.
Spray test with a garden hose starting from below the window — add height until water appears, and that locates the entry.
What you need
- Siliconized acrylic latex caulk
- Caulk gun
- Caulk removal tool
- Painter’s tape
- Toothpick or compressed air (for weep holes)
- Garden hose (for spray testing)
Step 1: Spray test to locate the entry
Have someone spray the exterior of the window with a garden hose while you watch inside. Start below and work up. When water appears inside, the spray is near the entry.
Step 2: Clear weep holes
Inspect the sill at the exterior bottom of the window frame. Look for small slots or holes — these are the weep holes. Clear any blockage with a toothpick or shot of compressed air. If weep holes are clear but the sill is still leaking: the issue is the caulk or flashing above.
Step 3: Remove and replace failed caulk
Scrape out all old caulk from the exterior window frame perimeter with a plastic caulk removal tool. Clean the surface, let dry. Apply fresh siliconized acrylic caulk: seal the head, both jambs, and the sill — but leave small gaps at the weep holes at the sill corners.
Smooth with a wet finger. Let cure 24 hours.
Step 4: Inspect the weatherstripping
Open the window and inspect the sash weatherstripping. Cracked or compressed weatherstrip: peel off the old weatherstrip, clean the channel, press new adhesive-backed weatherstrip in. Available at hardware stores in rolls.
Step 5: Test after repairs
Wait for a rain event or repeat the hose test. No water entry = repair successful. Still leaking above the window: head flashing may need evaluation.
Related guides
- How to Caulk Around Windows — full exterior caulking guide
- How to Seal Drafty Windows — weatherstripping and air sealing
- How to Fix Wood Rot — repairing rot around the window frame
- Spray test to locate entry
On a dry day, use a garden hose to simulate rain — have a helper watch inside while you spray the window zone in sections. Start at the bottom of the window and work up: sill first, then sides, then head (top). Mark with painter's tape where water first appears inside. This determines whether the leak is from failed caulk, blocked weep holes, damaged flashing, or the window frame itself.
- Clear the weep holes
Vinyl and aluminum windows have small weep holes at the bottom rail of the outer frame — these drain water that gets into the frame channel. Blocked weep holes cause water to back up and overflow into the interior. Use a thin tool (a bent wire, a toothpick, or compressed air) to clear each weep hole. Run water on the outside while watching for drainage from the weep holes — water should exit freely.
- Remove and replace failed caulk
Cut out all existing caulk from the joint between the window frame and the exterior wall cladding (siding, stucco, brick) using a utility knife or oscillating multi-tool. Clean the joint with a brush and remove all debris. Apply foam backer rod in joints wider than 3/8 inch. Apply a continuous bead of silicone or paintable polyurethane exterior caulk — no gaps. Tool smooth. Do not caulk the bottom sill weep holes closed. Allow to cure 24 hours before testing.
- Inspect the head flashing
The head flashing is a metal or vinyl piece above the window that directs water away from the frame. If it is missing, bent upward, or not lapped over the water-resistant barrier (house wrap), water runs directly behind the cladding and into the window frame. Examine the flashing by removing a section of siding above the window if accessible. Bent flashing can often be straightened and re-sealed; missing flashing requires installation of new aluminum or vinyl head flashing slid behind the siding and over the window casing.
- Test after repairs
After caulk has cured, repeat the hose test from Step 1. Start at the bottom and work up zone by zone, holding the hose on each area for 2 minutes. A properly sealed window should show no interior moisture even when the spray is directed at the head and sides. If water still enters after completing caulk, weep hole, and flashing repairs, the problem may be inside the wall — water-resistant barrier damage — or the window unit itself needs replacement.
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