This is a very common issue, especially in windy, stormy climates. When rain is driven sideways by wind, it can find its way into places that normal vertical rain cannot. Here’s why this happens:
1. Wind Pressure Forces Water Into Gaps
Wind-driven rain doesn’t just fall down — it’s pushed horizontally against your home’s exterior.
- Even a tiny gap in the window frame, siding, or caulking can let water in.
- Pressure differences (high pressure outside, low pressure inside) can force water through pinhole gaps, screw holes, or weak weatherstripping.
2. Failed or Missing Caulking
- Over time, sealant around window frames cracks, shrinks, or peels due to sun exposure and temperature changes.
- When the wind blows rain sideways, it can enter through these cracks and run behind the siding or window frame, showing up as leaks around or under the window inside.
3. Poor Flashing or Missing Drip Edge
Flashing (the thin metal or vinyl barrier installed around windows) directs water away from the wall.
- If flashing is missing or improperly installed, wind-driven rain can be blown behind the siding and then back into the wall cavity.
- Similarly, a missing drip edge allows rain to cling to the underside of trim and seep in.
4. Design of the Window Itself
- Some older windows, like single-hung or aluminum sliders, have built-in weep holes to drain out rainwater. If these weep holes are clogged, water can back up and overflow inside.
- Casement windows (the kind that crank open) are particularly vulnerable if the seal or weatherstrip is worn, because wind pressure can push the sash slightly open on one side.
5. Wall or Frame Leaks, Not the Window
Sometimes what looks like a window leak is actually water coming from elsewhere:
- Wind-driven rain enters siding joints, roof flashing, or above the window, then runs down the sheathing and exits at the window edge.
- This is why professional inspectors often perform water intrusion tests — spraying specific spots to trace where water actually enters.
6. How to Prevent Sideways Rain Leaks
Here’s what helps:
- Reseal all perimeter caulking with high-quality exterior-grade silicone or polyurethane.
- Clear and test weep holes at the bottom of window frames.
- Inspect and repair flashing and trim above and below windows.
- Add or replace compressible weatherstripping on operable sashes.
- Check that drip caps are installed over the top casing to deflect water away.