Water Leaking Around a Window — Causes

The most common cause. External silicone between frame and wall shrinks and cracks over 8–12 years. Water follows the crack, enters the cavity and appears on the internal sill. Fix: rake out old silicone, apply new low-modulus (LM) silicone. £30 in materials, one afternoon.

uPVC frames have drainage slots at the base that let any moisture inside the frame escape outward. If those slots block with debris or paint, water backs up and enters the room. Fix: clear the slots with a thin blade, flush with water, check drainage runs freely.

Rare but possible on windows over 20 years old. Water tracks down the outside of the glass, past a failed bead gasket, and inside. Fix: professional bead removal and re-gasketing.

If the external cill has cracked, or if the pointing in the brickwork above the window has failed, water enters the wall above and tracks down to appear near the window — which looks like a window leak but isn't. Fix: repoint the affected area first and re-test.

On cavity-wall properties, the cavity tray above the window guides moisture out through weep vents. If the tray has failed (missing on older installations) water pools inside the wall and exits at the window opening. This is a serious repair — usually requires opening up the brick work above.

Wait for the next heavy rain and watch where the water enters. Trace it back with a torch from inside during the rain. Most leaks show themselves clearly if you observe them mid-storm. Don't pay for a repair based on assumption — a 20-minute observation saves hundreds.