Water leaking from the bottom of a window frame? What to check

Perished silicone around the frame

Every uPVC window is sealed to the surrounding wall with a bead of silicone, both to keep water out and to allow slight movement without cracking. Silicone has a working life, typically shrinking, cracking or pulling away from the frame after some years of UV exposure and weather. Once that seal splits even slightly, wind-driven rain can track behind it and appear inside on the wall or sill well away from the actual gap.

Failed drainage slots

uPVC window frames are designed to let a small amount of water in deliberately, then drain it straight back out through small slots along the bottom of the outer frame. That is normal and by design. Problems start when those slots get blocked with dirt, insect nests, or paint from a previous redecoration, because then water that should have drained away backs up inside the frame and eventually finds its way over the sill and into the room instead.

• Check the underside of the outer frame for small horizontal slots
• Clear visible debris gently with a thin tool, without forcing anything into the frame cavity
• Watch during the next rain to see if water now drains outside instead of pooling

Cill and pointing

Outside the window, the cill and the pointing where brickwork meets the frame do a lot of the actual weatherproofing work. A cracked or poorly pitched cill can let water sit and soak inward rather than shedding it away from the wall, and pointing that has crumbled or was never sealed properly against the frame gives water a direct path behind it. This is particularly common on older properties where a window has been replaced but the surrounding brickwork detail was not fully made good.

Condensation mistaken for a leak

Not every wet patch near a window is rainwater. Heavy condensation running down the inside of the glass can pool on the sill and soak into surrounding woodwork or plaster, looking very much like a leak days later once it has caused staining. The giveaway is timing: condensation appears on cold, still mornings regardless of whether it has rained, while a genuine leak tracks with actual weather.

How to trace it

Watch the window during heavy rain if you can do so safely, and note exactly where water first appears, since the entry point outside is often higher or to one side of where it shows up inside. Feel around the external silicone line for gaps or softness. Check the cill outside for cracks or a flat rather than sloped profile. If none of that is obvious, it is worth getting it looked at properly, because tracing a leak from the inside alone can be misleading.

How window drainage works

uPVC windows are designed to let a little water into the bottom of the frame and drain it straight back out through small slots on the outside face. If those slots fill with dirt or insects, water backs up and spills over the inside edge, which looks like a leak but is really a blockage.

A thin wire or pipe cleaner pushed gently through each slot from outside usually clears them.

How to fix rain leaking between window and frame

If water appears at the joint between the window frame and the wall, the external silicone has usually cracked or pulled away. Cutting out the old bead and applying fresh exterior-grade silicone to a clean, dry surface often cures it. Water appearing between the panes or with misting inside the unit is a failed sealed unit, which needs replacing rather than resealing.