Why does water collect in the bottom of my window frame?

A uPVC window is designed to let a small amount of rain past the outer gasket and drain it away. When the drains block, that water backs up, comes over the inner gasket and appears on the window board — which people reasonably interpret as a leak.

What you'll need

• A stiff wire or a cable tie
• A vacuum with a crevice tool
• A cup of water
• A small screwdriver

Step by step

1. Look on the outside, at the bottom.Two slots, sometimes three on a wide window, cut through the front face of the outer frame near each end. They are often behind small plastic covers or hoods.
2. Check the sash as well.Opening lights have their own drainage in the bottom of the sash, draining into the frame channel below. Both sets have to be clear.
3. Clear them with a wire.A cable tie or a piece of stiff wire pushed through and worked about. Do not use anything that could scratch or enlarge the slot.
4. Vacuum the internal channel.Open the window and vacuum the rebate the sash closes into. Grit, dead insects and pollen collect there and wash into the drains.
5. Test with water.Pour a cup of water into the bottom channel of the open frame. It should run out of the external slots within a few seconds. If it sits there, the drain is still blocked or the frame is not falling correctly.
6. Check the hoods are the right way up.The external covers have a small flap that lets water out and stops wind driving rain back in. Fitted upside down, they hold water in the frame.
7. Look for paint over the slots.Windows that have been sprayed, or where render has been applied around the frame, frequently have the drains sealed by the coating.
8. Check the external cill.Water should run off the cill and clear of the wall. A cill with no fall, or a blocked drip groove underneath it, keeps water against the frame permanently.
9. Rule out the gasket only after this.If the drains flow freely and water still appears inside, then investigate the gaskets and the glazing seal. In practice the drains are the cause far more often.
10. Put it on the annual list.Two minutes per window, twice a year. It prevents a repeated leak and prolongs the life of the sealed units.

Tips

• Do this in autumn after leaf fall and again in spring after pollen. Those are when the slots block.
• If the cill inside is stained but the window has never leaked in a storm, condensation is a more likely explanation than drainage. Check which weather produces the water.
• Never seal a drain slot to stop water coming in. It has the opposite effect and it is the single most common well-intentioned mistake with windows.

Common mistakes

• Sealing the drainage slots with silicone — The slots are the outlet for water that is supposed to get into the frame. Blocking them means the chamber fills, and the water then finds its way over the internal gasket and onto the window board — a much worse leak than before.
• Assuming water inside means a failed seal — Failed gaskets, blocked drains and condensation all produce water on the internal cill. Replacing gaskets when the drains were blocked achieves nothing and costs an afternoon.

If it doesn't work

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