Sealing uPVC windows

Apr 09, 2026 Last reply: 3 months ago 5 Replies

In the storms earlier this year, we had some water ingress from all three upstairs windows. They're uPVC and there's no sign of any previous damage, though here in the south west the storms were exceptional.



It seems that some sealant had worked free in all three windows in the same lower corner. Of course, I replaced this. A neighbour mentioned something about mortar which I didn't quite catch, but since then I've started noticing that many uPVC window round here seem to have a fillet of mortar round the edges, presumably covering silicone sealant.



I've never noticed this before but haven't lived here very long and never had uPVC windows before.



Is it normal?


Trowel Mastic? Not normal for window installers, but I've used it.

That may be it thanks. Never heard of it, though the stuff I see here is grey. Seems like it might be more secure than silicone.

What did you use it for?

Sealing around uPVC windows (after removing silicone sealant), and repointing Accrington Red bricks laid with tiny amount of lime mortar.

It doesn't adhere to PVC.

I've actually uses decorators caulk - acrylic - because it is paintable, Silicone isn't

Once painted it holds up pretty well

The problem could be the wrong silicone has been used. Low modulus silicone is advised for glazing owing to the movement of the uPVC in comparison to the opening the window is set in. I would scrape out what is there and replace with the more flexible low modulus silicone.

Richard

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required