Sealing the Gap Between Window and Sill?

21 Aug 2023
Reaction score
United Kingdom
Water is getting into our conservatory and has ruined part of our windowsill. The water pools in one corner.

Somebody advised me to apply CT1 to seal the gap between our conservatory windows and the sill on the outside to stop the leak. It didn’t work, and now somebody else has said that I should remove the CT1 as this gap is in fact for drainage.

I can’t figure out who is right and if this gap should be sealed or not.

To be sure, the windows I sealed don’t open—I know about those little holes beside the hinges of windows that do open and how they allow water to drain out of the frame.

I can’t see why these windows that don’t open would need the gap, but that doesn’t mean they don’t.

We are already getting the polycarbonate roof replaced, so if the source is the roof that might solve the problem, but I don’t think so as I check when it rains and the vertical seam in this corner is always dry.

But even if we stop this particular leak I want to know if the gap has a function.

Thank you!


  • 64bf631c-90db-4a5a-8f02-7a5b21e6473b.jpeg
    148.2 KB · Views: 152
  • IMG_8061.jpeg
    318.7 KB · Views: 183
Sponsored Links
Do you have ' drainage caps ' stuck on the face of the bottom of the frames on the outside ? If not definetly get that CT1 out of that gap between the cill and frame , as what you can't see is the drainage holes in the base of the frame , they won't just be under the opening windows! . It would be very unlikely for water to be getting under the frame , tracking up the cill , over the rear ledge of the cill and onto the internal window cill anyway... don't see that is your issue.
Water pools at the bottom, so this could be coming in anywhere ... as a guess ( and its a guess we,ve only got pics of the cills ) , it will either be coming in a roofbar OR the eaves beam, look for any sealant peeling or gaps on the outside in that corner near the top
Thanks for your advice!

Here in a photo of the whole sill. I don’t see any drainage caps. I will try to remove the CT1 (it doesn’t look good anyway…. was great when I applied it and took that photo, then it caught a bunch of dirt as it dried).

We also think the source of the leak is in the polycarbonate roof/frame, which is going to be replaced with something insulated. Hopefully that will stop us getting so much water in.

But sill, if the gap was designed to be there I don’t want it blocked.


  • IMG_0082.jpeg
    412.6 KB · Views: 94
I scrapped the CT1 out of the gap all along our conservatory window sill… as soon as I cut it at the spot where the water pools it poured out from behind the seal, even though indoors the leak is dry at the moment. So clearly it was a mistake to seal the gap.


  • 0a71cfa3-175c-4cc7-8583-c67c0f749e39.jpeg
    185.2 KB · Views: 90
  • 03ab94ec-18a1-4252-b325-ce1265b8eb58.jpeg
    104.3 KB · Views: 96
  • 77224248-e926-4e22-bc65-31c3753c207f.jpeg
    135.6 KB · Views: 99
Sponsored Links
A few years ago, I watched someone fit uPVC windows. They filled the drainage gap where the sill meets the windows with expanding foam...
Picture 1 in your latest post.. so the water came out the bottom of the corner post? That could just have been all the water that's collected from where the hidden drainage couldn't drain. I'd still be looking at the top of that post, it's a hollow 70mmx70mm post with a upvc ' sleeve' and will not necessarily be sealed at the top , and if it was when rhe conservatory was constructed it may well of come off since.
As i said in my last post , look above the windows at the eaves beam and roof bars

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local