Ask a tradesman


Thickness of silcone sealant under between plastic external window sill and original window sill.

We have just had new windows fitted. They have fitted a lipped window sill externally (previously it was flat) over a brick window sill. This gap has now been filled with over 2 inch thick silicone sealant - they say to prevent water ingress. Is this the norm or just used to cover up holes not filled prior to fitting the sill and because difficult to fill as a small gap

2 Answers from MyBuilder Window & Door Fitters

Best Answer


Personally I wouldn't seal any gap with silicone bigger than 7-10 mm.

The gap under the window should have been pointed with sand and cement. First I would question why there is a 2 inch gap to fill. Was the window measured correctly?

Hope this helps.



Answered 15th Jun 2014

I'm assuming the reason for the 20mm gap is because the "brick sill" is slanted?
If this is the case, then a neat strip of facia trim could cover the unsightly "mastic" and finish the job correctly.

And I agree with Grant, anything over 10mm should be questioned.
I hope this helps



Answered 27th May 2020

Post your job to find high quality tradesmen and get free quotes

Can’t find an answer? Ask a new question

Question Categories