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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

1 Answer from a MyBuilder Window & Door Fitter

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

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