DIY Doctor

Bay window leak and plywood sill

Postby CornishJon » Mon Jun 08, 2020 7:30 pm

Hi all,

I'm new here and would appreciate any advice..

I have a bay window (this style might be termed bow window).

I recently found it had been leaking for some time and completely rotted the plywood underneath the plastic interior sill (no idea how long it had been going on for!). I removed it all, spent a very long time diagnosing the ingress and found the cause was poor sealing on the outside (I bought house 18 months ago - windows were installed 5 years ago).

Anyway, I pretty sure I've sorted the leaking by redoing all the exterior sealant... but i'm still nervous to put it all back (including new plywood) in case there is any additional small leak or condensation.

My question is A. Could I, as an added measure use waterproof expanding foam in the gap between under the window and fibreglass? Is that a stupid idea?

B. Can I use something other than plywood for the sill, something more waterproof? I was wondering if I could use insulating board? There is white plastic covering that goes on top it's just the board under that worries me with dampness/chance of rot.

Thanks in advance for any help
Sketch of window
Gap under window that goes back about 3 inches
Fibreglass frame where plywood rotted
Window exterior
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Simply Build It

Postby stoneyboy » Tue Jun 09, 2020 9:19 pm

Hi cornishjohn
Buy some white upvc faced board and use this for the cills. You may have to use facia board which is thicker. Bed this on foam with packers and weights on top while the foam sets.
Regards S
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