DIY Doctor

Leaking UPVC window causing MDF to blow?

Postby mark.johnson3000 » Fri Nov 20, 2020 4:46 pm

We have been in our house for 15 years and I noticed the mdf window sill in one of our dormers has expanded in two areas. I dug out the plaster and repaired this on the inside LH corner two weeks ago and have re-sealed the frame outside but water must be getting in elsewhere as the mid point of the sill is also damp.

I have just chiselled the mdf sill away and its damp in these two areas (see dark patches in images). I have checked outside (light was fading) and the lead flashing under the sill and sealant all looks to be intact. I have also checked the drainage channel on the opening window which works ok. Could water be getting trapped in the fixed pane and not draining externally? Any ideas on the point of ingress or solution for repair would be gratefully received!
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Simply Build It

Postby stoneyboy » Sat Nov 21, 2020 9:36 pm

Hi mark.johnson3000
There could be two issues, the dormer cheeks have a defective DPM which is causing damp in the corners. The lead flashing continues under the frame and into the area under the cill with either rain ingress or damp formation.
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