Upvc window bottom mechanical fixings through lead window tray?

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5 Mar 2009
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Kent
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United Kingdom
I’m putting in some triple glazed Upvc windows -
2.4 metres wide into our new dormer room -
Which is timber frame, sheathed in OSB, tyvek, battens and plastic cladding. My building surveyor says building control will expect a lead window tray , to create a flashing over the cladding under the cill. I’m worried if we mechanically fix the window at the bottom (Ie screw it down) as my window guy wants to do - the screws will perforate the lead tray which seems silly. The window guy says he’ll put a big blob of CT1 or sikafix where the screws will go through (where the handle on the opening panels will be to resist the thump and wobble on closure) so it can’t ever leak there.

I wonder if we can simply fix the window at the sides and through the header, so as to preserve the waterproofness of the lead tray. Anyone know?

I’ve asked the manufacturers of the window (formula one) who say they have no idea / recommendations on how the windows should be fixed / fitted !?!?! I’m guessing this must be a not too uncommon problem - how to mechanically fix a window without perforating weathering detail beneath it …
 
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