Is lead flashing under a PVC window OK?

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We are having a small ground floor pitched roof extension built at rear of house. Drawings show the top line of new roof tiles running immediately underneath the cill of a replacement uPVC bedroom window (which is on order), with lead flashing above the tiles to run along the cill line (to be cut in below the cill, I presume).

The builder now says he can't take the roof up to that level as it's not done to flash into window openings as "you can never get a good seal around a window". He's saying he will have to lower the roof (height, not pitch) in order to leadflash at a couple of courses below the window opening. This will change the external look and also have a knock-on effect for internal ceiling height.

Is he right? Any comments or advice appreciated. Thanks.

PS. We can't change the existing bedroom window dimensions, say to higher it above the new roof line.
 
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He may be correct in that he can never get a good seal around a window.

But many others could

He should be able to insert lead under the cill and seal it well, and also fit lead into the window reveals.

It may mean a small section of the roof below the window is adapted into a flat area where the window effectively cuts into the roof.

Don't change the roof height if its a problem. There is no need to
 
You cannot have the top course of tiles tight under the window cill. The flashing requires an upstand of minimum of 100 mm .

Flashing under the cill of window that already is installed can be done, but it is better if it is a course or two below the cill.

In your case you say the window is on order which is ideal as the new window will sit on a lead tray fitted before the window is fitted.

I'm sure cavity trays are being fitted where need?

To change the heights and cock up ceilings is mickey mouse.. small flat aera below cill to give correct upstand will be the way to go.. as previously said
 
Thanks woody and data. Your speedy info is much appreciated. Both your replies make complete sense and are very helpful.

I have a sinking feeling I may be back with more questions like that - so look out for me! :(
 
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Hi. This the lead type of lead tray i would use to combat your situation. However the one pictured is for use on vertical cladding you would need a longer lap over slates.
 
Thanks justlead. Seeing the picture makes it even clearer for me to understand what could be done. Thanks again to all for your time and trouble.
 

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