What is the best way to get watertight seal between flat roof and house wall

Hi everyone I am getting a new flat roof done because my old one was leaking I am getting the falls changed and they were wrong and had the window change what is the best most water tight seal I can get between the flat roof and house wall is it led flashing or beading? I’m confused any help or advice would be great thank you, the window has been changed to a smaller one to allow for a better seal

 

Its simple really and the same for any roof covering.

The flat roof material is turned up vertically at wall abutments (a special trim maybe requied depending on material used).
Then a chase is cut in the wall and lead flashing is then dressed down over the flat roof upstand.
There is need fot any seal as such behind the lead.
Lead seal mastic can be used to seal the chase instead of mortar - there are special lead flashing clips to grip the lead in the chase, might be good -its important the lead gets in the chase deep enough

Pipe penetrations are tricky if close to the wall - mske it difficult to seal behind.

You may take advise on whether cavity trays are needed -not necessarily on rendered walls, but NHBC recommend it.

 

Ok thanks for the advice

 

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