Follow-up question: how to attend to the detail where roof join?
The flat- roofed extension in question will be attached to a dormer bungalow. The joists for the flat roof will share the wall plate with the first floor joists of the dormer bungalow. The top one or two rows of tiles will need to be removed.
How to join the roofs? A neighbour (who has built his own house) suggested taking the flat boarding up to the existing rafters, then removing the bottom row of (large, interlocking conrete) tiles to be retained, continuing to board up the rafters for a foot or so, fibreglassing the lot, then putting the edge of the roof underdrawing back over the sloping bit of the glassed board, and replacing the bottom row of tiles. That is, no separate flashing material but continuous fibreglassed board. Does the team agree, or have alternatives?
Regards Richard