Flat roofs can help where height is tight
A well-designed warm roof can keep the external profile low while still providing insulation. Outlets, upstands and rooflight kerbs then become critical details.
Pitched roofs need enough fall and headroom
Low roof pitches can restrict tile choice and may bring the ridge into a first-floor window. Draw the full section rather than choosing roof covering after the walls are built.
The wall junction is usually more vulnerable than the middle
Lead flashings, cavity trays, upstands and abutments have to stop water where the new roof meets old masonry. A large sound roof surface does not compensate for a poor junction.