Wall plate on a flat roof

You can do either but if you use a wall plate you can fix the joists more easily which in term makes them more sturdy.

Ideally you would install a wall plate fixed down to the block/brick work it would provide a good base for the joists to be installed and fixed

You don't need a wall plate for a flat roof

Wall plate not required on Flat roof, roof gets fixed down with steel straps

a timber frame sat on top of brickwork then tied to inside wall of new extension using steel wall ties would be the preferred method this keeps the trusses lower and looks neater when fascia and flat roof trim are added to complete the project hope this helps!