overhang on a flat roof

Hi Maybe a silly question. Flat roof - will overhang the outskin by approx 32cm. Of that - 20cm is a gully ( epdm lined ). All works a treat on paper :). My question - do the joists need to finish at the start of the cavity or can they extend into the cavity as long as they don’t touch...

Where putting on our flat roof incorporating two skylights, size approx. 5x3m. 1st question: along the sides do we need to have an overhang to incorporate a soffit, or put the ladder the wall, then clamp a timber on the outside brickwork tight and overboard with our PVC facia. Just have a soffit...