Short answer
In a mechanically fastened PVC roof the membrane is fixed through the insulation into the deck along the overlaps, and the next sheet is hot-air welded over the fasteners. Fastener density is calculated for wind uplift and increases at edges and corners.
Fixing in the laps
Fasteners with washers are placed in the lap of each sheet; the next sheet covers them and is welded. With standard overlap fixing on steel decks the membrane runs across the deck ribs (NSAI 06/0262).
Wind zones
Suction is highest at corners, then edges, then the field. The wind uplift calculation sets fastener spacing per zone — uniform spacing across the whole roof is a common error.
Fastener capacity
SINTEF TG 2010 lists design capacities of 627–912 N per fastener for Protan SE 1.2 depending on the fastening system and substrate, with a partial factor γm = 1.5. On weak substrates the fastener-to-deck connection can govern, so pull-out tests are done on refurbishment.
Seams
Laps ≥ 120 mm side and ≥ 80 mm end; finished hand weld 20–40 mm. Every seam is probed; machine welding is peel-tested (NSAI: every 200 m).
See Protan membranes compared or the parapet detail.