osb3

I've recently installed a floating floor over a concrete slab in a garden workshop. Due to height concerns (door threshold/plug sockets) I went for 25mm PIR under 18mm T&G OSB (with DPM and slip layers). I've got a lot more floor deflection than expected, would 3.6mm plywood laid perpendicular...

I need to replace Garage Roof, am going for EPDM, the existing doof boards are shot. Looking at OSB3 for the roof, but never done this before, whats the best way to fit this? Should I go for the tongue and groove type and glue joins with PVA? OR Should I just cut OSB3 to fit the joists...