Bakersdozen

Hi Adsibob You are correct in thinking fibre glass is incredible strong if used the right way - after all the build yachts with the stuff. However, it doesn't like sharp angles, you need to add some fillets so the fibreglass mat can flow in a supported curve. As for the decking, yes plywood needs a small expansion gap that needs reinforcing with embedded fibreglass tape - think of the scrim tape plasterer's use on plasterboard. T & G also moves but the tongue protects the gap rather than the tape. Marine ply isn't waterproof only the glue is, hence the need for varnish or an epoxy coating in boat building. The edges especially need treating to keep out moisture. Why did your architect choose that over structural ply ( EN314-3 - Class 3 for anyone who is interested)? As you need to take the roof off anyway to tape it can you replace the ply with OSB 3 T&G, add fillets in the correct places, and use the ply somewhere else- or sell it on Gumtree?