Hi - you seem knowledgable. I'm in the process of having 2 large (cold) dormers built. They will be GRP, tile hung cheeks and fronts.
Builders have used OSB3 instead of ply on the spec, loads of gaps between OSB - minimum 4mm gaps, some larger. They said gaps are because they got OSB from 2 places and had to cut the tongue off some to fit.
Should they double board the OSB - overlapping the gaps.