Hi Most of the kits you buy have either 6, 4, 2, 1, mm ply. 6 mm for keel, frames, 4mm for super structure, 2mm decks, 1mm for curved parts. Thickness will depend on size of boat the ones I mentioned will be OK for up to 1000mm. Birchply won't warp good for cutting. I hope this help. Ps 3/8 ply about 9mm.