The invention concerns a rough timber boarding comprising a body made of insulating material, for example expanded polystyrene, delimited by an upper surface (12), a lower surface (14), and two opposite end edges (16, 18) and wherein the upper surface extends through two support edges (20, 22) which extend mutually in parallel and between the two end edges to form each a support rebate designed to be nested on a fixed support, in particular on a beam heel. The upper surface (12) delimits ribs (48) which extend in a direction globally parallel to the end edges (16 and 18), thereby enabling to reduce the minimum thickness of a concrete slab subsequently cast on the rough timber boarding and to reinforce the support. The invention is useful for constructing concrete floors in the building sector.