Abstract: A building sheet suitable for roofing and siding provides a weatherproof lap joint having an antisiphon gap when installed partially overlapping an adjacent like building sheet by spacing the entire overlapping portion of one sheet above the lapped portion of an adjacent sheet. Parallel strengthening ribs formed into the building sheet are structurally modified along one edge to form an overlapping member and along the opposite edge to form a lapped member. The overlapping member includes an intermediate strengthening bead and an exterior downwardly projecting sealing lip which rests upon the lapped member of an adjacent sheet and spaces the entire overlapping member of one sheet above the lapped member of the adjacent sheet.