Place the plywood roof sheathing panels with the face grain perpendicular to the roof rafters or trusses. The panel end joints are staggered for better tying traction and spaced at least 1/8 inch (2-3 mm) to prevent expansion buckling. Plywood joints between rafters are supported by blocking nailed between the rafters under the joints, or H clips attached to the plywood joints that run at right angles to the rafter. This prevents damage to the roof covering. Wood and asphalt shingles require 9.5 mm or 12.5 mm plywood thicknesses when the rafter spacing is 400 mm or 600 mm. Slate, tile and mineral fibre shingles require 12.5 mm or 15.5 mm plywood thicknesses when the rafter spacing is 400 mm or 600 mm. 19 mm by 38 mm nailing strips are placed with their centres under the butt end of each shingle when wood shingles or shakes are used on plywood sheathing less than 12.5 mm thick. Nailing strips also reduce condensation build up in the roof by providing ventilation under the wood shingles.