Roof Decking, Substrate Preparation, and Underlayment
Decking
Decking carries loads and holds fasteners, so it must be sound, dry, flat, and thick enough for the rafter spacing. Typical minimum panel thickness is 3/8 in. at 16 in. on center, 7/16 in. at 20 in., and 15/32 in. at 24 in. Fasten with 8d common or deck nails - about 6 in. on center along panel edges and 12 in. in the field - driven into framing, not just into the panel below. Leave roughly 1/8 in. between panels for expansion and support all edges. Wood shakes and shingles may use spaced 1x4 sheathing instead of solid panels, but the board spacing must match the exposure so fasteners land in wood.
Preparation
Replace soft, delaminated, or rotted sheathing; renail loose panels; correct sagging or wavy framing so water does not pond and shingles do not telegraph. Sweep the deck before membrane work; dust and moisture defeat adhesion.
Underlayment
Underlayment is the secondary water barrier: one layer of No. 15 felt (or equivalent synthetic) on steep slopes, but two layers - or one No. 30 - from 2:12 to 4:12. Side laps run about 2 in., end laps about 6 in., and laps must face downhill. Apply an ice-dam membrane along eaves where ice is likely, extending at least 24 in. inside the warm wall line, and in valleys. Synthetics are lighter and stronger but must follow the maker's lap and fastener pattern.
Traps
Nailing underlayment only to old shingles, skipping ice barrier at a valley, and laying felt over a wet deck are classic distractors.
Worked example
Rafters at 24 in. on center: specify 15/32 in. plywood or OSB, 8d nails at 6/12 in., panel gaps 1/8 in.