When a roof needs a whole new deck
Three situations call for sheathing a roof from scratch. New construction and additions start with bare rafters or trusses. A re-roof on an older house sometimes uncovers board decking that cannot take a modern roof. And a deck soaked through by years of condensation can be too far gone to patch, so it is replaced while the roof is open anyway. A few soft sheets found during a tear-off are a different job, roof decking repair, which is priced very differently since the shingles above have to be replaced with it.
This estimate assumes the framing is exposed and ready. It carries the panels, 8d ring-shank nails to fasten them and drip edge for the roof edges, plus about 16 person-hours for checking the rafters, staging and cutting the sheets, nailing them down and fitting the edge metal. Sistering a cracked rafter or two is inside those hours. Tearing off the old roof is not (that is roof removal), and neither is stripping old boards, nor the underlayment and shingles that go on top.
A 4 by 8 sheet covers 32 square feet, and the order includes a waste allowance because sheets are trimmed at the ridge, hips and valleys and the joints are staggered from row to row. A plain gable wastes little; a hip roof with dormers wastes more. The roof sheathing calculator counts sheets for your roof area and chosen thickness.
OSB or plywood
Both are rated structural panels, and building codes and shingle makers accept either under asphalt. OSB (oriented strand board) is cheaper and more uniform from sheet to sheet, which is why most new houses get it. Plywood costs more but copes better with getting wet: it dries out faster and is less prone to swelling at the edges, where OSB can raise a ridge that telegraphs through thin shingles.
So the choice comes down to exposure. If the deck will sit open to weather for a while, as on an addition framed late in the year, or the attic has a history of condensation, plywood is the safer buy. Where the deck is dried in with underlayment the day it goes down, OSB does the same job for less. The panel line here covers either, which is part of why its range is wide.
Thickness, spacing and H-clips
Panel thickness follows the framing. With rafters at 16 inches on center, 7/16-inch OSB or 1/2-inch plywood is standard. At 24 inches, common with trusses, 5/8 inch is the safer choice, because a thinner panel flexes over that span and the shingles show it as waves. Each panel is stamped with a span rating, so check the stamp against your spacing rather than going by thickness alone. Tile and slate may need a thicker deck and a look at the framing before anything is ordered.
H-clips are small metal clips that slip over the long edges of two neighboring panels between rafters. They hold up the edge where it does not land on framing, so the two sheets cannot sag separately, and they set the small gap that lets panels swell in damp weather without buckling. The material list here does not include them. On 24-inch framing, ask whether they are in the quote.
Nailing is where a new deck is won or lost. Nails go closer together along the panel edges than across the middle, and every one has to hit a rafter. Nails that missed the framing show as shiny points in the attic, and they are worth pointing out before the underlayment hides the deck. In high-wind regions, some building departments inspect the deck nailing before anything covers it.
Old plank decking: cover it or strip it
Houses built before plywood was common often have decks of 1x6 or 1x8 boards. If the boards were laid tight and are still sound, asphalt can go back over them, though wide gaps, cupped boards and knot holes print through the shingles in time. If they were spaced apart, the way decks were built under wood shingles so the wood could dry, they cannot take asphalt as they are, because shingles need a continuous surface.
There are two fixes. The cheaper is to overlay the boards with 7/16-inch OSB or 1/2-inch plywood, nailed through to the rafters, which stiffens the deck and gives the shingles a smooth base. The extra thickness changes the edge details, so the drip edge and any skylight flashing have to be set for the new height. The other is to strip the boards and sheathe the rafters directly, which is the right call when boards are rotten, split along the nail lines, or too thin to hold a nail. Ask for board removal as its own line, since it is labor this estimate does not carry.