OSB vs plywood roof sheathing

What the span rating on the stamp means

Every APA-rated sheet, OSB or plywood, carries a span rating such as 32/16. APA defines the left number as the maximum recommended centre-to-centre spacing of supports, in inches, when the sheet is used as roof sheathing; the right number is the same figure for subfloor. So a 32/16 panel can go on a roof framed at up to 32 inches, or a floor framed at up to 16. The rating is a performance number, and it is the same scale for both materials. The code does not ask which one you bought. It asks what the stamp says.

The IRC sends roof sheathing (§R803.2.2) to Table R503.2.1.1(1). That table gives each rating a minimum nominal thickness, a maximum span with and without edge support, and the live load the panel can carry at 16 and 24 inches on centre. The loads assume a 10 psf dead load. If the roof covering is heavier than that, the table says to reduce the live load to match, which is why a slate roof or a tile roof is a question for the framing and the deck, not only the roofer. Roof deflection is held to L/180 under total load and L/240 under live load, and the spans assume the panel runs continuous over two or more supports with its long dimension across the framing.

Read the live-load column against your snow. A 24/16 sheet on 24-inch trusses is good for 40 psf of live load; a 32/16 on the same trusses is good for 70. Where the roof snow load comes out above what the lighter panel carries, the thicker panel is not an upgrade. It is the minimum.

Roof sheathing thickness by rafter spacing

Turn the table round and it answers the question estimators actually get: what thickness of OSB or plywood does this roof need? On 24-inch trusses, the most common residential layout, the lightest panel that spans with no clips is 24/16 at 7/16 inch. Drop to 24/0 at 3/8 inch and the same spacing is only legal with edge support.

These are minimums, read from the code table. The shingle maker can ask for more (see below), and so can a local amendment. When you replace a few sheets during a re-roof, match what is there; when you re-deck, size to the framing and the snow.

H-clips and panel edge support

Panels go on with the long edge across the rafters, so the end joints break on framing and the long joint between courses crosses open bays. That unsupported edge is what the two span columns are about. The table defines edge support as lumber blocking, panel edge clips (one midway between each support, except two equally spaced between supports where the span is 48 inches), tongue-and-groove panel edges, or another approved type of edge support. On a roof deck that nearly always means H-clips.

By the table, 7/16 OSB rated 24/16 spans 24 inches without clips, and 3/8 rated 24/0 does not: it is limited to 20 inches unsupported. Go to 32-inch framing and a 32/16 sheet needs edge support (28 inches without), while 40/20 spans it bare. Many crews clip every roof anyway, because the clip also holds the gap between courses and stops a soft edge showing through the shingles. Whether your inspector expects them on a 24/16 deck is a local question. Ask when you pull the permit.

Water, edge swell and the wet deck

This is the real difference between the two. Ordinary roof sheathing in either material is classed Exposure 1: exterior adhesive, built to ride out construction delays, not to live in the weather. The IRC allows Exposure 1 for roof sheathing whose underside is exposed, but a panel designed to be permanently exposed outdoors has to be Exterior. That matters at open soffits and rake overhangs, not on the main deck.

Wet OSB swells at the edges. It is the material’s known weak point, and the clearest proof is that manufacturers sell around it: Georgia-Pacific puts a moisture-resistant edge coating on its Blue Ribbon OSB, and Weyerhaeuser markets a proprietary edge seal that resists edge swell. A swollen edge on a roof deck reads as a raised line under the shingles along every joint. Plywood is less prone to edge swell, but its veneers can separate after long wetting, which is the delamination a tear-off finds under a long-running leak.

Nail holding and the fastener schedule

APA describes fastener holding as a strength of both products. The code treats them as one: IRC Table R602.3(1) has a single row for wood structural panel roof sheathing from 3/8 to 1/2 inch and another from 19/32 to 3/4, and neither row asks whether the sheet is OSB or plywood.

• Fastener: 8d common, 2½ in × 0.131 in, or RSRS-01 ring shank, 2⅜ in × 0.113 in (roof sheathing ring shank).
• Spacing: 6 in at panel edges, 6 in at intermediate supports.
• High wind: 4 in on centre at gable-end framing and at intermediate supports within 48 in of roof edges and ridges, where the ultimate design wind speed is over 130 mph in Exposure B or over 110 mph in Exposure C.
• Overhang: the sheathing may not cantilever more than 9 in past a gable end wall without overhang framing (§R803.2.3).

The shingle nails are the other half. The code asks for 0.75 inch of penetration into the deck or right through it, and Malarkey’s instructions say the same. A 7/16 or 1/2 deck is thinner than that, so on a normal roof the nails go through, and the points you see from the attic are correct, not a fault. The full sequence is in asphalt shingle installation.

What shingle warranties ask of the deck

Shingle makers set their own deck minimum in the application instructions printed on the bundle, and here the two materials are not treated alike. Malarkey’s laminate instructions call for a deck that is solidly sheathed with at least 3/8 inch exterior-grade plywood, 7/16 inch OSB, or nominal 1-inch seasoned lumber. They also warn that deck materials that are not properly conditioned can move, damage the roof covering, and may void the warranty. So 3/8 OSB, which the code would allow on 16-inch framing, falls under that maker’s floor.

The panel has a warranty of its own, and it has conditions too. Georgia-Pacific’s OSB warranty excludes failures from panels not installed in line with APA’s Engineered Wood Construction Guide (Form E30) and applicable codes. Read both documents before the bid, not after the claim; the roofing warranty guide sorts out whose warranty covers what.

Weight, price and the cost of a re-deck

Weight follows thickness more than material. A 4 × 8 sheet of either is a one-person carry up a ladder, and it is rarely the deck that makes framing marginal. It is the covering. At the lumberyard OSB is usually the cheaper sheet, but both trade as commodities and the gap moves, so price the day’s quote rather than a rule of thumb.

Installed, the material barely shows. Deck replacement runs $2 to $8 a square foot, which is $64 to $256 a sheet, and that spread is access, quantity and labour, not OSB against plywood. The decking cost guide shows how to carry it as a per-sheet line, and a tear-off rather than an overlay is the only way anyone sees the deck at all.