Two panels can look identical stacked at the lumberyard and behave completely differently after ten years under shingles. Roof sheathing — the structural skin of OSB or plywood fastened over your rafters — is specified by span rating, thickness, and nailing schedule, and getting any of the three wrong produces sagging panel edges, popped shingle nails, or a deck that peels off in a windstorm. This guide covers the spec side of the question: how OSB and plywood actually compare, what thickness your rafter spacing requires, and what the building code says about clips, gaps, and fasteners. (If your question is instead “the roofer says my deck is rotten — now what?”, start with the homeowner-focused companion piece, our roof decking guide.)
Part of our Roofing guide — explore the full guide →
OSB vs. Plywood: The Real Differences
Oriented strand board is engineered from compressed wood strands in cross-oriented layers; plywood laminates thin veneers with alternating grain. Structurally, at the same span rating they are equals — code and the APA (the engineered wood association) treat them interchangeably for shear and load. The differences show up around water and weight:
- Moisture behavior. The deciding factor. Plywood wets and dries relatively gracefully; OSB absorbs water slowly but holds it longer, and its cut edges swell permanently — the raised “ridging” you can see telegraphing through shingles on cheap builds. Modern OSB with resin-sealed edges is far better than 1990s product, but plywood still wins the durability contest in leak-prone and humid conditions.
- Cost. OSB runs $15 – $25 per 7/16-inch 4×8 sheet against $25 – $45 for 1/2-inch CDX plywood. On a 60-sheet roof that spread is $600 – $1,200, which is why roughly three-quarters of new construction decks are OSB.
- Weight and handling. A 7/16-inch OSB sheet weighs about 45 pounds; 1/2-inch plywood about 47. Effectively a wash on the roof.
- Nail holding. Comparable when dry; wet-cycled OSB loses withdrawal strength faster, which compounds the moisture story.
Practical spec: OSB is a sound, code-approved choice under a properly detailed roof in most climates. Spend up for plywood on low-slope sections, coastal and high-humidity zones, and anywhere ice dams are a way of life.
Reading the Span Rating Stamp
Every structural panel carries an APA grade stamp with a two-number span rating such as 24/16 — the left number is the maximum rafter/truss spacing in inches for roof use, the right for floor use. The stamp, not the thickness, is the legal spec. Common roof panels: 3/8-inch (24/0), 7/16-inch (24/16), 15/32- and 1/2-inch (32/16), 19/32- and 5/8-inch (40/20). The stamp also lists exposure class — Exposure 1 panels tolerate construction-phase weather, which is what you want, since decks routinely sit through rain before dry-in.
Thickness Requirements by Rafter Spacing
Minimums under the IRC, assuming typical snow loads and panels run perpendicular to framing:
- 16-inch on-center framing: 3/8-inch minimum — but nobody should build this; 7/16-inch is the practical floor
- 24-inch on-center framing: 7/16-inch OSB or 15/32-inch plywood minimum with edge support (H-clips); 19/32-inch without
Minimum is not optimum. The industry-wide quiet upgrade is stepping 24-inch-o.c. trusses from 7/16 to 19/32-inch: about $10 more per sheet buys a deck that does not flex underfoot, holds nails through multiple reroofs, and never shows sagging between trusses. Heavy roofing changes the math too — concrete tile and slate loads typically demand 5/8-inch minimum plus an engineer’s check of the framing, and high-snow-load zones (50+ psf ground snow) should be specified locally, not from a national table.
H-Clips, Gaps, and Panel Layout
Three installation details do most of the quality work. H-clips — small aluminum spacers snapped between panel edges mid-span — provide the edge support that lets thinner panels meet their rating on 24-inch framing; one clip between each pair of framing members, two for 48-inch spans. Gapping: panels expand, so code and APA call for a 1/8-inch space at all edges and ends (H-clips set this automatically at clipped edges). Tight-butted panels are the number-one cause of the buckled, wavy decks visible on hot days. Layout: long dimension perpendicular to framing, end joints staggered by half a sheet, every panel bearing on at least two spans, and the grade stamp facing down into the attic where the inspector can read it.
Code Nailing Schedules
The IRC baseline fastening for roof sheathing is 8d common nails at 6 inches on-center along supported panel edges and 12 inches in the field, with nails driven flush — not overdriven through the face ply. High-wind jurisdictions tighten this substantially: coastal and 130+ mph wind zones typically require 8d ring-shank nails at 6/6 or even 4/6 spacing, and Florida’s re-nailing rule requires bringing the entire existing deck up to current schedule during any reroof. Ring-shank nails are the cheapest wind insurance in the assembly — roughly double the withdrawal resistance for about $15 more per roof — and FORTIFIED-standard construction requires them everywhere. Staples are prohibited for roof sheathing in most current codes. If a reroof is how you are touching the deck, the fastening upgrade happens at the decking stage described in our shingle roof replacement walkthrough.
Sealed-Deck Upgrades and Special Panels
Two spec upgrades worth knowing. Taped or integrated-membrane decks — ZIP System sheathing or taped seams under standard underlayment — create a secondary water barrier so a roof that loses shingles in a hurricane sheds rain instead of soaking the house; insurance-backed FORTIFIED programs credit this. Radiant-barrier OSB (LP TechShield and similar) laminates foil to the underside and knocks 20 – 30°F off attic temperatures in cooling climates for about $5 – $8 extra per sheet — worthwhile in the Sun Belt, pointless in Vermont. Fire-retardant-treated sheathing is a third category, required at some property lines and townhouse party walls.
Sheathing Over Existing Plank Decks
Older homes with 1×6 or 1×8 board decking raise a recurring spec question during reroofs: overlay or leave alone? Shingles install acceptably over solid plank decks with gaps under 1/8 inch, but wider shrinkage gaps, cupped boards, and split ends make nail placement a lottery — and metal roofing, low-profile tile, and some warranties require a continuous panel substrate. The standard fix is overlaying the planks with 3/8- or 7/16-inch panels, fastened through into the rafters on the code schedule, which adds roughly $1.50 – $2.50 per square foot to a reroof but produces a flat, uniform nailing surface that outperforms either layer alone. Watch two details: the added thickness changes fascia and drip-edge reveals slightly, and total dead load should be sanity-checked on marginal framing, though an added 7/16-inch panel runs only about 1.5 pounds per square foot. True skip sheathing — boards spaced inches apart under old wood shakes — is not a shingle substrate at all and always gets paneled over or replaced.
Moisture Management and Panel Life
Sheathing rarely dies of load; it dies of water cycles, from one of three sources with three different fixes. Roof leaks concentrate damage in telltale stripes below valleys and penetrations — fix the leak, replace the affected sheets. Attic humidity condenses across broad areas of the panel underside in winter, frosting and then soaking the wood; the fix is air-sealing the ceiling plane and balancing intake and exhaust ventilation, not new wood. And construction-phase wetting matters more than builders admit: panels that sat saturated for weeks before dry-in can swell at edges permanently, which is why Exposure 1 rating and a reasonable dry-in schedule belong in any new-build conversation. A moisture meter reading below 16 percent before underlayment goes down is the cheap verification. Panels kept in that regime routinely serve 50-plus years — the sheathing itself is never the reason a well-detailed roof gets replaced.
Spec Summary
For a typical 24-inch-o.c. trussed roof in 2026: 19/32-inch Exposure 1 OSB (or 1/2-inch CDX plywood in damp climates), H-clips at unsupported edges, 1/8-inch gaps, stamps down, 8d ring-shank nails at 6/12 — 6/6 near coasts — with taped seams if you want the resilience credit. That deck will outlast two or three shingle cycles, stay flat, and never be the reason a roof fails. Keep the attic below it ventilated — condensation rots more sheathing than rain does, and our roof vents guide covers the balanced airflow that keeps panel moisture in check.
