Roof Sheathing Thickness: OSB vs Plywood Guide (2026)

Roof sheathing thickness is the one sheathing decision that cannot be fixed after the shingles go on. Too thin, and the deck sags between rafters, telegraphing waves through the finished roof. Too thick, and you paid for stiffness the roof never needed. Our roof sheathing calculator counts the panels once the thickness is chosen, and this guide covers the choice itself: OSB versus plywood, thickness by rafter spacing, and which material earns its premium.

The short answer is that thickness follows rafter spacing, and material follows climate and budget. The long answer is below, because the two choices interact: OSB and plywood do not behave identically when wet, and a roof that gets rained on during construction is a different roof from one that stays dry.

Thickness by Rafter Spacing

The governing rule is span: the wider the gap between rafters, the thicker the panel must be to bridge it without sagging. At 16 inches on center, 7/16 inch OSB or 15/32 inch plywood is the usual minimum. At 24 inches on center, step up to 19/32 inch OSB or 23/32 inch plywood. These are minimums, not targets: going one step thicker is cheap insurance on a roof you expect to last 25 years.

Rafter spacing	Minimum OSB	Minimum plywood	Span rating
16 in. on center	7/16 in.	15/32 in.	24/16
24 in. on center	19/32 in.	23/32 in.	32/16
24 in. on center, heavy snow	23/32 in.	23/32 in.	40/20

Span ratings are printed on the panel edge stamp, and they are worth reading. The first number is the maximum rafter spacing for roof use, and the second is the maximum floor joist spacing. A panel stamped 24/16 is rated for 24 inch roof spacing, while a panel stamped 32/16 is rated for 32 inch roof spacing. If the stamp and your spacing disagree, the stamp wins, and you need a thicker panel.

OSB: What It Is and Where It Wins

OSB, oriented strand board, is wood strands pressed with resin into a uniform panel. Its advantages are price, consistency, and availability: every sheet is the same, and it costs roughly 18 to 32 dollars per 7/16 inch panel against 30 to 50 dollars for comparable plywood. For a 77-panel roof, that gap is over a thousand dollars.

OSB wins on roofs that are dried in quickly and stay dry. Its weakness is edges: when OSB edges soak, they swell, and swollen edges do not fully shrink back. That swelling telegraphs through shingles as ridges along every panel joint. If your construction window is tight and the roof will be covered within days, OSB is a sound, economical choice.

Plywood: What It Is and Where It Wins

Plywood is thin wood veneers glued in alternating grain directions, and that cross-grain construction is why it behaves differently when wet. Plywood swells less at the edges than OSB, it dries back closer to its original thickness, and it holds fasteners better after wetting cycles. It is also lighter per panel, which matters to a crew carrying 77 sheets up a ladder.

Plywood wins on roofs with a history of leaks, long exposed construction windows, low slopes that hold water, and any roof where the deck may get rained on before it is covered. It also wins on re-decks over older framing, where fastener holding in questionable rafters matters more than the panel price gap. The premium buys forgiveness, and forgiveness is what a roof deck sells.

OSB vs Plywood: Head to Head

Factor	OSB	Plywood
Price per 4 x 8 panel	About 18 to 32 dollars	About 30 to 50 dollars
Weight per panel	Heavier	Lighter
Wet-edge swelling	More, partly permanent	Less, mostly reversible
Fastener holding when wet	Weaker	Stronger
Consistency	Very uniform	Varies by grade
Best for	Quick dry-in, dry climate	Leak history, slow dry-in, low slope

Neither material is wrong at the right thickness. The mistake is choosing on price alone for a roof that will get wet, or paying the plywood premium for a roof that will be covered in two days and never leak. Match the material to the roof in front of you, not to the one in the advertisement.

Thickness Mistakes That Show Through Shingles

The classic thickness mistake is 7/16 inch OSB on 24 inch rafters. It meets no span rating, and it sags between every rafter pair, producing a roof that looks quilted from the street. The fix is a full re-deck at the right thickness, which costs several times what the thicker panels would have cost the first time.

The second mistake is mixing thicknesses on one roof: a few thicker panels where the supplier ran short, the rest at minimum. Mixed thickness creates steps at every joint, and steps create shingle wear lines. If the supplier substitutes, substitute the whole roof, not a few sheets. Uniform thickness is part of the specification, not a detail.

Illinois Notes: Snow, Ice, and the Thicker Panel

Lake County snow loads argue for the thicker panel on every spacing. A 24 inch rafter spacing with 19/32 inch OSB meets code, but a wet spring snowpack loads that deck harder than the rating assumes, and the upgrade to 23/32 inch costs little against the whole roof. On low-slope roofs that hold snow for weeks, the thicker panel is not optional in practice, whatever the minimum says.

Ice dams add a second argument: water backing up under shingles soaks panel edges, and soaked edges are where both OSB swelling and plywood delamination start. Ice-and-water shield at the eaves, extended at least 24 inches inside the heated wall line, protects those edges, and it belongs in every Illinois re-deck quote regardless of material.

Related reading: our roof sheathing calculator counts panels for the full roof, plus roof decking calculator and how to calculate sheathing for a roof. Every tool is in the roofing calculators hub.

Frequently Asked Questions

What thickness should roof sheathing be?

Use 7/16 inch OSB or 15/32 inch plywood for rafters spaced 16 inches on center, and 19/32 inch OSB or 23/32 inch plywood for rafters spaced 24 inches on center. Check the span rating stamped on the panel edge to confirm it matches your spacing.

Is OSB or plywood better for roof sheathing?

Plywood holds fasteners better when wet, swells less at the edges, and is lighter, but it costs more. OSB is cheaper and more uniform, and it performs well when the roof is dried in quickly. For leak-prone or slow-drying roofs, plywood is the safer choice.

Can I use 7/16 OSB on 24 inch rafters?

No. 7/16 inch OSB is rated for 16 inch spacing, and on 24 inch rafters it sags between supports and produces a wavy finished roof. Use 19/32 inch OSB or 23/32 inch plywood for 24 inch spacing.

What does the span rating on a sheathing panel mean?

The span rating is stamped on the panel edge: the first number is the maximum rafter spacing for roof use and the second is the maximum floor joist spacing. A panel stamped 24/16 is rated for 24 inch roof spacing.

Does thicker sheathing cost much more?

The step from minimum to next-thicker panels adds a modest amount per panel, and a small share of the whole roof cost. Against the price of a re-deck to fix a sagging roof, the thicker panel is cheap insurance.