Roof Decking: OSB vs Plywood, 7/16" vs 1/2", Edge Swell, and What Code Requires

Decking is the structural skin your shingles fasten to, and getting it wrong shows up 20 years later as waves, nail pops, and sagging between rafters. Here''s the real trade-off between OSB and plywood, and when each one is the right answer.

The Two Products

OSB (Oriented Strand Board)

Compressed wood strands glued in cross-laid layers, made in 4x8 sheets. Introduced in the 1980s, dominant in residential sheathing since the 1990s. Manufacturers: LP (OSB Radiant Barrier, TopNotch, TechShield), Weyerhaeuser (Edge Gold), Huber (ZIP System, AdvanTech), Georgia-Pacific (Blue Ribbon).

Plywood (CDX typically)

Veneer plies of actual wood laminated in cross-grain, made in 4x8 sheets. Older product. Grades from CDX (construction grade) to AC (one smooth side) to BC (both sanded).

7/16" vs 1/2": What''s Actually Required

The 2021 IRC Table R503.2.1.1(1) and most amendments require a minimum thickness based on rafter/truss spacing:

Rafter spacingOSB minimumPlywood minimum 16" on center7/16" (0.437")3/8" (0.375") 19.2" on center15/32" (0.469")15/32" (0.469") 24" on center19/32" (0.594")19/32" (0.594")

Most new homes in 2027 use 7/16" OSB on 16-inch-on-center trusses. That''s the code minimum and what tract builders specify.

Why 1/2" Still Gets Installed

• Custom builders spec 1/2" as an upgrade.
• High-wind zones (Florida, coastal TX) require 15/32" minimum per amendment.
• Heavy roofing (tile, slate) requires 5/8" often.
• Metal panel systems sometimes spec 1/2" for purlin spacing.

1/2" costs roughly 15-20% more per sheet than 7/16" OSB. For a reroof on existing 7/16" decking, you''re not replacing the whole deck to upgrade; you''re replacing damaged sheets to match the existing thickness.

Price Gap

ProductThicknessPrice per 4x8 sheet (2027) OSB standard7/16"$18 to $24 OSB standard1/2"$22 to $28 OSB standard5/8"$28 to $36 Plywood CDX1/2"$38 to $55 Plywood CDX5/8"$48 to $68 LP TechShield7/16"$26 to $34 ZIP System7/16" + tape$42 to $55 AdvanTech1/2"$38 to $48

Plywood is 60-100% more expensive than OSB of the same thickness. On a 2,400 sq ft roof (roughly 75 sheets) that''s $1,200 to $2,400 extra material cost.

OSB Edge Swell: The Real Complaint

OSB''s big weakness is moisture exposure at the edges. When a sheet gets wet (rainstorm during dry-in, sloppy drip edge, leaky flashing) the strand matrix swells perpendicular to the sheet, usually 8 to 15% thickness increase. When it dries, it doesn''t fully return to flat. The edge becomes a ridge that telegraphs through shingles for the life of the roof.

You''ve seen it: a 10-year-old asphalt roof with visible ridges every 4 feet running horizontally. That''s OSB edge swell from the sheathing getting wet during the original install.

Premium OSB products (AdvanTech, ZIP System) use higher-grade resins and waxes that resist edge swell significantly. Standard OSB is the one that warps.

Plywood''s Advantages

• Better edge moisture resistance. Plywood swells and shrinks but largely returns to original dimensions.
• Holds fasteners better, especially in nail pull-through tests.
• Feels firmer underfoot during install.
• Accepts staining/painting if ever left exposed.

OSB''s Advantages

• 60-100% cheaper.
• More consistent stiffness (no knot/defect variation).
• Manufactured in larger and thicker formats than plywood.
• Engineered OSB (AdvanTech, ZIP) performs like plywood or better.

When I Specify Each

1. Standard OSB 7/16": tract residential new construction, reroof matching existing OSB, budget-conscious homeowner.
2. Premium OSB (AdvanTech, ZIP) 7/16" or 1/2": custom build where "OSB" is the spec but moisture exposure is a risk. Slightly higher cost, dramatically better durability.
3. CDX plywood 1/2": high-end custom homes, historic restoration, owner specifically requests plywood.
4. CDX plywood 5/8": tile roofs, slate roofs, cedar shake, any heavy covering.
5. LP TechShield (OSB + radiant barrier): hot-climate new construction where a radiant barrier is part of the energy strategy.

Code Compliance on Reroofs

Reroofing existing 7/16" OSB: match with 7/16". Replacing damaged sheets needs to match, not upgrade. An entire deck replacement lets you upgrade.

Florida HVHZ (Miami-Dade, Broward): 15/32" minimum and specific nail patterns. Check FBC.

Wildland Urban Interface (California, Colorado, Oregon): Class A roof assembly required, affects decking choice.

Fastening Spec

Roofing nail penetration into decking must be 3/4" minimum per IRC R905.2.5. Using a 1.25" roofing nail through 7/16" OSB gets you 0.81" penetration, barely legal. Through 1/2" OSB you get 0.75" exact minimum. On 5/8" decking you need longer nails (1.5" minimum).

This is why sheathing thickness and nail length have to be coordinated. A cheap bid that used 1" nails on any decking thicker than 3/8" failed code.

Common Failures

• OSB deck delamination near eaves: ice damming or improper ice and water shield, water sat in the decking for years. Tear off, replace.
• OSB edge swell telegraphed through shingles: dry-in got rained on before shingles went down. Ghost of the install still visible.
• Nail pops every 4 feet: decking dimensionally moved, fasteners held their position. Common on early-2000s OSB.
• Cracked plywood between trusses: trusses spaced 24" with 3/8" plywood (under-spec). Should have been 1/2" minimum.
• Fungal growth on underside of decking: inadequate attic ventilation. Not really a decking problem, a ventilation problem showing up on the decking.

Decking Inspection on Tear-Off

Every reroof needs a decking walk before shingles come off. Soft spots, delamination, water staining on the underside (check from the attic) are replacement candidates. Plan to replace 2-5% of decking on a typical reroof; budget more on older homes.

Replacement sheets in RoofKnockers get auto-added to the material list when the field tech flags decking damage during the inspection. Reduces the "extra charge" conversation with the homeowner later. Features, pricing.

Internal Links

Also read the ice and water shield guide since it''s what protects the decking at vulnerable edges, and ridge vent vs box vent because bad ventilation is what kills decking from below.

FAQ

Can I go from OSB to plywood on a partial reroof?

Physically yes, but the shingles laid over mixed decking types show waves where one material meets the other because of different deflection characteristics. Match existing, or replace the whole plane.

How do I tell if decking is damaged during a walk?

Walk the whole roof. Soft spots, crunchy sounds under foot, or visible sag between rafters are all replacement candidates. From the attic, look for water staining, dark discoloration, fungal growth.

Is ZIP System worth the premium?

On new construction in wet climates, yes. The tape system makes the whole deck a drainage plane. For reroofs, the budget usually doesn''t support the upgrade unless the homeowner specifically asks for it.