Roof decking is the structural sheathing that sits on top of your rafters or trusses, forming the foundation for everything above it: underlayment, shingles, and flashing. If the decking fails, the entire roof system fails.
OSB vs Plywood Comparison
OSB (oriented strand board) and plywood are the two materials used for roof decking in modern construction. Both are approved by building codes, but they have important differences in performance, moisture behavior, and cost.
OSB
70% Market SharePlywood (CDX)
30% Market ShareWhy OSB Dominates the Market
OSB costs 25-40% less than plywood and offers consistent structural performance. It is manufactured from fast-growing, small-diameter trees, making it more sustainable and less dependent on old-growth timber. For standard residential roofing in dry climates, OSB performs well.
When Plywood Is the Better Choice
High-humidity climates
Southeast U.S., Pacific Northwest, coastal areas. Plywood handles moisture cycles better than OSB.
Roof-over (layered) applications
Plywood's superior stiffness provides better support when adding a new layer over an existing roof.
Cedar shake or tile roofs
Heavy materials benefit from plywood's greater stiffness and nail-holding capacity over long spans.
Unventilated assemblies
If attic ventilation is inadequate, plywood tolerates moisture accumulation better.
Premium builds
When longevity is the priority and the cost difference is justified by the overall project budget.
Cost Comparison at a Glance
For a typical 2,000 sq ft roof requiring 63 sheets of 7/16" or 1/2" sheathing:
OSB
Plywood CDX
The installed cost includes cutting, fitting, nailing, and H-clips. Labor represents 60-70% of the total cost, so the material savings from choosing OSB over plywood are smaller than the raw material price difference suggests.
The moisture problem with OSB: When OSB absorbs water, the edges swell and do not return to their original dimensions when they dry out. This irreversible edge swell creates ridges visible through shingles, known as "ghost lines" or "telegraphing." Once this happens, the affected panels must be replaced. Plywood, by contrast, swells and then returns close to its original dimensions as it dries, making it much more forgiving when exposed to rain during construction or from a minor leak.
Thickness Requirements by Rafter Spacing
Roof decking thickness must match your rafter or truss spacing. Using thinner panels than specified causes sagging between rafters, poor nail holding, and potential code violations.
Thickness by rafter spacing: The table below shows the minimum requirements per the IRC and APA standards. Always check the APA span rating stamp on the panel to confirm it matches your rafter spacing.
| Rafter Spacing | OSB Minimum | Plywood Minimum | APA Span Rating | Notes |
|---|---|---|---|---|
| 12" OC | 3/8" | 3/8" | 24/0 | Uncommon; older construction |
| 16" OC | 7/16" | 3/8" | 24/16 | Most common residential spacing |
| 20" OC | 7/16" or 15/32" | 15/32" | 32/16 | Less common; check span rating |
| 24" OC | 7/16" or 15/32" | 1/2" | 32/16 or 40/20 | Common with trusses; H-clips required |
| 32" OC | 5/8" (19/32") | 5/8" (19/32") | 48/24 | Rare; requires engineered spec |
Best practice: Many experienced roofers recommend going one thickness up from the code minimum, especially for 24" OC spacing. Using 15/32" or 1/2" OSB instead of the minimum 7/16" at 24" OC provides a noticeably stiffer deck that holds nails better and is less likely to sag over time.
Decking Material Estimator
1,792
Adj. Area (sq ft)
17.9
Squares
60
Bundles
APA Ratings Explained
The APA (American Plywood Association, now The Engineered Wood Association) stamp on every sheet of structural sheathing tells you exactly what that panel is rated for.
Span Rating
The two-number span rating (e.g., 32/16) is the most important marking. The first number is the maximum rafter spacing in inches when used as roof sheathing. The second number is the maximum joist spacing when used as floor sheathing. A panel rated 32/16 can span up to 32 inches on a roof but only 16 inches as a floor.
Exposure Rating
Exterior
Waterproof adhesive. Can handle permanent weather exposure. Used for siding and permanent outdoor applications.
Exposure 1 Standard for roofs
Moisture-resistant adhesive. Handles temporary construction exposure but not permanent weather exposure.
Interior
Not moisture-resistant. Never use for roof decking.
Common Panel Grades
- CDX Plywood: C-grade face, D-grade back, Exposure 1 glue. The standard for roof decking. The "X" stands for exposure-rated glue, not "exterior." CDX is rated for temporary moisture exposure during construction but is not an exterior panel.
- Structural 1 (Rated Sheathing): Enhanced cross-panel strength and stiffness. Required or recommended in seismic zones (California, Pacific Northwest) and high-wind regions (hurricane coast). Costs 10-15% more than standard rated sheathing but provides significantly better racking resistance.
- Rated Sheathing: Standard structural panels (OSB or plywood) meeting minimum APA performance standards. The most common and affordable option for roof decking.
H-Clips: What They Are and When You Need Them
H-clips are small metal clips shaped like the letter H placed at the unsupported edges between two adjacent decking panels, midway between rafters. They serve two purposes: providing edge support to prevent differential deflection between panels, and maintaining the required 1/8" expansion gap.
$0.10-$0.15
Cost each
1
Per unsupported edge
18-20 ga
Galvanized steel
24" OC
Required at this spacing
When H-Clips Are Required
- 24" OC rafter spacing: H-clips are required at all unsupported panel edges unless the decking is tongue-and-groove or the panels are supported by blocking between rafters.
- Panels thinner than 1/2": When 7/16" panels span 24" OC, H-clips are essential for stiffness.
- Building code: The IRC (R803.2.2) requires panel edges to be supported by H-clips, blocking, tongue-and-groove, or other approved methods.
Common mistake: Skipping H-clips to save time. This causes panel edges to deflect under load, creating visible waviness in the finished roof and reducing shingle life. At $0.10-$0.15 each, they are one of the cheapest components in a roofing system and should never be skipped.
Signs of Roof Decking Damage
Damaged decking must be addressed before new roofing is installed. Here are the signs organized by severity and visibility.
Visible from Inside the Attic
Daylight visible through the deck
Gaps or holes where light penetrates are obvious failure points requiring immediate replacement.
Delamination
Plywood layers separating or OSB strands lifting from the surface. These panels have lost structural integrity and must be replaced.
Mold or mildew
Black, green, or white growth on the underside of decking indicates chronic moisture and poor ventilation. May still be structurally sound but requires treatment and source elimination.
Water stains
Dark marks on the underside of decking panels indicate current or past leaks. Even dried stains mean the wood has been compromised and should be evaluated.
Visible from Outside or by a Roofer
Soft spots
When a roofer walks the roof and the deck flexes or bounces underfoot, the decking has deteriorated. Roofers check for this during every re-roof and mark panels for replacement.
Sagging between rafters
A wavy roofline between rafter lines indicates decking that has lost stiffness due to moisture damage, age, or insufficient thickness. Often the first sign visible from ground level.
Edge swelling (OSB)
Ridges visible through the shingles at panel edges, caused by irreversible moisture swelling. This is an OSB-specific problem and affects both appearance and shingle performance.
Nail pops
Nails pushing up through shingles indicate the decking below has softened or lost its grip. A few may be normal on older roofs; widespread nail pops signal decking failure.
Replacement Costs
Roof decking replacement is typically done during a re-roofing project, when the old shingles have been removed and the deck is exposed for inspection.
| Scope | OSB Cost | Plywood Cost | Notes |
|---|---|---|---|
| Per sheet (4x8) during re-roof | $75-$100 | $100-$150 | Includes labor to cut, fit, and nail |
| Partial (5-10 sheets) | $375-$1,000 | $500-$1,500 | Common during typical re-roof |
| Full deck (2,000 sq ft) | $4,000-$7,000 | $5,000-$10,000 | 63 sheets at standard 4x8 |
| Emergency spot repair | $200-$500/sheet | $250-$600/sheet | Higher due to shingle removal/replacement |
Typical re-roof: Most projects find 3-10 sheets needing replacement, adding $225-$1,500 to the total project cost. Get a per-sheet price in your roofing contract before work begins so there are no surprises.
Decking Replacement Cost Estimator
Sheets Needed
63
Material Cost
$1,000 - $1,800
Installed Cost
$4,000 - $7,000
When to Replace vs Repair
Not every issue requires full deck replacement. Here is how to make the decision.
Spot Repair
$75-$150 per sheet during re-roof
- ✓ Isolated damage around a former leak
- ✓ Fewer than 30% of panels damaged
- ✓ Damage limited to specific areas
- ✓ Structural members are sound
Full Replacement
$4,000-$10,000 for 2,000 sq ft
- ✗ More than 30% of panels damaged
- ✗ Widespread mold / chronic moisture
- ✗ Substandard original decking
- ✗ Previous overlay excess weight
- ✗ Upgrading from planks to panels
The 30% rule: When more than 30% of the decking needs replacement, most contractors recommend replacing the entire deck. At that threshold, the labor savings of partial replacement are minimal compared to the consistency and warranty benefits of a completely new deck. A new deck also allows the roofer to inspect every rafter and correct framing issues.
Building Code Requirements
Roof decking is regulated by the International Residential Code (IRC) and enforced by local building departments.
IRC Section R803 (Wood Roof Framing)
R803.2.1
Structural wood panels must be manufactured to DOC PS 1 or PS 2 standards, or conform to APA performance standards.
R803.2.2
Panel edges must be supported by H-clips, tongue-and-groove joints, blocking, or other approved methods.
R803.2.3
Fastener schedule: 8d common nails (2.5") or 8d deformed shank nails at 6" OC along panel edges and 12" OC in the field for standard roofs.
High-Wind Zones
In areas with design wind speeds of 130+ mph (hurricane zones), enhanced nailing schedules are required:
- 8d ring-shank nails at 4" OC along edges and 6" OC in the field
- Minimum 15/32" (or 7/16" with specific span ratings) panel thickness
- Some jurisdictions require Structural 1 panels
Seismic Zones
- Structural 1 rated panels are recommended or required in Seismic Design Categories D, E, and F
- Structural 1 panels cost 10-15% more but provide enhanced racking resistance critical for earthquake performance
- Panel layout (staggered joints, blocking) follows specific engineering requirements
Expansion Gaps
A 1/8" gap is required between all panel edges to allow for thermal and moisture expansion. Butting panels tight causes buckling as the wood expands. H-clips automatically create this gap; without H-clips, use 8d nail spacers during installation.
Installation Best Practices
Stagger joints
Offset panel end joints by at least one rafter spacing (minimum 16") from adjacent rows. Prevents a continuous weak line and improves structural performance.
Face grain perpendicular to rafters
Plywood strength ratings assume the face grain runs across the supports. Parallel installation reduces load capacity significantly.
Acclimate panels
Store panels flat and covered on-site for 24-48 hours to adjust to local temperature and humidity conditions.
Do not over-drive nails
Nail heads should be flush with the surface. Over-driven nails break through face veneer and lose holding power. Adjust pneumatic nailers accordingly.
Keep panels dry
Cover installed decking with underlayment or tarps quickly. OSB suffers irreversible edge swell from rain exposure. Even one heavy rain before felt is installed can cause visible damage.
Permits and Inspection
Roof decking replacement typically requires a building permit since it is a structural component. The building inspector will verify panel thickness, nailing pattern, H-clips, and proper edge support before the underlayment and roofing are installed. Failing inspection at the decking stage means tearing off finished roofing to make corrections, which is extremely costly.
Frequently Asked Questions
Is OSB or plywood better for roof decking?
How thick should roof decking be?
How much does it cost to replace roof decking?
Can you roof over damaged decking?
What is CDX plywood?
What are H-clips and do I need them?
How long does roof decking last?
Do I need a permit to replace roof decking?
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Roof Ventilation Guide
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