Choosing the right plywood thickness for a roof is critical for structural strength, weather resistance, and code compliance. This guide explains common plywood thicknesses for roof decking, how span, roofing cover, and building codes affect selection, and practical installation tips for safe, durable roofs. Key factors include load, rafter or truss spacing, and the roofing material.
| Plywood Thickness | Common Use | Typical Rafter/Truss Spacing |
|---|---|---|
| 1/2″ (12mm) | Light-duty, short spans, underlayment in some regions | 12″ to 16″ o.c. only with restrictions |
| 5/8″ (15mm) | Standard roof sheathing in many homes | 16″ o.c.; sometimes 24″ with stiffer panels or higher grade |
| 3/4″ (18mm) | Heavier loads, long spans, 24″ o.c., and roofs with heavy coverings | 12″, 16″, or 24″ o.c. depending on load |
Understanding Roof Sheathing And Plywood Thickness
Roof sheathing transfers live, dead, and wind loads from the roofing material to the rafters or trusses. Plywood thickness directly affects stiffness and load capacity, so selecting the correct thickness prevents sagging, reduces deflection, and supports roofing materials like shingles, tiles, or metal.
Manufacturers rate panels by ply and span rating. Common residential plywood (CDX) is used for roof sheathing, while OSB is an alternative. Both materials follow building code guidelines, but thickness choice should reflect the specific performance needs of the roof.
Building Codes And Span Ratings
Most U.S. building codes reference the International Residential Code (IRC), which provides span tables based on panel type, thickness, and rafter or truss spacing. Span ratings such as 32/16 or 48/24 indicate load capacity for roof and floor use.
For example, a 5/8″ plywood panel may be acceptable at 16″ o.c. rafters, while 3/4″ panels are often required for 24″ o.c. rafters. Always check local code amendments and the IRC span tables to confirm requirements for the specific project.
Common Plywood Thickness Recommendations
Typical thickness choices depend on rafter/truss spacing and roofing material. Here are practical recommendations commonly used in American construction:
- 1/2″ Plywood: Used in limited situations; best for 12″ o.c. rafters or as underlayment beneath a thicker structural layer.
- 5/8″ Plywood: The most common choice for residential roofs with 16″ o.c. rafters and asphalt shingles.
- 3/4″ Plywood: Preferred for 24″ o.c. rafters, heavy roofing materials (tile, slate), or long-span rafters where stiffness is critical.
OSB Versus Plywood For Roof Decking
Oriented Strand Board (OSB) and plywood are the two major sheathing materials. OSB is cost-effective and performs well when protected from prolonged moisture, while plywood offers higher shear strength and moisture tolerance.
OSB panel edges swell more than plywood if exposed to moisture, which can affect roof flatness and shingle installation. Building codes treat both materials similarly for many applications, so the choice often comes down to local conditions, budget, and contractor preference.
Roofing Material Impacts On Sheathing Thickness
Heavier roofing systems require stiffer decking. Ceramic or concrete tiles, slate, and heavy metal systems often necessitate 3/4″ plywood or engineered sheathing to distribute loads and limit deflection.
Conversely, asphalt shingles are lighter and commonly used with 5/8″ plywood. Synthetic underlayments and modern fasteners can influence the overall system performance but cannot substitute for insufficient sheathing thickness.
Rafter And Truss Spacing Considerations
Rafter and truss spacing is a primary determinant of required plywood thickness. Closer spacing (12″ or 16″ o.c.) allows use of thinner panels, while wider spacing (24″ o.c.) demands thicker sheathing or structural panels.
For retrofit projects, if rafters are widely spaced and replacing decking with same thickness causes noticeable deflection, upgrading to a thicker panel improves stiffness and extends roof life. Span tables from the panel manufacturer are the authoritative reference.
Fastening And Edge Support Best Practices
Proper fastening pattern and edge support affect the installed strength of sheathing. Use the recommended nail type, spacing, and pattern from the IRC or the panel manufacturer to achieve rated capacity.
- Common practice: 8d nails or equivalent staples at 6″ o.c. along panel edges and 12″ o.c. in the field for 5/8″ panels at 16″ o.c.
- Ensure panels are installed perpendicular to rafters with supported edges on rafters or blocking.
Ventilation, Moisture, And Plywood Performance
Roof ventilation and moisture management are essential for plywood longevity. Insufficient ventilation or prolonged moisture exposure can warp or delaminate panels, reducing structural integrity.
Install breathable underlayment, maintain attic ventilation (intake and exhaust balanced), and protect sheathing from exposure during construction. For humid climates or frequent storms, consider plywood over OSB for improved moisture tolerance.
When To Use Structural Roof Sheathing Or Tongue-And-Groove
Structural roof sheathing (e.g., 5/8″ or 3/4″ APA-rated panels) and tongue-and-groove decking are options for higher performance. Tongue-and-groove boards provide continuous support and are commonly used in older homes and heavy-roof systems.
Sheathing rated for roof application should have the appropriate span rating and be installed per manufacturer guidelines. For high-wind zones or complex roof geometry, structural sheathing prevents uplift and improves stiffness.
Retrofit And Repair Guidance
When reroofing, assessing existing sheathing condition is crucial. Replace any rotten, delaminated, or excessively sagged panels and consider upgrading to thicker decking if original panels are undersized.
Small repairs may allow patching with the same thickness, but for consistent performance and code compliance it is often better to replace full panels and maintain continuous support and fastening patterns.
Cost, Availability, And Practical Trade-Offs
Thicker plywood costs more and weighs more, which affects handling and labor. However, the additional cost is often justified by increased durability, reduced deflection, and longer roof life.
OSB may save on material costs but may require faster installation and better moisture protection. Local lumberyards and big-box stores typically stock 1/2″, 5/8″, and 3/4″ panels; specialty sizes or higher grades may need special orders.
Inspection And Professional Advice
Building inspectors and structural engineers can confirm the correct panel thickness for atypical designs or heavy loading. For complex roofs, unconventional spans, or when changing roofing materials to heavier products, consult a structural engineer to verify sheathing thickness and fastening details.
Local building departments can provide specific code references and acceptable material lists, ensuring the roof meets safety and performance standards.
Quick Selection Checklist
- Check Rafter/Truss Spacing: 12″–16″ o.c. often allows 5/8″ panels; 24″ o.c. usually needs 3/4″.
- Consider Roofing Material: Heavy tiles/slate require thicker decking.
- Verify Local Codes: Use IRC span tables and local amendments.
- Choose Material: Plywood for moisture tolerance, OSB for cost-effectiveness.
- Follow Fastening Specs: Proper nails, spacing, and edge support are essential.
Resources And References
Authoritative resources include the International Residential Code (IRC), APA — The Engineered Wood Association span tables, and local building departments. These sources provide span ratings, fastening schedules, and material specifications for safe roof construction.
Manufacturers’ technical documents and local code amendments should be checked before finalizing material selection to ensure the plywood thickness meets all structural and safety requirements.