Roof sheathing, or decking, is the structural layer installed over the rafters or trusses, beneath the roofing materials. Most people use plywood, OSB, or sometimes just plain wood boards for this. The material and thickness depend on the spacing of your rafters, the loads your roof must handle, the panel rating, and local building codes. Fastening also matters because the decking provides structural support for the roofing system. When you understand these details, you’ll have a better idea of which material works best and when it’s time to swap out any decking that’s seen better days.
Key Takeaways
- Roof sheathing vs. decking usually refers to the same layer beneath the roofing, although the terms can vary between contractors and construction settings.
- Plywood, OSB, wood boards and structural panels are common options. The right material depends on the roof design, framing and project requirements.
- Sheathing thickness depends on rafter or truss spacing, roof loads and panel span ratings. Do not choose a thickness based on a general rule alone.
- Building codes can set requirements for thickness, fastening and panel spans. Check local requirements before installing or replacing roof decking.
- Water damage, rot, sagging and delamination can weaken the decking. Inspect the existing material before installing new roofing.
What Is the Difference Between Roof Sheathing and Decking?
In most roofing discussions, roof sheathing and roof decking describe the same basic layer. It sits over the rafters or trusses and provides the surface where roofing materials are installed.
| Aspect | Roof Sheathing | Roof Decking |
| Difference | The structural material installed over the rafters or trusses to form the roof surface. | The layer beneath the roofing materials that supports the roof covering and helps form the roof surface. |
| Main Purpose | Provides structural support and a surface for the roofing system. | Supports the underlayment and roofing materials installed above it. |
| Where It Is Installed | Directly over the roof rafters or trusses. | Between the roof framing and the roofing materials. |
| Common Usage | More commonly used in construction, structural, and building-code discussions. | More commonly used by roofers, contractors, and homeowners. |
| Thickness | Depends on framing spacing, roof loads, panel span rating, and code requirements. | Depends on the material used, framing layout, roof loads, and applicable requirements. |
What Types of Roof Sheathing and Decking Are Available?
Several materials can be used for roof decking. The right choice depends on the roof design, framing spacing, loads, and applicable requirements.
Plywood
Plywood is made from layers of wood veneer bonded together. The cross-layer construction gives it good strength and stability. Roofing plywood comes in different grades and thicknesses. The panel needs to be rated for the intended structural use and installed according to the applicable requirements. Plywood can also be more expensive than some alternatives. Its suitability depends on the specific roof rather than the material name alone.
OSB
Oriented strand board, or OSB, is made from wood strands that are arranged and bonded into panels. It is widely used for roof sheathing. OSB is available in different thicknesses and panel ratings. The correct product depends on the framing spacing and roof requirements. OSB can be exposed to moisture during construction, but prolonged water exposure can cause problems. The panels should be protected and installed according to the manufacturer’s instructions.
Wood Boards
Older roofs may use solid wood boards instead of plywood or OSB. These boards can run across the rafters and provide the base for the roofing material. Existing board decking does not automatically need to be replaced simply because it is an older material. Its condition and ability to support the new roofing need to be considered. Rot, splitting, movement, or large gaps can create reasons for repair or replacement.
Structural Panels
Some roofs are composed of other structural panels for specific applications. Different span ratings, fastening requirements and installation details may apply to these products. Just because a panel looks like plywood or OSB doesn’t mean it can be used for roof sheathing. Be sure to check the product structural rating and installation instructions before using any product.
What Thickness Should Roof Sheathing or Decking Be?
There is no single thickness that works for every roof. The required thickness depends on the framing and the loads the roof needs to support.
Rafter and Truss Spacing
The distance between rafters or trusses affects the support available to the sheathing. Wider spacing may require a thicker or differently rated panel. Wider spacing can require a thicker or differently rated panel. Panel span ratings help determine whether a particular product is suitable for the framing layout. The panel markings can provide information about the spacing the material is designed to span.
Measure the framing before selecting replacement sheathing. Installing a panel that is too thin for the spacing can create unnecessary deflection.
Roof Load Requirements
Roof sheathing also needs to handle the loads expected on the roof. Snow, wind, roofing materials, and other loads can affect the design. A roof in an area with heavier snow loads may have different requirements from one in an area with little snow. The roof structure also needs to be considered as a whole. Do not choose sheathing thickness based only on what was used on another roof. Framing and load conditions may differ. The loads and framing may be different.
Manufacturer Specifications
Panel manufacturers provide information about approved uses, span ratings, fastening, and installation. These specifications should be checked before installation. The roofing material can also have its own requirements. Some roofing products specify particular deck conditions or fastening methods. Use the product information along with the applicable building requirements when selecting the sheathing.
What Building Codes Apply to Roof Sheathing and Decking?
Building codes can affect the type, thickness, span, and fastening of roof sheathing. The exact requirements depend on the location and the roof design.
Minimum Thickness Requirements
Codes may establish minimum requirements for structural roof panels based on the framing arrangement and other design factors. A common thickness should not automatically be treated as a universal code requirement. The applicable table or local provisions need to be checked for the specific roof. This is especially important when replacing decking during a roofing project.
Fastening Requirements
Sheathing needs to be attached properly to the rafters or trusses. Fastener type, size, spacing, and placement can all matter. Improper fastening can allow panels to move or contribute to roof problems. Wind exposure can also affect fastening requirements in some areas. Follow the applicable code and panel manufacturer’s fastening instructions rather than using a general nail pattern.
Panel Span Requirements
Panels are designed for specific spans between supports. A panel’s span rating helps indicate whether it is suitable for the spacing below it. For example, a roof panel marked for a particular span should not automatically be used where the framing exceeds that span. Check the panel markings before installation. If the framing layout does not match the panel rating, a different panel or structural solution may be needed.
Local Code Requirements
Building requirements vary between jurisdictions. Local authorities may also have requirements related to wind, snow, roof construction, and permitted materials. Check the applicable local code before replacing or installing structural roof sheathing. A roofing contractor or building official can help clarify requirements when the situation is unclear.
When Does Roof Sheathing or Decking Need to Be Replaced?
Roof decking does not always need to be replaced during a reroof. The condition of the existing material should be checked first.
Water Damage and Rot
Water can rot wood sheathing over time. Look for dark staining, soft spots, rotting wood or other signs of long-term exposure to moisture. New roofing should not be put on rotten decking. You have to cut out the damaged section and replace it before putting on the new roof.
Sagging or Deflection
A sagging roof deck can indicate a structural or moisture-related problem. Panels may also deflect when they are not adequately supported for the framing spacing. Do not assume that new shingles will correct a sagging deck. The cause needs to be identified first. A professional inspection may be needed when the cause is not obvious.
Cracked or Delaminated Panels
If plywood sits in heavy moisture for too long, the layers start peeling apart. OSB isn’t much better; it swells and breaks down if it stays wet for a while. So before you lay down new shingles, check those panels. This is because cracked, swollen, or delaminated sections can weaken the roof deck and affect the roofing above. Damaged panels should be checked before new roofing is installed.
Repeated Moisture Damage
A roof that has leaked a few times might have more extensive damage to the decking than is visible from inside the building. When the old roofing is stripped off, areas that appear to be sound from the underside may have deteriorated around fasteners or roof penetrations. Fix the source of moisture and replace the affected decking. Otherwise the new substance could have the same problem.
Conclusion
Learn the difference between roof sheathing vs. decking to choose the best roofing material for your project. Both terms are general terms for the structural layer beneath the roofing materials. Plywood, OSB and wood boards are popular but the right material and thickness depend on roof framing, loads, panel ratings and local requirements.
Water damage, rot, sagging, or delamination should be corrected before new roofing is installed. If you need help inspecting or replacing damaged roof decking, contact Precision Roofing for professional roofing advice and roofing service.
Frequently Asked Questions
Is roof sheathing the same as roof decking?
In most cases, the terms mean the same layer of material that is installed over the rafters or trusses and below the roofing. For different areas, manufacturers and contractors may use different terminology
Is plywood or OSB better for roof sheathing?
Neither is automatically better for every roof. Both can be suitable when the correct product is selected for the framing, loads, and installation requirements. Cost, availability, panel rating, and project conditions can also affect the choice.
What thickness should roof decking be?
The required thickness depends on variables such as the spacing of rafters or trusses, roof loads, the panel span rating and local building codes.