Roof Sheathing Guide | Roof Decking Materials & Installation

Roof sheathing (decking) provides a stable substrate, bracing and weather protection during construction. Selecting the right material and thickness—and fastening it correctly—is vital for structural performance and roof longevity.

Materials

Plywood

Exterior-grade structural plywood is common. Specify thickness by rafter spacing and loads. Consider edge support and H-clips where required.

OSB

Engineered from oriented strands; cost-effective with good strength-to-weight. Moisture management is critical to avoid edge swell.

Tongue and Groove Boards

Traditional solid wood for appearance applications; ensure adequate thickness and fastening.

Metal and Concrete Decking

Used in commercial/industrial structures; coordinate with membrane roofing and fire ratings.

Thickness and Span

Match thickness to rafter/truss spacing using span tables and AS 1684 requirements. Alpine regions may require additional stiffness for snow loads; coastal wind zones require uplift resistance.

Span Example (described)

At 600 mm rafter spacing, 17–19 mm structural plywood with H-clips provides adequate stiffness for metal roofing under AS 1684 guidance.

Installation

Lay sheets with long dimension perpendicular to rafters
Stagger joints; maintain 3 mm expansion gaps
Use corrosion-resistant fasteners at specified spacing
Provide edge blocking where required

Interfaces with Roof Coverings

Coordinate sheathing flatness and fastener heads with metal, tile or membrane systems. For membrane roofs, ensure clean substrate and compatible primers.

Costs

Plywood 15–21 mm: $28–$55/m² material; $65–$120/m² installed
OSB 15–21 mm: $22–$42/m² material; $58–$105/m² installed

Common Issues

Moisture damage and delamination from inadequate protection
Inadequate fastening patterns leading to uplift or drumming
Insufficient gaps causing buckling in heat

Case Studies

Re-roof, Northern Beaches: Upgrading to 19 mm plywood improved wind performance and reduced noise under heavy rain with metal roofing.

Conclusion

Follow span tables, select appropriate materials for environment and roof covering, and execute fastening and gaps correctly for a durable, code-compliant roof deck.