Roof sheathing (also known as sarking) is the layer of boards or sheet material fixed directly to the rafters or trusses, creating a solid, flat substrate on which the roof covering and underlayment are installed.
Materials used for roof sheathing
Traditional roof sheathing consists of solid timber boards, typically softwood planks of 18 to 25 mm thickness, nailed across the rafters. In modern construction, engineered sheet materials are more common. These include oriented strand board (OSB), plywood, and moisture-resistant chipboard. Sheet materials are faster to install and provide a consistent, stable surface with fewer joints.
The choice of sheathing material depends on the roof type, the covering to be applied, and the structural requirements. For example, a flat roof with a bituminous membrane typically uses moisture-resistant OSB or plywood, while a pitched roof with tiles may use either battens (without full sheathing) or a sheathed and counter-battened system.
Function of roof sheathing
Roof sheathing serves several important purposes. It provides a rigid, flat surface for the roof covering or underlayment to be attached to. It adds structural rigidity to the roof by bracing the rafters together, helping the roof resist wind loads and racking forces. In cold roof designs, it also supports the breathable membrane that protects the insulation below from wind and moisture.
Without sheathing, roof coverings like felt, single-ply membranes, or liquid coatings cannot be installed, as they require a continuous substrate.
Application
Roof sheathing is installed during the structural phase of roof construction, after the rafters or trusses are in place. Boards or panels are laid perpendicular to the rafters and fixed with nails or screws at regular centres. Joints between panels should be staggered and supported on a rafter.
When renovating an older roof, it is important to inspect the sheathing for rot, insect damage, or sagging before installing new roof covering. Damaged sheathing must be replaced to ensure a sound base.
Related terms
- Rafter
- Roof covering
- Underlayment
- Roof construction
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