Sarking boards are fixed to the outside of the rafters of pitched roofs and can provide structural support. They are typically 150mm wide and between 19mm to 22mm thick in timber form. Some other types of sarking, such as wood fibre boards, are available in larger widths and greater thicknesses. Sarking boards are more common on roofs in some areas of the country than others due to weather conditions. Specifying the right type of sarking board can bring advantages to a pitched roof as we shall explore below.
What are sarking boards made of?
Sarking boards can be made of the following materials.
- Sawn softwood - This is the traditional material used for sarking made from sheets of sawn softwood.
- Wood fibre boards - These are rigid boards made from wood fibre bonded with synthetic resins or using the natural glues from the wood itself. Some wood fibre sarking boards primarily offer weatherproofing and additional insulation to the roof.
- Exterior grade plywood - These are made by gluing together thin sheets of wood veneer with each layer rotated at 90 degrees from the previous layer.Plywood is available in different grades. It is worth noting you should only specify boards that have been certified for use in humid conditions – known as ‘service class 3’ for sarking board applications
What is the role of sarking boards?
The primary role of a sarking board is to enhance weather protection, insulation, and airtightness. In some systems, it also provides structural support. Commonly used in pitched roof construction, sarking boards help prevent wind-driven rain ingress, improve thermal performance, and contribute to the roof’s overall durability. Depending on the material and application, they may be structural (e.g., sawn softwood or plywood) or non-structural, supporting breathable and diffusion-open roof designs.
The additional mass added to the roof by sarking boards can help absorb or reflect airborne sound and, depending on the board composition, dampen vibrations to further reduce the transmission of outside noise. When materials of varying densities are layered - such as combining lightweight insulation with denser wood fibre sarking - sound waves encounter increased resistance at each interface. This disrupts their energy and reduces overall transmission. This multi-layered approach can be especially effective in mitigating both airborne and impact noise in roof structures.
Some sarking boards such as those made of wood fibre, are diffusion open. These sarking boards can allow moisture from the building to escape whilst still blocking liquid water, helping to prevent condensation within the roof structure.
Where are sarking boards positioned in a roof build-up?
Sarking boards are positioned above the rafters and below the outer waterproofing layer of the pitched roof.
In the construction sequence of a typical tiled warm pitched roof, the sequence would be.
- Fit insulation either between and under the rafters, or between and over the rafters. This should be at a thickness determined by the insulation manufacturer based on the thermal requirements of the roof.
- Fix sarking boards to the rafters – leaving a 2mm to 3mm gap between boards in line with the sarking board manufacturer’s guidelines.
- Install a breathable membrane over the sarking boards to protect them from exposure to moisture.
- Fix counter battens to allow for drainage of water.
- Fix tiling battens.
- Lay and fix roofing tiles and any other coverings such as vents or solar tiles.
Where are sarking boards most commonly used?
As sarking boards can provide structural support and add another layer of weather protection, they are mainly used on buildings in areas where the roof will be exposed to periods of high winds and heavy rain.
In the UK, sarking boards are therefore more commonly used on roofs in areas like Scotland due to the higher frequency of wind and rain when compared to England and Wales.
This can be illustrated by looking at the Mean UK Wind Speed Annual Average map produced by the Met Office and is linked below[1]. The darker the shade of blue, the windier the location.
Can sarking boards be fitted without gaps?
Sawn wood and plywood sarking boards should be fitted with gaps to allow the roof to breathe by providing a path for any trapped moisture to escape. However, as we have just mentioned, wood fibre sarking boards are diffusion open. This means that as water vapour will diffuse through the sarking boards they can be fitted without gaps. They therefore create a continuous protective layer against moisture and wind, as well as helping to increase the thermal performance of the roof.
Where can I find out more information about sarking boards?
STEICO has a range of sarking and sheathing boards suitable for use in pitched roofs. They are all diffusion open and boards with a tongue and grooved (T&G) profile can often be used without any additional weathering membrane. This helps to provide a continuous strengthening and protective layer that allows the fabric of the roof to breathe.
With a compressive strength of 200 Kpa and a lambda value of 0.043 W/(m*K) STEICOuniversal dry is a sarking board that can provide structural support as well as potentially supporting the thermal performance of the roof. STEICOsafe is a T&G sarking board that has a directly bonded membrane for increased weather protection.
To enquire about using our sarking boards on your next project, please contact us and our expert team will be happy to help.
[1]weather.metoffice.gov.uk/learn-about/weather/types-of-weather/wind/windiest-place-in-uk