12.5mm Siniat Weather Defence External Sheathing Board - 1200mm x 2400mm

12.5mm Siniat Weather Defence External Sheathing Board is a high-performance, non-combustible sheathing board designed for use in steel frame and timber frame constructions. It is used to weather-protect the building envelope before the final cladding—such as rain-screens, brickwork, or render—is installed. Often recognised on-site by its iconic purple colour, the Siniat Weather Defence External Sheathing Board is a glass-mat faced gypsum board. It has transformed the industry by providing the speed of drywall installation with the weather-shrugging performance of a structural cladding.

CHARACTERISTICS

• One of its greatest strengths is its ability to be left exposed on the frame for up to 12 months before the final façade is applied. This allows internal trades (plumbers, electricians, and plasterers) to start work much earlier, as the building is effectively weather-tight.
• It carries the highest fire safety rating, Euroclass A1 Non-Combustible. It is a vital component in modern high-rise buildings where fire-spread prevention is non-negotiable.
• Weather Defence can be scored and snapped with a standard utility knife. It is up to 50% lighter than traditional cement boards, making it much faster and safer to handle at height.
• The board is highly vapour-permeable but exceptionally airtight. When the joints are sealed with Weather Defence Tape, it creates an incredibly efficient air barrier, helping the building meet strict Part L energy requirements.
• It has extremely low moisture movement. Unlike timber-based sheathing (such as OSB), it will not swell, bow, or delaminate when wet.

APPLICATION

• Steel Frame Systems (SFS): Providing the primary external skin over steel studs in commercial and multi-storey residential projects.
• Timber Frame Construction: A fire-safe, moisture-resistant alternative to OSB or plywood sheathing.
• Rain-Screen Cladding: Acting as the stable, fire-proof backing for various cladding systems, including stone, metal panels, and high-pressure laminates.
• Modular Construction: Its lightweight and durable nature makes it perfect for off-site manufacturing and transport to site.
• As the structural sheathing layer on buildings over 11 or 18 metres where A1 non-combustibility is mandatory.
• It can be used as the inner leaf sheathing in a traditional brick-and-block cavity wall.
• Because it is lightweight and easy to install from a mast climber or scaffolding in high-rise Façades.

INSTALLATION

• No power saws required. Simply score the purple face with a sharp knife, snap it over a straight edge, and cut through the white glass-fleece on the back.
• The board must be fixed with the purple side facing outwards. Use specialist Siniat Weather Defence Screws, which are designed to sit flush without tearing the glass-mat surface.
• To achieve the 12-month weather protection and airtightness, all board joints, screw heads, and penetrations must be sealed using the dedicated Siniat Weather Defence Tape.
• Ensure a 3mm to 5mm gap is left at floor and ceiling junctions to allow for structural movement, then seal these gaps with a compatible flexible sealant or tape.