What is a mineral composite board?

A fibre carrier holds a thin mineral matrix together. During manufacture the carrier is coated or impregnated; the binder then hardens. The composite combines a low build-up height with a continuous mineral cross-section. It is neither automatically a load-bearing board nor a roof waterproofing.

AllShield BarrierSheet is one such comparison case. The product data sheet of April 2026 states 2.5 mm thickness and 3 kg/m² mass per unit area, each with ±10 % tolerance. The layer is intended beneath a new roof waterproofing. These values belong to this specific third-party product. [1]

Magnesium phosphate and cement: different routes to a mineral matrix

Magnesium phosphate binders harden through the reaction of a magnesium component with a phosphate and water. A cement-bound matrix hardens predominantly hydraulically: the reaction of the cement with water forms the binding phases. Fillers, reactive fines, fibres and other additives influence workability, pore structure and strength.

The publicly available BarrierSheet documents describe a mineral composite but do not disclose a complete commercial formulation. A material family is therefore not to be equated with an exact formulation.VlamShield SHEET is a cement-bound board with glass-mesh reinforcement on both faces. Selection follows the task in the roof, not the similarity of the surface.

Why a thin layer lets heat through

Non-combustibility describes a material's own contribution to the fire. It does not mean that its rear face stays cool. For a simplified steady-state view, R = thickness / thermal conductivity. Using the BarrierSheet data sheet values of 0.0025 m and around 0.6 W/(m·K) gives approximately 0.0042 m²K/W. That is a worked example for thermal resistance, not a prediction of temperature in a fire test. [1]

Under fire, material properties and moisture change. Heat storage, evaporation, joints and radiation come into play. Greater thickness can delay heat transfer for comparable material; a blanket ranking of different products does not follow from this.

What the ZAG report actually shows

The published report P 448/25-560-1 distinguishes several build-ups. In test AS1 the layer limited fire spread, yet the EPS insulation melted locally through its full thickness. Residual flames were actively extinguished after around 30 minutes. This shows the difference between a barrier against direct flame contact and a thermal shield. [2]

For AS2 the report describes, among other things, burn-through or weak points at joints and damage to the underlying construction. The results must be condensed neither into blanket failure nor into clearance for every roof. What counts for selection are the individual layers, connections and observed temperatures.

One or two layers: read the build-up first

The Efectis report 2025-R000982 includes two BarrierSheet layers: one above and one below the PIR insulation. That is not the same as two boards lying directly on top of each other beneath the roof membrane. Nor does this report imply a general requirement to install every project in two layers. [3]

A second layer changes material quantity, joint layout, fixing and cost. Compare the complete roof package therefore. Our cost example shows how a second layer changes the calculation; the cover board comparison puts the stiffer alternatives in context.

For thin layers, compare underside temperatures, joints and the required number of layers alongside the reaction-to-fire class.