Surface mass times area

The added load of a layer is surface mass (kg/m²) times roof area. For the DEK reference board the Sika data sheet (June 2026) gives 5.9 kg/m² at 6.4 mm and 9.8 kg/m² at 12.7 mm. On 5,000 m² that is roughly 29.5 tonnes and 49 tonnes respectively. For the WEER reference board around 10 kg/m² applies at 10 mm, but it only sits at details: calculate with linear metres and board height, not with the roof area.

What the structural engineer does with it

Added permanent load comes on top of self-weight, PV system, ballast, snow and water. Whether the roof can carry it follows from the reserve capacity in the structural calculation. That judgement is reserved for the structural engineer. We do supply the exact surface mass of the selected board so that the calculation works with real values. Thin is also not automatically light: a gypsum-based board of 6.4 mm and a cement-based board of 10 mm are closer per m² than the thickness suggests.

Temporary loads during installation

A pallet of boards in one spot is a concentrated load far exceeding the design value per m². Plan where pallets go, how they are spread across the roof and what lifting equipment is needed. That belongs in the method statement, not in first-morning improvisation.

Worked example: added roof load

Example values from the data sheets cited

5,900 kg

Only the added dry mass of the layer. Not a structural assessment; the structural engineer assesses capacity.

Give the structural engineer real kg/m², the total added weight and the temporary installation loads.