Correct loading of inverted roof insulation

Whilst a correctly installed Water Flow Reduction Layer (WFRL) prevents the majority of rainwater from getting beneath inverted roof insulation, the insulation board still requires ballasting down to prevent flotation. This is true for all forms of inverted roof insulation be they EPS, XPS, VIP or Cellular Glass, which should all be ballasted with a minimum load of 80 kg/m2.

In the case of finishes that are deemed to satisfy without fire testing by Commission Decision 2000/553/EC this is easy to achieve, with 450mm x 450mm x 50mm paving slabs weighing 118.52kg/m2, and 50mm of 20/40mm rounded ballast weighing 80kg/m2, but what about other finishes such as porcelain tiles, aluminium & composite decking and green roofs?

20mm thick porcelain tiles are becoming a more common specification as architects turn to higher end finishes than the municipal paving slab. The porcelain tiles are typically installed on either paving supports or rail systems on supports. Neither system provides enough weight to ballast
the insulation sufficiently.

The same is true of composite decking, which should always be checked to ensure it has passed a BS EN 13501-5 fire test and achieved a Broof(t4) fire rating. Most composite decking does not achieve this fire rating and should not be used on roofs. Whilst most aluminium decking does achieve a Broof(t4) fire rating, it is equally not heavy enough to ballast down the insulation. In both cases sufficient gravel ballast should be installed on the WFRL, around the paving support, at the appropriate depth to ensure the combined weight of gravel and finishes achieves a load of 80kg/m2.

When it comes to green roofs it depends on whether a fire test exists for the system or not. Where no specific fire test exists, the green roof needs to comply with the requirements in Approved Document B to meet fire requirements – a minimum 80mm substrate depth and less than 50% organic content. Any substrate meeting this requirement should also have a dry weight of 80kg/m2. Where a fire test exists, the loading the tested system achieves needs to be checked.

There are however two further considerations that need to be taken into account. Firstly, the location and height of the roof can require a load greater than 80kg/m2. The only way to check this is by wind load calculation, something QI can do for its customers. Secondly, the fire advice above is specific to roofs, roof terraces, enclosed balconies over heated space and insulated walkways. It is only relevant to inverted roof balconies if FOAMGLAS INVATHERM is used, as this is the only non-combustible inverted roof insulation board on relevant buildings over 18m.

Next month: How does the NHBC Chapter 7.1 update affect roof insulation?