Is EPS insulation suitable for a flat roof?

An inverted roof has a different layer sequence, and the thermal insulation is exposed to water differently. Ordinary roof EPS should not simply be carried over to it. There are EPS grades with documentation covering such applications, but the specific build-up must be verified. The name “roof-floor” alone is not a sufficient basis for such a change.

The insulation is subjected to the weights of the layers, equipment and people carrying out work. A photovoltaic installation or air-conditioning unit can increase pressures at selected points. For this reason it is not enough to calculate the average weight of the whole roof. The bearing areas and load-distributing elements must be checked, as well as the load-bearing capacity of the structure below.

CS(10) is a test parameter at 10% deformation. Permissible long-term performance requires a separate assessment. Deflection or localised indentation can impair drainage and put stress on the covering. Service walkways should allow access without damaging the insulation. It must not be assumed that every EPS roof can be walked on anywhere.

The thickness results from the requirements for the whole roof and the lambda adopted. A lower thermal conductivity value helps where the height at the parapet or outlet is limited. It does not, however, remove the need for proper detailing. Too low a height of the waterproofing upstand may force a change of detail, even if the calculated thermal resistance is good.

In a tapered roof the thickness may vary across the surface. The thinnest points must be taken into account, along with the correct method of calculating the component. A simple average of two heights is not always sufficient. It is worth knowing the position of the outlets, the lengths of the falls and the constraints at thresholds and skylights already at the design stage.

Yes, cut EPS elements are used to direct water to outlets or edges. The wedges are designed for a specific roof plan. They should be marked and have a laying plan, because turning an element the wrong way can create ponding. The fall is therefore not a decorative slope but part of the drainage system.

It is worth checking the continuity of drainage also at equipment and parapet corners. The insulation should not close the water path or create local thresholds. In a refurbishment the actual deflections and unevenness of the old substrate must be taken into account. A design prepared without surveying the existing roof may fail to reproduce its actual geometry.

The waterproofing is selected taking into account contact with EPS. Not every membrane or substance can lie directly on polystyrene. Sometimes a separating layer is needed. Work involving heat requires a technology intended for the whole covering. Exposed polystyrene must not be treated as a substrate resistant to flame.

The reaction-to-fire class of the board does not by itself describe the fire resistance of the roof. The classification for external fire exposure also relates to a specific roof covering. The complete layers, the substrate and the fixing details must be checked. Replacing one element with a seemingly similar one may go beyond the scope of the documentation.

EPS can be a good choice when its properties match the design and the build-up is correctly executed. The low weight of the material makes transport onto the roof easier, but light packs must be protected from the wind. Work organisation should allow successive areas to be protected quickly against rain. It is not worth exposing a larger area than can be safely closed.

EPS materials for roofs are offered by, among others, Swisspor, Styropmin, Termo Organika and Knauf Therm. Individual boards differ in parameters and permitted application.

Before buying, compare the insulation, covering, fixings and detailing as one set. Also ensure that the outlets and service penetrations can be inspected during use. Efficient water drainage and rapid repair of minor damage are just as important for the durability of the roof as the initial choice of a good board.