Last edited 12 Nov 2017

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Specifying insulation for inverted roofs



[edit] Introduction

EPS (expanded polystyrene) and XPS (extruded polystyrene) insulation are specified in three different grades - 200, 300 and 500.

The load imposed on a roof by plant for air handling units is typically 1 tonne. This will be supported by 6 spacer pads at 150 mm diameter giving the load exerted onto the insulation as 73 kN/m². Only exceptional loads require over 90 kN/m².

A habit of over specification of inverted roof insulation has meant that until recently the minimum grade used was 300.

When an exceptionally heavy load is required, a roof can be designed using 300 grade for the specific load area and 200 grade for the rest of the roof. This approach offers a number of benefits; it is cost effective, lighter and helps to achieve BREEAM as 200 grade is A+ rated in the BRE Green Guide to Specification.

EPS has other strengths compared to XPS when tested for permanent deformation under cyclic loading:

  • XPS showed a permanent deformation of 25%.
  • XPS loses elasticity at around 45% compression.
  • EPS recovers back to 98% of its original thickness when compressed up to 50%.

[edit] Drainage

Drainage, and how to achieve compliance to BS 6229 and BS EN 12056-3:2000, is probably the most contentious issue about an inverted roof design.

A minimum fall of 1:80 is best practice, but the problems of using a screed layer mean that many roofs are constructed with a zero fall.

Tapered insulation bespoke-designed to the roof, removes the requirement of a screed layer to create the fall for drainage. The insulation boards can be laid straight onto a concrete deck and add just 2% the weight of a screed layer.

A major benefit of a bespoke tapered insulation compared to the screed solution is that there is no drying time for the insulation, which can add up to 40 days to a roof installation.

[edit] Thermal performance

The specified thermal performance of the inverted roof is achieved through the roof design and the thickness of the insulation used. U-values as low as 0.10 can be achieved.

[edit] 200 and 300 Grade

For most inverted roofs with typical pedestrian traffic, a 200 grade EPS insulation board for inverted roofs is a cost-effective product that will provide the required compression performance.

300 grade insulation should be specified when a roof has exceptionally heavy loads. Another option is to design a combination roof that gives performance and cost benefits.

[edit] Tapered

Jablite launched the first tapered insulation for inverted roofs. This enables a roof to be installed according to best practice to achieve the required fall without the addition of a screed layer.

[edit] 1mm increments

Insulation boards for inverted roof are available in 1 mm increments. In some cases this can mean up to 19 mm less material on a large inverted roof giving a substantial saving in cost and load weight. This option eases roof design constraints especially around door thresholds onto balconies and roof terraces and on inverted roofs with public access.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki



Are the loads given in the introduction for AHU correct 73-90 kN/m² seem so large that I wonder whether a decimal point might be missing? (I.e 7.3-9.0 Knus/m²).


Thank you for your comment. The loads given are correct as this is specific to our high density product for inverted roofs application. I hope this answers your query. Many thanks,

Jablite Team