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Last edited 12 Nov 2017
Specifying insulation for inverted roofs
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.
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 remains elastic up to 65% compression.
- EPS recovers back to 98% of its original thickness when compressed up to 50%.
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.
 200 and 300 Grade
 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.
 Find out more
 Related articles on Designing Buildings Wiki
- Domestic roofs.
- Flat roof.
- Heat transfer.
- Inverted roof.
- Inverted roof defect - case study.
- Limiting fabric parameters.
- Roof coverings.
- Roof insulation.
- Roofing defects.
- Solid wall insulation.
- Thermal bridge.
- Thermal insulation for buildings.
- Types of roof.
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