Roofconsult Website Specifying Insulation for Inverted Roofs by David Teasdale, Jablite
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Inverted roofs, or protected membrane roofs, as they are often called, are an increasingly popular way to design a flat roof construction. Clients choose it because the waterproofing membrane is protected by the insulation, this extends the lifetime of the roof and reduces the maintenance required.
The inverted roof design raises a number of questions for specifiers and contractors, David Teasdale, Sales & Marketing Director at Jablite Insulation looks at the issues.
Compressive strength
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 150mm diameter giving the load exerted onto the insulation as 73kN/m. 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 remains elastic up to 65% compression
  • EPS recovers back to 98% of its original thickness when compressed up to 50%
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,  the problems of using a screed layer mean that many roofs were left with a zero fall.
The Jablite 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.
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.
Engineering the best value into an Inverted roof
At Jablite, our technical innovations team recognised the advantages offered by the inverted roof design and set out to develop a range of insulation products that would provide the specifier and the contractor with flexibility and choice when designing and building an inverted roof so as to provide the very best value solution.
200 and 300 Grade
For most inverted roofs with typical pedestrian traffic, a 200 grade Jablite 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.
We are very proud to have 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.
Although not best practice, we recognise that for some projects a zero fall roof is the only option and Jablite has the first BBA-certified zero-fall insulation for inverted roofs.
1mm increments
Jablite insulation boards for inverted roof are available in 1mm increments. In some cases this can mean up to 19mm 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.
BBA Accreditation
Jablite insulation for inverted roofs is BBA accredited and achieves compliance with European standards.
BRE Green Guide to Specification
The BRE Green Guide to Specification awards all expanded polystyrene (EPS) insulation an A+ rating except for 300 grade because of the additional material content.
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