| 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/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 remains elastic up to 65%
compression
- EPS recovers back to 98% of its original
thickness when compressed up to 50%
|
| 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,
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. |
| Tapered |
| 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. |
| Zero-Fall
|
| 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. |
| |
