January 2014: The Code of Practice states an appropriate fall shall be accumulated within the flat roof design (BS 6229) and a roof is adequately drained in accordance with BS EN 12056-3:2000.
A minimum fall of 1:80 is required to meet BS 6229. Creating this required fall for drainage has always been a challenge…
Jablite’s inventor* of tapered insulation for inverted roofs, Conrad Newberry, who was a concrete technologist and is now a technical advisor in Jablite’s innovations team, explains the issues:
“The majority of commercial roofs are concrete and it is very difficult to create a fall to the rainwater outlets in concrete. Consequently the fall is created by using a screed layer on top of the concrete,” explains Conrad.
Unfortunately, this process of using a screed layer is beset with problems and despite BS 6229 only around 20% of commercial roofs have the screed layer although it is specified on about 50% of roofs.
The problem with using screed to create fall on inverted roofs
1. After installation the screed needs to have dried enough to receive coverings. Screed typically dries at a rate of 1mm per day, if there is no rain during this drying out phase.
2. Screeds are typically semi-dry and this zero slump workability often results in an open surface texture which absorbs rainfall more easily than well compacted concrete. A hair hygrometer is the best way to determine whether a screed is dry. The worst methods are guess work or a visual inspection!
3. If the screed is not dry, any moisture in the screed is drawn to the surface by the hot applied weatherproofing system. This may create small bubbles in the waterproofing layer.
4. Screed adds weight to a roof. Previous RCI articles highlight screed weight has clear implications for building frames and foundations leading to further costs. Reducing the number of storeys in a building can impact on its commercial value.
5. Screed weight can make it unsuitable for refurbishment projects.
6. The surface regularity to BS 8204 of a semi-dry sand/cement screed is 10mm over a 2m straight edge.
7. The additional time associated with installation and drying of the screed and the cost of suitable structural fibres should the designer wish to expand the joints beyond 6m bays (to BS8204) all add to the total build cost.
All in all, it is not surprising that many contractors and specifiers choose not to include a screed fall.
Many instead opt for a zero fall roof; this is outside the scope of best practice; however Jablite has an insulation product for a zero fall flat roof that is BBA certified to meet this application.
Nevertheless, at Jablite we always recommend best practice and finding a solution to this issue exercised the mind of Conrad Newberry: “The issue of screed falls and inverted roofs has never been far from my mind.
“I realised that the manufacturing processes used by Jablite for its insulation for inverted roofs could be adapted and developed to manufacture a tapered insulation system for an inverted roof.
“In addition, I knew that Jablite had the capability to design bespoke tapered roofs as we already do that for range of insulation products designed for the warm roof single ply market.”
Conrad quickly realised that his idea would solve the problems of using a screed layer to create a 1:80 fall on an inverted roof. With a bespoke-designed tapered insulation, an inverted roof could be specified to benefit from all the advantages of a protected membrane installation – and still meet BS 6299.
The benefits of Jablite tapered insulation for inverted roofs over a screed layer are:
• Allows contractors and specifiers to comply with the relevant Codes of Practices and regulatory compliance
• Saves time with the removal of the screed installation phase
• Has no drying time, Jablite insulation boards can be laid straight onto the membrane directly over the concrete deck
• Offers superior design to the client and specifier
• Is a cost effective roof drainage solution – screed costs £10/m2 more than Jablite’s tapered insulation product for inverted roofs
• Meets BS 6299
• Weighs 2% the weight of screed
Accreditation and Testing
Jablite has been awarded BBA accreditation for this latest innovation to its inverted roof insulation range.
During testing of this product to the European Technical Approval Guidelines (ETAG) 031 for inverted roofs, Jablite’s innovations team discovered some interesting aspects about the performance of its inverted roof insulation.
“The BBA testing found no water penetration through our inverted roof insulation system,” says Conrad.
“However, as laboratory testing rarely reflects real life usage, we include an assumption of a 2.5% rainwater cooling factor in our u-value calculations for our inverted roof insulation – to ensure they are completely reliable.”
Additional Benefits of Jablite Insulation
Jablite EPS (expanded polystyrene) insulation for inverted roofs has a dimensional stability of 0.5.
The 2% dimensional ‘instability’ of an XPS insulation board can cause significant and unwelcome changes to the roof installation.
During a period of temperature changes such as we saw in December 2013 – relatively warm days and below zero temperatures at night, the boards will contract and expand.
This is the reason why XPS is rarely used in a single ply membrane system and EPS is selected for this application
Jablite insulation for tapered roofs can deliver any specified thermal performance as part of a designed roof structure.
BRE Green Guide to Specification
Jablite tapered insulation for inverted roofs is manufactured using expanded polystyrene (EPS) which has an A+ rating in the BRE Green Guide to Specification.
*The Tapered Inverted Roof Board Patent Application is due to be published in early 2014.
You can find more indepth and technical information about flat roofs at Roofconsult