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DOW BUILDING SOLUTIONS - Rock Wins Full Marks For Tackling Deflections

Roof waterproofing specialist Rock was called in to provide technical and practical support on a major schools building project – par for the course for a market-leader, but it was unaware that it would end up solving a testing industry issue at the same time.

When a major contractor was commissioned to rejuvenate and rebuild over 20 schools in the North West it turned to 40-year roofing veteran Rock to install traditional inverted roofs on the first two projects to get the green light. With both buildings set to open for the new term in September - and construction being managed on existing school sites close to residential areas - it was vital that the project was safely managed within tight timescales. Cost-effectiveness and client satisfaction were of course also key.

With large expanses of flat roofing to cover – and energy efficiency as well as waterproofing a core client demand – the kit specified was Rock’s Flexiphalte triple protection system, a fully bonded high performance waterproofing system combining three core elements: a high performance membrane, overlain by durable Flexiphalte Pommar polymer modified mastic asphalt and covered with a 180mm thick layer of ROOFMATE SL-A insulation from Dow Building Solutions. The system was covered with ballast and paving slabs to produce a standard inverted roof finish.

“We recommended Flexiphalte triple protection for this job because we believe it gives a level of protection that cannot be matched by any other type of roofing system or membrane, giving “lifetime” performance equal to the substrate upon which it is laid,” said Mike Goddard, Rock’s Construction Director. “We felt ROOFMATE SL-A was the right insulation choice because it’s readily available, robust with a high compressive strength, easy to work with and we’ve seen its long-term performance first-hand.”

However, it was prior to installing the ROOFMATE insulation that Rock, in collaboration with the lead contractor, wanted to ensure adequate drainage - without having to resort to the cost of laying additional screed on what had already been designed as a ‘zero pitch’ roof.

Although BS 6229:2003 recommends that an appropriate fall be accommodated on roof designs where there are continuously supported coverings, the British Board of Agrément (BBA)’s Information Bulletin No.4 acknowledges zero pitch roofs, emphasising the need to ensure drainage is sufficient and effective. Both designs are considered correct practice as long as drainage is adequate – but for zero pitch roofs this means any possibility of deflections must be designed-out in order to ensure water will not pond on the roof.

With a project already running to tight timescales and within strict cost limitations, the estimated six figure additional cost associated with adding screed and potentially raising parapet height was not an option. Rock’s solution was to carefully survey the roof for deflections and add additional Flexiphalte Pommar asphalt to potential problem areas, enabling a completely flat finish and ensuring water would run off to drainage outlets already specified and installed.

“The additional asphalt fuses entirely with the existing layer meaning we can still provide a fully-bonded system - but with no deflections,” continued Mike. “We’ve developed a solution that can either be built in at design stage where a zero pitch roof is favoured for cost reasons, or as a retrofit solution if deflections need to be addressed once the asphalt waterproofing has been completed.”

“This project was a great example of a main contractor, specialist roofer and materials supplier coming together to develop a solution for the ultimate benefit of the client,” said Richard Powell, Roofing Manager for Dow Building Solutions. “It’s thanks to that positive approach that the industry has found a method of addressing deflections in a cost-effective way based on existing, proven solutions.”

For more information about how Rock’s Flexiphalte triple protection system can be used to help prevent deflections and information about Rock’s other roofing solutions, visit For product and stockist information on ROOFMATE SL-A visit or email below.

Note: JPA Technical Literature has produced a paper entitled “A review of recent guidance on inverted roof construction” which summarises European Technical Approval Guidance 031 (ETAG 031) on inverted roofs and BBA Information Bulletin No. 4. To download a copy visit the Dow Building Solutions website at

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