Daniel Bosworth, Design & Technical Manager at SIG Design & Technology (now AccuRoof), and Steven Vincent, Head of Design at Eco Green Roofs, discuss best practice for waterproofing new-build school roofs with Architecture Today’s Technical Editor John Ramshaw.

Quality, longevity and sustainability are central to the success of any roof design – not least those intended for education buildings. The historic use of RAAC (reinforced autoclaved aerated concrete), for example, highlights only too clearly what can happen if the wrong constructional system is specified – in this case leading to issues of pupil and teacher safety, academic disruption, and costly remedial works.

SIG Design & Technology is currently taking the lead in providing an innovative, robust, and eco-friendly flat roof waterproofing solution for new-build Department for Education (DfE) schools. Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, and Steven Vincent, Head of Design at Eco Green Roofs, discuss the origins of the system, its principal components and typical build-up, as well as the key benefits and feedback received so far, with Architecture Today’s Technical Editor John Ramshaw.

School roof under construction. The ECO Green filter layer (visible) is installed above the green roof water attenuation layer, which is then followed by the IKO Enertherm water control layer, IKO Enertherm XPS insulation, and IKO Permatec hot melt waterproofing systemm including PermaGuard-F protection layer (ph: Eco Green Roofs).

How did SIG and Eco Green Roofs become involved in developing a robust waterproofing system for DfE new-build schools?

DB: SIG Design & Technology was asked by a leading main contractor to engage with them on their bid for DfE new-build schools as we have been their preferred roofing partners for more than ten years, part of their supply chain, and critically, have an agreed Design Framework Agreement document, which clearly defines the actions and responsibilities we assume for flat roof waterproofing system design. This is backed by our robust insurances, which gives a high level of comfort to the main contractor that the system will be designed in accordance with the waterproofing British Standards, the requirements of the projects, and their own protocols.

SV: We’ve worked with SIG for a number of years across all roof finish options, including blue roofs, green roofs, recreational areas, and of course our SolarNature products, to fulfil the DfE’s high criteria for biosolar.

The EGR SolarNature system is a non-penetrative PV mounting system that is integrated into the green roof and uses the growing medium to ballast against wind uplift (ph: Eco Green Roofs)

What is the main concept behind the waterproofing solution?

DB: We were tasked with providing a robust waterproofing system in line with the DfE requirements for the latest net zero carbon schools. This led us to specify an IKO Permatec hot melt inverted roof waterproofing system applied to a IKO Permascreed mastic asphalt screed, with the system including IKO Enertherm XPS insulation (to achieve a U-value of 0.12), an IKO WCL (water control layer) then a built-up green roof system with photovoltaic panels installed above.

Completed roof showing photovoltaic panels in place (ph: courtesy of DfE)

What are the key components and how do these work together?

DB: All the roof components needed to be compatible and able to work together in harmony. To this end,  SIG Design & Technology worked with one of its approved waterproofing manufacturing partners, IKO for the Permatec system. There was also a need to maintain maximum footprint coverage for both the green roof and PV element over the entire roof area. To achieve this, we needed to integrate these two key net carbon zero requirements. The solution was to use ECO Green’s Solar Nature system, which employs a green roof drainage mat as part of the PV frame mounting system, so the green roof is installed under the PV Panels. This is calculated by ECO Green Roofs to ballast the frame in place and is designed to include plant species that flourish in such conditions.

SV: The EGR SolarNature system is a non-penetrative PV mounting system that is integrated into the green roof and uses the growing medium to ballast against wind uplift stresses. Not only does the SolarNature scheme ballast the insulation layers, but we design a full wind map with the unique loadings attributed to each row, all dependant on the roof location, orientation, row length, proximity to the perimeters and corners, to ballast the PVs.

Typical roof plan

What is the ‘typical’ roof build-up using the system?

DB: The schools are designed with suitable structural decks to 1:40 falls: concrete for the main roof areas and lightweight metal decks for sports halls and main halls (with an appropriate substrate installed over in order to provide a level surface for the hot melt waterproofing). The substrate is primed using the IKO Permatec Primer, then Permatec Anti-Root waterproofing, including Permaflash-R reinforcement, a Permaguard-F protection layer, IKO Enertherm XPS inverted roof insulation to achieve a U-value of 0.12, and then IKO’s waterflow reducing layer (WFRL) called IKO WCL.

For the Solar Nature green roof, Eco Green Roofs specify a robust 25mm drainage/water retention layer installed over the WFRL, followed by its substrate (which is calculated to precise depths in order to ballast the inverted roof system and PV array), then the preferred plants species either sedum matting or more often a rich in biodiversity wildflower seed mix suitable for shade. Detailing to Solar Nature areas include 20-40mm pebble margins and fire breaks as per ‘GRO’ guidelines, as well as 50mm thick paving Slabs for trafficked walkway areas sat on proprietary supports.

Detail showing typical roof parapet

What are the main benefits of the solution?

DB: Hot melt waterproofing systems are among the most robust available, and the Permatec hot melt system is BBA certified as having a service life equal to the roof in which it is incorporated. It is also BBA certified to be used with green roofs, and along with BroofT4 fire test compliance, is the natural first choice for the waterproofing element.

Permascreed has a considerable benefit over other screeds, such as sand and cement, in that it is manufactured from selected bitumens, limestone filler and specially graded aggregates, similar to that of mastic asphalt waterproofing, meaning that once the Permascreed has been installed, it can resist water ingress, is fully compatible with the Permatec waterproofing system, and can be waterproofed over once cool. The latter means that the duration between laying the screed and installing the waterproofing can be as little as a couple of hours, rather than days.

Detail showing typical plank level change

Both Permascreed and Permatec are manufactured in the UK, which not only supports our national manufacturing industry, but also reduces the carbon footprint of the system, compared to products manufactured overseas.

Permatec is the only hot melt system to have an integral anti-root property as part of the waterproofing itself (other hot melt systems need to include an anti-root membrane above the waterproofing layer). It also has a melting wrapper to each Permatec block, which is included in the hot melt boiler as part of the heating process, so the wrappers do not need to be discarded as waste. This, backed by the IKO Single Point Guarantee, means that there are huge benefits to the main contractor and DfE for the waterproofing system.

SV: The system utilises the already planned for green roof build up to ballast the PVs and does so in a manner that allows enough light around the PVs, and crucially, enough water to track back under the PV to aid plant growth.

Detail showing typical rainwater outlet

Is the system being rolled out on site, and if so on which schools to date?

DB: We have provided this design for a number of schools across the Midlands and the North West at present, and have future projects in the South West and North East.

What feedback have you received so far?

DB: Feedback from the roofing contractors has been positive in terms of the inverted roof system – notably the speed to which the Permascreed can be installed over, and the benefits of the Permatec’s low temperature flexibility. We have experienced some obstacles along the way in terms of unforeseen circumstances as part of the build, however with the experience of the design team, they have been overcome.

SV: Feedback form the PV installers indicates that the panels are both quick and easy to install.

Contact Details
If you require design assistance on a new-build school project, please call, email, or visit the SIG Design & Technology website.