Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses best practice design for multi-material roofs on the Lowfield Green residential development in York with Architecture Today’s Technical Editor John Ramshaw

Commissioned by developer Shape Homes York and due to complete in 2021, Lowfield Green is a major housing development in Acomb, North Yorkshire. The scheme comprises 165 dwellings ranging from semi-detached bungalows to a substantial apartment building organised around a ‘village’ green. Central to the project has been SIG Design & Technology’s role in specifying, detailing and supplying a ‘suite’ of highly compatible roofing systems across the whole development. Daniel Bosworth, SIG Design & Technical Manager, in discussion with Architecture Today’s Technical Editor John Ramshaw, explores the reasons behind the material choices, what benefits they bring, and how key details have been resolved.

The Lowfield Green residential development in York is due to complete in 2021 (CGI used with kind permission of City of York Council and @ShapeHomeYork)

How did SIG become involved in the project and what is its relationship with the main contractor?
SIG Design & Technology has a longstanding professional relationship with Wates. It brought the project to us because we provide a comprehensive design service and are able to offer a collateral warranty to the client. The latter ensures that the roof design is robust and can be guaranteed in terms of the products and material build-ups. Furthermore, if there are any issues or problems, the client can contact SIG directly to find a quick solution. We were also able to provide a single-point guarantee for the roofing systems in terms of product longevity and workmanship.

Lowfield Green apartment building with zinc roof (CGI used with kind permission of City of York Council and @ShapeHomeYork)

What was SIG’s brief and how did it develop the detail design of the roofs?
The brief was to provide a cost-effective and robust design for the development’s zinc and flat roof areas. Starting with the flat roofs that link the semi-detached houses, these had originally been specified with cold-applied liquid waterproofing. SIG proposed its SIGnature Torch On Bituminous membrane as a more resilient alternative that would provide better value for money. Specified with a plywood deck and laid to 1:40 falls, the system is also fire-rated to achieve BROOF(t4).

Detail section showing fully encapsulated house parapet (detail: SIG Design & Technology)

Full encapsulation of the houses’ flat roof parapets formed an important part of the design process. The architect’s original detail showed only a capping system at the top of the parapet for waterproofing. SIG opted for a more robust approach, which takes the waterproofing membrane from the flat roof and runs it up the vertical face and across the top of the parapet to the outside edge. The parapet is then closed off using a SIG A2 Versapanel Cement Particle board with a galvanised metal support. Again, this detail maximises toughness and longevity.

Detail section showing gutter condition between sloping zinc roof and single ply flat roof (detail: SIG Design & Technology)

Moving onto the apartment block’s zinc roof, the original specification showed a concealed 200mm wide zinc gutter transitioning into the surrounding cold-applied liquid flat roof areas. Our solution was to use a concealed IKO Armourflow 1.2mm laminated metal gutter between the elZinc Rainbow Red zinc roofs and IKO Armouplan PSG bonded single ply flat roofs. By using the same ‘family’ of products we were able to eschew the need for a drip edge and tricky installation whereby the roofing contractor would have had to squeeze into the 200mm wide diameter gutter in order to carry out the workmanship.

The revised detail shows the single ply membrane fixed in place by the Armourflow gutter and then fully sealed with a hot air welded IKO Armourplan P membrane strip. In essence, the gutter and the flat roof form a single, seamless and easy-to-build waterproofing detail. These is also a cost-saving associated with eliminating the zinc gutter, which was always concealed and therefore of no aesthetic value.

Detail section showing fully encapsulated apartment block parapet (detail: SIG Design & Technology)

SIG also designed a robust fully encapsulated parapet detail for the apartment block. Crucially, this includes a separation layer between the PVC-based waterproofing (taken up and over the top of the parapet) and the EPDM capping system, due to issues of material incompatibility.

Detail section through apartment balcony showing balustrade and pitch pocket (detail: SIG Design & Technology)

For the apartment balconies, the architect had again specified cold-applied liquid waterproofing. However, Wates was concerned about the durability of this solution with regards to residential foot traffic. SIG proposed a fully bonded SIGnature Torch-on Bituminous membrane laid on a screeded-to-falls concrete deck for reasons of cost-effectiveness and robustness. The architect had detailed the balcony balustrade, but again Wates was concerned about long-term weather performance. SIG was able to provide a tougher solution by suggesting galvanised steel pitch pockets, which will be filled on site with a liquid IKO Permatec Compound to fully encapsulate the balustrade base plates. In common with other areas of the roof design, both the SIGnature Torch-on Bituminous membrane roof and IKO Permatec Compound pitch pockets are fully compatible, resulting in a multiple product application with a reliable guarantee.

What advice would you give architects that are wanting to benefit from working with specialist suppliers, such as SIG?
Seeking early engagement with SIG, ideally at the initial design stage, is one of the most important points to remember. This will allow us to contribute to the roof details as they are produced, rather than when the project is on site. SIG is also product agnostic, so it can make recommendations for changes to roof designs and specifications based purely on performance and cost grounds. Overall, our approach is to work with the project team, providing the right product(s), expert design advice, proper installation ­– through the SIG Design & Technology Accredited Contractor Scheme (DATAC) ­– and reliable guarantees that minimise risk.

Contact Details
For more information please visit the SIG website.