Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses the approaches to and benefits of working with a specialist supplier on roofing designs with Architecture Today’s Technical Editor John Ramshaw

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Roof design forms a critical aspect of most construction projects. The finished structure must not only be weathertight, adequately insulated and durable, but in many cases, it also has to look right and incorporate aesthetically pleasing details. While the underlying principles of good roof design have changed little over the years, the proliferation of new products and systems has made specification and detailing increasingly complex and onerous ­– particularly in the case of multi-material roofs. So how can architects ensure they are specifying the right product for the right application? And how do they meet the needs for robust detailing, correct onsite installation, and dependable guarantees? Daniel Bosworth, Design & Technical Manager at SIG Design & Technology, discusses these questions and more with Architecture Today’s Technical Editor John Ramshaw.


London Business School comprising 41 roof areas totalling 2681m² (ph: Adam Coupe Photography)

What are the benefits of using a specialist supplier, such as SIG Design & Technology, when it comes to roof design?

First and foremost, SIG Design & Technology is ‘product agnostic’. This means that we can recommend the right roofing system for the application from an extensive portfolio, which includes cold applied liquid offerings, hot melt products, single ply membranes, bituminous felt and GRP. We would never propose a waterproofing solution where the product used was not completely fit-for-purpose or robust.

Specifiers should avoid relying on a sole manufacturer for all their roofing requirements as they are likely to recommend their own products exclusively for any application. I’ve seen projects where the architect has specified a single ply membrane manufacturer for every roof surface, including plant areas with multiple penetrations. Here, SIG would recommend changing the specification for these locations to a more robust solution, such as an inverted hot melt roof or a multi-layer felt build-up.


Some of the other benefits of working with SIG are outlined in the company’s useful guidance document: 8 Steps to the Perfect Roof. They include design expertise, confidence in the supply chain (SIG has a national network of more than 120 outlets and a fleet of hundreds of vehicles), full compliance with statutory requirements, and experienced contractors that are fully trained in the products they use.

The SIG Design & Technology Accredited Contractor Scheme (DATAC) provides assurance that our roofing systems are installed correctly and wont leak! Furthermore, we have a team of field technicians who can monitor installations on site. Post-installation, SIG can provide an appropriate planned maintenance regime to ensure the validity of our roofing guarantees.

Re-waterproofing of 1950s concrete shell roof at Plymouth Market (ph: Terry Smith)

Why is it important to seek early involvement with a trusted specialist supplier?

Early engagement with SIG means that we are able to check that the roof specification is adequate, fit-for-purpose, and can be guaranteed. I’ve seen, for example, a proposed build-up where the architect specified extruded polystyrene insulation for a warm roof application. Typically used on inverted roofs, XPS insulation is not as thermally efficient as warm roof insulation, and therefore should not be used on this type of application.

It is important to spot potential errors and problems early on before the roof design gets too advanced or the project goes to site. As a trusted advisor, SIG’s role is to help, advise, and if need be, make changes to the specification. Overall, early engagement provides the benefit of working in accordance with British Standards, which results in more robust roofs designs.

Mastic asphalt re-roofing of Westminster Cathedral in London (ph: IKO)

What are the key issues architects should consider when designing multi-material roofs and how can SIG Design & Technology help with this?

Specifiers must ensure that they have a robust, recommended and approved detail for every interface. Careful consideration should also be given to material compatibility. For example, linking or bonding windows and doors with EPDM seals to a PVC single ply waterproofing layer needs to be highlighted and detailed appropriately, particularly with regards to adhesive compatibility. Roof penetrations and balustrade fixings also require close attention, with SIG generally recommending proprietary roof products, such as those made by Nicholson.


Grade II listed ogee dome at the former Bristol General Hospital roofed in Armourplan PSG single ply membrane with profiles to replace the original lead roof (ph: Terry Smith)

What are the advantages of using a single source of design expertise, and how does this affect onsite installation and material/system guarantees?

The main advantage is a single point of contact and responsibility. Everything is covered by one party, instead of multiple companies with different opinions and interests. SIG can supply a wide range of products and is fully aware of how the British Standards and codes of practice apply to each of them. It also has expertise in product compatibility and interfacing where multiple roofing materials are used on a single construction project.

Balcony liquid waterproofing for 116 apartments at Riverwalk in Pimlico, London (ph: Adam Coupe Photography)

Many manufacturers provide design and specification services for their own products, but if they are unable take responsibility for the whole roof, there is the potential for clashes between their elements and those of other suppliers. This in turn can lead to crude or unsightly details that are more susceptible to failure. By contrast, SIG is able to provide a one-stop supply solution covering design, installation and guarantees. Added to this, product interfaces are covered by SIG’s design, eliminating potential disputes between different roofing manufacturers working on the same project. And if anything does go wrong, specifiers have a single point of responsibility to ensure a quick resolution.

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Click the link to read SIG’s informative guide: 8 Steps to the Perfect Roof. For more information please visit the SIG website.