Christa Coe, Lead Validation Manager at SIG Design & Technology, discusses the do’s and don’ts of multiple material roof design with Architecture Today’s Technical Editor John Ramshaw

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Multiple material roofs can provide a wide range of benefits – not least increased building performance, reduced costs and improved aesthetics. Unfortunately they can also add complexity and risk to construction projects, particularly for architects and their clients. So how do specifiers ensure that these types of roof are materially compatible, correctly detailed, properly installed and adequately guaranteed? Christa Coe, Lead Validation Manager at SIG Design & Technology, discusses these questions with Architecture Today’s Technical Editor John Ramshaw.

AT: What are the benefits of multiple material roofs?
The main benefit is that they allow architects to combine different roof forms, such as curves, pitches and flat elements within a single development. This is important not only in terms of achieving the overall design objective, but also for ensuring that the materials used are fit-for-purpose. For example, liquid roofs would not be advisable on sloping areas due to the risk of slump. However, they might be favoured over say, felt systems, for overlooked flat areas due to their seamless appearance. SIG Design & Technology’s approach is to determine the most robust solution for each roof, taking into account aesthetic considerations and of course cost.


Single ply and zinc roof interfaces at Premier Inn Jersey St Helier (ph: SIG)

AT: What are the main issues to be aware of when using more than one roofing material?
One of the most important factors is to ensure that the roofing products are materially compatible. Zinc and copper, for example, should never be placed in contact with each other for reasons of product contamination. Other materials are also incompatible, for example PVC single ply membranes will degrade in contact with bituminous membranes. Aesthetic issues relating to long-term performance and wear should also be considered. Bitumen, for example, can ‘bleed’ over time, potentially staining adjacent materials and fixings.

Multiple material roofs may also require multiple roofing manufacturers and subcontractors. This can have serious implications for interface design and its attendant responsibilities, quality control on site and establishing liability in the case of failure. By contrast, SIG Design & Technology supplies a wide range of different roofing materials from membranes and metals, to liquids and green and blue systems, which are installed by the appropriate Design and Technology Accredited Contractors (DATAC), who are in turn monitored by on-site engineers. This one-stop shop approach to materials, specification and installation ensures high levels of quality while also reducing risk for the client and project team.


Armourplan to hot melt separation curb detail (SIG)

AT: What is the best way for architects to approach multiple material roof design?
Architects should discuss the design of their multiple material roofs with a reputable manufacturer and/or supplier at the earliest opportunity. SIG provides unbiased, product-neutral advice, guaranteeing complete system build-ups. Crucially, it can supply both standard and bespoke details covering all aspects of the roof design. This can be particularly useful when resolving complex multi-material interfaces. In addition, the company can assist with acoustic, wind-uplift and U-value calculations, as well as condensation risk analysis, and cut-to-falls insulation designs. SIG’s material resources and detailing knowledge can also help with budget issues. For example, cost savings on expensive, large-scale zinc roofs can be made by using liquid-applied gutters instead of metal ones. SIG is able to supply both of these products, safe in the knowledge that they are compatible and can be warranted as a complete system.


Tile to SIGNature Torch on System detail (SIG)

AT: What detail conditions should architects pay particular attention to?
Ridges, gutters and upstands are all clearly important, however, it is the interfaces between different materials that are perhaps the most critical areas to address. Typically, where more than one manufacturer and/or subcontractor is involved neither party will be willing or able to take responsibility for the interface design, increasing risk for the entire project team.

Tile to SIGNature Torch on System detail in use (ph-SIG)

SIG works with a range of manufacturers to ensure material and product compatibility at critical junctions. A good example is the detail condition where metal stanchions penetrate a single ply roof deck. The preferred solution is to use a liquid-applied roofing product around the penetrations to ensure a weathertight seal. Approved for use together by both manufacturers, the membrane and liquid roofing products specified by SIG enable it to guarantee the whole roof system.

SIG worked with Nicholson to develop a fixing for Armourplan PVC roofs (ph: Nicholson Rooftrak)

SIG also works with specialist fixings companies, such as Nicholson, to create purpose-designed components for interfacing specific materials and systems. One example is a balustrading anchor designed for Armourplan membrane roofs. The fixing incorporates an Armourplan-faced flange that is ‘welded’ to the roof to weatherproof the junction.


VITA student accommodation in Newcastle combines zinc shingles with anodised aluminium dormers and a stone facade (ph: SIG)

AT: Can architects still obtain a warranty when more than one roof material is used?
Most roofing companies insist on sole use of their products as a warranty prerequisite. However many of these do not have a wide enough range of materials or systems to complete and warranty an entire multiple material roof contract. SIG has the resources and is able to unify the responsibilities for large and small multiple material roofs alike. As the name suggests, the SIG One Warranty Scheme covers the entire roof, providing that a complete system is specified. This protects both the architect and client, while also eliminating potential disputes between material suppliers and installers.

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