EXTERNAL ENVELOPE 65
Roofs also play a key role in limiting heat loss from a building. For new build roofs, the Notional Dwelling/Building specifi cation (contained in the relevant Approved Documents or Technical Handbook) provides a good starting point when deciding what U-value should be achieved. On retrofi ts, the worst case U-value the roof should achieve will depend on whether it is just the existing waterproofi ng system being replaced (roof refurbishment) or the deck itself is being replaced (full re-roof). The regulations in England, Scotland and Wales all provide some fl exibility in these U-values for refurbishment projects to allow for potential refurbishment challenges that the projects can pose. In the case of fl at roof retrofi ts, careful inspections will also be needed to confi rm the condition of the existing deck, waterproofi ng system, and any insulation that is present. If any of these are retained, they must be sound and capable of supporting the dead load of the insulation, and the potential condensation risk should be considered by an expert. In all cases, site teams will need to ensure the deck is in good condition before the insulation is fi tted, with care taken to ensure the fi nished system falls to drainage points.
Tapered solutions
The traditional approaches to establishing a fall on a fl at roof are either to lay screed to a fall or to install angled timber fi rrings beneath a plywood deck. The insulation layer is then fi tted above this, in a separate operation. This can be labour and time intensive, require careful planning from the roofi ng contractor to ensure the correct fall, and can lead to substantive constructions with the thickness of insulation needed to achieve the required U-values. Alternatively, tapered insulation systems can help to streamline processes, ensure correct drainage and reduce the overall depth of the construction. These systems typically comprise three types of rigid insulation boards: tapered, hip and valley, and fl at ‘packer boards’ (which sit below the other types of board). In some cases, systems can be mechanically fi xed, meaning no time is lost waiting for the system to dry – and a much lighter alternative to screed. In addition, as the insulation boards themselves are used to create the fall, the overall system thickness can be slimmer than with the alternative approaches. Most tapered systems are manufactured from PIR
ADF OCTOBER 2025
insulation; however, some manufacturers provide them with vacuum insulation panel (VIP) packer boards, achieve much lower thermal conductivities than standard PIR packer boards and can make it possible to achieve the necessary U-value with a reduced depth of insulation. To ensure tapered systems provide both the required U-value and fall, manufacturers typically offer dedicated tapered design services. For new build roofs, these designs can be developed based on the roof plans. For refurbishments, the manufacturer may offer to survey the existing deck. The surveyor will review key aspects, for example, the position of features such as roofl ights and the location of the rainwater outlets which the system will need to be designed to fall to. From this, the designers can then develop a tailored system layout which will provide effective drainage and meet your target area weighted U-value with the slimmest possible construction. The design will also typically include a condensation risk analysis.
A clear way forward
Ensuring what is designed is built is a major hurdle on any project. The tailored design approach for tapered roofi ng systems means architects and specifi ers can have greater confi dence that the fi nished system will match the design, and deliver on the key performance targets.
Matthew Evans is director of technical and regulatory affairs at Kingspan Insulation
WWW.ARCHITECTSDATAFILE.CO.UK
Ensuring what is designed is actually built is a major hurdle on any project
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