With ensuring effective drainage and insulation being essential for all flat roof projects, Matthew Evans of Kingspan Insulation explores how tapered systems can be used to meet both requirements, and adhere to designs.
As we head towards the winter months, flat roofs across the country are set to be put to the test from wind, rain, sleet and freezing temperatures. When working on any project involving a flat roof, whether it be a new build or refurbishment, ensuring the surface will drain properly is essential for the lasting health of the roof. At the same time, these constructions also need to be effectively insulated to limit the heating demand for the building. Tapered insulation systems combine both insulation and a fall for drainage in a simple, lightweight solution which is designed especially to meet the demands of your project.
Despite their name, flat roofs are laid to a slight angle (fall), which allows any moisture to drain down to internal or external guttering and be channelled off the roof. The finished fall should be at least 1:80 in all areas of the roof, including at the hip and valley joints. Where the fall is either insufficient or fails to channel water to the correct points, it can lead to water ponding on roofs. This standing water can contribute to mould and algae growth, increase the thermal stress on the waterproofing layer during freeze thaw cycles, and add additional weight to the deck – potentially causing it to deflect over time, shortening the lifespan of the roof.
Roofs also play a key role in limiting heat loss from a building. For new build roofs, the Notional Dwelling/Building specification (contained in the relevant Approved Documents or Technical Handbook) provides a good starting point when deciding what U-value should be achieved. On retrofits, the worst case U-value the roof should achieve will depend on whether it is just the existing waterproofing 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 flexibility in these U-values for refurbishment projects to allow for potential refurbishment challenges that the projects can pose.
In the case of flat roof retrofits, careful inspections will also be needed to confirm the condition of the existing deck, waterproofing 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 fitted, with care taken to ensure the finished system falls to drainage points.
Tapered solutions
The traditional approaches to establishing a fall on a flat roof are either to lay screed to a fall or to install angled timber firrings beneath a plywood deck. The insulation layer is then fitted above this, in a separate operation. This can be labour and time intensive, require careful planning from the roofing 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 flat ‘packer boards’ (which sit below the other types of board). In some cases, systems can be mechanically fixed, 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 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 rooflights 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 roofing systems means architects and specifiers can have greater confidence that the finished system will match the design, and deliver on the key performance targets.
Matthew Evans is director of technical and regulatory affairs at Kingspan Insulation