Changes to Building Regulations have recently altered the guidance for SIPS panel installation for flat roofing, in that these are no longer recommended. In line with guidance from the Structural Timber Association in conjunction with BS6229:2018, the majority of national roofing contractors have completely banned SIPS projects.
What is a SIPS Panel?
SIPS stands for Structural Insulated Panels, and these are prefabricated, lightweight components used in the construction of floors, walls, and roofs for both residential and commercial buildings. Each panel comprises two high-density facings, most commonly Oriented Strand Board (OSB), which are bonded to either side of a low-density, cellular foam core. The typical construction involves sandwiching a rigid foam plastic insulation core between two structural skins. In the UK, SIPs are available with a variety of insulation cores, including expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanurate (PIR), and polyurethane (PUR). OSB is generally used as the outer facing material. These panels are manufactured in tightly controlled factory environments and can be tailored to specific design requirements.
Where does this impact the roofing industry?
According to BS6229:2018, a SIPs panel is defined as a form of structural deck suitable for flat roofing, and has previously been categorised as a “hybrid” roof system. The standard highlights the importance of exercising caution when specifying this type of deck, due to the challenges involved in creating a continuous Air and Vapour Control Layer (AVCL) beneath the panel. Particular attention is required around penetrations and joints, where maintaining continuity is essential to prevent the build-up of moisture.
What are the changes to SIPS panel guidance for flat roofing?
Some manufacturers of flat roofing membranes, such as Sika, have in the past, allowed waterproofing systems to be applied directly onto SIPs panels. This approach has been especially popular with single ply membranes, as these can be profiled to mimic the appearance of standing seam or lead roofing at a significantly lower cost. However, as the use of this method became more widespread, the industry began to see numerous failures, largely attributed to condensation issues.
In response to these failures, due to condensation issues, the Structural Timber Association has issued a guidance note specifically addressing the application of SIPs, outlining appropriate usage and installation practices.
Notably, the association now reclassifies the use of SIPs in roofing applications as a form of “cold” roof construction. It also advises that membranes should not be applied directly to the panel surface. While the introduction of firrings to create a ventilated void between the membrane and the panel is technically allowed, this solution is often seen as impractical and economically unviable, particularly when considered alongside the guidance in BS6229, which limits cross ventilation to a maximum of 5 metres.
If clients request this form of construction, it’s important to refer to the guidance note issued by the Structural Timber Association, alongside BS6229:2018, to help inform designers why this approach is no longer recommended. While organisations such as the BBA, SPRA, LRWA, and NFRC are likely to update their own guidance in due course, our intention, here at Enviroply, is to lead the way by adhering to the most current and appropriate recommendations.