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Permo extreme RS SK2 from Klober – a leading European manufacturer of high-performance roofing membranes, ventilation and accessories – is a waterproof, tear-resistant underlay, designed for use on low pitched roofs.

Suitable for profile tiles pitched at 12.5o or more, 15o for slates and 22.5o  for plain tiles; it is manufactured from a bondable two-layered underlay with a shrink-resistant TPU coating on a robust PES (polyethersulphone) fleece which allows vapour to escape, while keeping rain water out.

Double self-adhesive edge strips provide a permanent seal and immediate weatherproofing against wind, rain and snow; while the PES fleece also incorporates a capillary stop to control the formation of condensation. This means that, unlike traditional vapour permeable underlays which provide limited weather protection if left, Permo® extreme can provide a temporary weatherproofing layer if required.

With a high-temperature stability that peaks at 100°C and due to the fact that it is very UV-resistant, it also means that Permo extreme is highly suitable for installation under in-roof system solar panels.

Another benefit of Permo extreme is that it dispenses with the need for plywood or OSB support boards and counter battens – which the company estimates saves at least £8 per m2 in materials at least, not to mention time and labour costs.

This ability to self-support was established through in-depth testing at the Building Research Establishment (BRE). In the BRE tests, Permo extreme RS SK2 was laid at 12.5 degrees with no slates or tiles on top of it and a 10-to-15mm deep drape between rafters, in line with the current version of BS 5534. The underlay was fastened with tacks and 50mm Butylon tape fixed down the rafters to seal any penetrations. Battens were installed with fixing nails through the tape and the next layer of Permo extreme laid with a 100mm headlap so adhesive strips met.

The first test was a rain deluge designed to create a one-in-fifty-year condition. The second combined wind, rain and suction from underneath, with all three elements increasing every five minutes over a 55-minute period, again mimicking a one-in-fifty-year condition.

For context, most interlocking tiles start to leak at between 25 and 30 Pascals, while Permo extreme was tested at over 110 Pascals – without any water ingress at all.

It’s also the ideal choice for those seeking a bondable underlay for a rainproof sub-roof. Not only is it tear-resistant, highly weatherproof and avoids the need for sarking boards, it also – when laid as a key primary barrier with its associated accessories – comes with a 10-year guarantee providing that it has been installed in accordance with Klober’s installation instructions.

Permo extreme RS SK2 is complemented by a range of accessories such as Permo extreme sealant which allows connection to rising structural components and eave flashings; an EPDM sealing collar suitable for the  sealing of pipe penetrations; and Butylon tape, which is ideal for creating seals around nail penetrations.