Risk of exposure

Relying on underlays to waterproof unprotected roofs can expose buildings to damage from water and wind. It’s time to protect the roof at all costs!

Roofing is now a year round business. The good news is that, while work is going on, vapour permeable underlays (VPUs) provide immediate, though temporary and limited, weatherproofing protection for the roof. However, far too much reliance is being put on them to resist gale force winds, rain and snow before the tiles are fixed. With reports from insurers confirming that claims for leaks on roofs with no more than a VPU to protect them  are increasing.

Manufacturers, trade associations and organisations such as the NHBC work extensively to improve standards across the industry and VPUs are a case in point. While the introduction of VPUs on the market removed the need to fix heavy, bituminous underslating felts, we should beware of expecting these excellent products to do the work of the tiles themselves. Of course, we should expect them to deal with the high amount of moisture which has to escape during a building’s drying out period (even if rafters and battens become wet), however there is a world of difference between that and protecting against torrential rain and storm force winds.


It’s inevitable there may be times when, having stripped the roof of tiles, there is a delay while we wait for the new ones to arrive. That’s why it’s vital to consider when exactly you start the job and factor in how long you’re going to have to leave the underlay exposed; when the roof is left uncovered the underlays start to degenerate – leave them for too long and that’s when problems can start. Ultimately, once the decision to start a job has been made it’s your job to protect the roof while the work is being done. Meaning you should always fix the slates or tiles quickly and if that’s not possible, protect the roof at all costs.


While it’s vital to fit the roof tiles as quickly as possible, a high quality VPU can give you some breathing space (no pun intended!). These days the extent of weatherproofing quoted by manufacturers covers UV and light rain and is generally around 3 months. Even so, with laps left open or if there is too great a drape between rafters a VPU may lift in high wind – leading to leaks. VPUs also have a saturation point  and heavy rain will cause it to  be reached pretty quickly. The  height and exposure of a property  will also increase the risk.

A product such as Klober Permo Extreme RS SK2 is the exception to the rule. You can use it as a waterproofing layer, as it’s designed for extension roofs below a tile’s recommended minimum pitch. The integral edge tapes will protect the roof space for up to 8 weeks.

This means that, whatever the reason, if there’s a delay in recovering the roof you do at the very least have a VPU in place that does what it says on the tin. That can mean the difference between a very successful outcome and a serious headache. Use a quality VPU and keep your risk of exposure to a minimum.