Avoiding potential underlay problems

Avoiding potential underlay problems

Mat Woodyatt, technical training manager of BMI UK & Ireland, looks at the consequences of allowing underlay to drape into gutters.

 There was a time when this particular error was seen everywhere on the eaves of pitched roofs. To ensure the flow of rainwater from the underlay to the gutter, it would be left to overhang the roof and draped into the gutter itself.

It seemed like a good idea at the time, yet it was a costly mistake. Most will now know that draping underlay into a gutter allows liquid to be drawn back into the roof. It will degrade the bottom edges of the membrane itself, eventually start to rot the fascia boards and ultimately will create an area of negative fall and ponding beneath the first few courses of tiles.

Contractors have spent years repairing and replacing underlay, facias – and even supporting timbers – thanks to the damage caused by this simple oversight. It’s taken some time, yet on the whole the sector has come to learn this lesson, and underlay support trays are now commonplace in the market. A simple low-cost solution to this problem, an underlay support tray creates a solid base for the bottom edge of the underlay to sit upon and replaces the old underlay drape as a method of allowing rainwater to drain off from the roof into the gutter.

Because it does involve some extra work and some (marginal) extra cost, there will still be a few types who choose to work in the old-fashioned, harmful way and one still has to inspect roofs where the drape is in place.

So please don’t take this shortcut. It is not only highly damaging to the long-term performance of the roof, it also highly noticeable, and is almost certain to be picked up during any kind of sign off or inspection, not to mentioning spoiling the overall look of the job.

In this day and age, where all roofs are fully fixed and all perimeters such as the eaves are twice fixed, this is no longer a simple solution to rectifying this problem. Having to disturb an entire double fixed row of tiles could result in having to not only supply new trays and tiles, but also fascias as a result of damage caused by existing penetrations. Use underlay support trays to avoid this unnecessary headache.

roofing BMI underlay support trays




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