Underlayment and Metal Roofing
I often get asked what the importance is of underlayment beneath a metal roof. What does it do and is it really necessary? Interestingly, over the past several years, we have seen significant changes in the types of underlayments being used. The typical underlayment being used today is a low weight polymer based synthetic underlayment which lasts longer and is easier to install than the old asphalt based felts or tar papers. We also see ice and watershield underlayments being used where required by code, usually on the roof perimeter and up the valleys. In all cases, underlayments beneath metal roofs should have smooth, non-granulated surfaces so they do not scratch the backs of the metal roof panels. In warmer climates beneath metal roofs, especially metal roofs that have direct contact with the underlayment, high temp underlayments are commonly called for. And, even if the metal roof is installed on battens or purlins, there should still be an underlayment on top of the roof deck beneath the battens. So, why are underlayments necessary? First, they are required by building code. My interpretation of the code also requires them even if the old shingles are left in place. Next, they provide a good surface for the roofing installers to work on. They also protect the structure from water intrusion before the roof project is completed. And, while condensation on the back side of metal roof panels is not a common thing, they will keep that condensation if it does occur from reaching lumber or getting inside of the building. Finally, if the roof should have a flashing or something that occasionally in certain extreme weather conditions bleeds a little water, a quality underlayment will prevent that water from getting into the structure as well.