If you're not a roofing professional, you may not be familiar with the term underlayment. It's simply the material used between the roofing on top, such as asphalt shingles, and the plywood deck below that gives the roof its structure. Underlayment protects the wood from any leaks that spring up between the shingles and prevents damage during windy and icy conditions. Depending on the roofing material you choose and where you live, you may need high temperature underlayment for a roof that lasts the years.
Your usual asphalt coated felt can't handle the hot temperatures that occur under metal roofing. While metal roofing can be coated with white and reflective coatings to reduce their temperature, there's still plenty of heat that becomes trapped between the roofing and the wood below it. The wood itself may not react much to this kind of heating, but the underlayment is prone to breakdown after years of daily baking during the summer. Metal roofing is very durable and requires little maintenance, so it's worth the investment in higher quality underlayment materials rather than rethinking your plan of using metal roofing.
Southern and Southwestern Homes
In some parts of the country where summer temperatures soar above the 100 degree F mark, you need high temperature underlayment regardless of what type of roofing material you use. Most of the homes that fall into this category are found in the deep South, such as most of Florida and southern Louisiana, or in the Southwest. The summer temperatures of the deserts of Arizona and Nevada are very hard on asphalt based materials like underlayment felt. Yet even advanced rubberized underlayment products can break down when there are back to back heat waves. High temperature underlayments are stable to temperatures up to 240 degrees F or higher, which your roof can reach during a heat wave.
Adhesive and Film Strength
High temperature underlayment needs to resist heat in two different ways. First, most modern underlayments are designed as self-adhesive products to speed up installation and limit the amount of nail holes made during installation. This means that the adhesives used must withstand the high temperatures on the roof without melting, breaking down, or allowing the material to slide around and bunch up. On top of adhesive strength, the film itself must withstand the wear and tear of heating up and cooling down hundreds of times over the course of each year. Woven polypropylene materials are much more durable than organic or synthetic felts.