Roofing felt has always been the underlayment of choice for many roofing contractors and do-it-yourselfers for many years for the use under metal roofs. It's low cost and familiarity with the product is why it has been around for so long. The two main functions of a roofing underlayment is to keep the roof deck dry before the metal roofing material is installed and to act as a secondary weather barrier in case the roof fails for some reason. One product that is quickly replacing the standard 30# felt paper is a line of synthetic underlayments. Their benefits greatly outweigh that of 30 # felt. The biggest benefit of a synthetic underlayment is its heat tolerance.
The temperature under a metal roof can get very hot and over time, 30# felt which is composed of asphalt will dry out quickly, leaving not much in the way of a secondary weather barrier. 30# felt doesn't come with any kind of warranty as opposed to synthetic underlayment which can come up to a 50 year warranty. Other benefits include better slip resistance, pre-printed nailing pattern and overlap lines to speed installation, and lays very flat on hard roof areas like ridges, valleys, and hips. Other benefits of a synthetic roof underlayment are:
- 10 times lighter than 30# felt
- 10 squares per roll versus 2 square roll of felt (less trips up the ladder)
- Can be left exposed to for up to 6 months with not degrading as opposed to felt which practically starts to degrade immediately
- Class A fire rating. Felt doesn't have a fire rating
- 20 times stronger than felt
- Can save up to 50% time in labor installation
You would think a product that seems so superior would be a lot more money to buy but that is not the case. For the most part, you can expect to pay maybe 5%-10% more for synthetic underlayment than 30# felt but for the benefits you gain with using synthetic underlayment, 5%-10% is hardly not worth paying a little more for a better product. Why not install a better roof underlayment with a superior metal roof?