When choosing a roof, many carefully choose their roofing product but ignore the underlayment. The underlayment is a thin layer that is applied under the roofing product you see but on top of the sheathing. The purpose of this application is to serve as a vapor barrier, preventing leaks and rot. The underlayment is a very important part of the roofing system.
The most common underlayment is felt paper. This is impregnated with asphalt and comes in various thicknesses. ASTM rated D226 #30 felt is the thickest rated felt paper. There are also many synthetic underlayments. synthetics are lighter weight but many roofers don’t like them because they can be slick to walk on and be a hazard. They are also newer on the market and not as tested.
My material supplier just informed me of a new product by CertainTeed called Diamond Deck. CertainTeed is advertising that this new product is “contractor friendly” because it is lightweight, slip-resistant, and cost-effective. We will be taking a look at it soon to determine if this is a better product than the D226 #30 felt which we currently use. So far, we haven’t jumped on board the synthetic underlayment craze as we like to stick with what works. However, with a trusted brand like CertainTeed, which is our brand of choice, we will consider it.