Traditionally, 15lb or 30lb tar-treated felt paper is installed between the plywood roof deck of your home and the actual shingles as a vapor barrier. The material is relatively cheap to produce, easy to install, and is highly resistant to corosion and mold. However, felt as a vapor barrier may not be the best option where maximum protection and roof lifetime are concerned.
While felt does have some fantastic properties, it has one major flaw as well: it tears very easily. This would not be a problem, one would think, if the installers are gentle with the material, but tar treated felt paper can develop small tears even after the shingles have been installed—especially if new plywood roof decking has been installed. After roof installation, the plywood decking beneath the roof will go through some changes; the wood can expand, shirnk, or warp deppending on environmental factors such as humidity and temperature. As the plywood changes, the fasteners used to hold down the felt paper and shingles can put stress on the felt due to the movements in the plywood, causing small tears that can let moisture through to the roof decking. As the roof decking becomes saturated with moisture over the years, it will continue to warp resulting in more underlayment tears and eventually leaking.
But this is all assuming that the felt was not torn at all during installation, which is nearly never the case, no matter how expert the installer. Even small tears caused by stepping on nails or dropping a hammer can compromise the integrity of your roof. Issues with felt paper take several years to manifest themselves, so when choosing a roofing contractor be sure to ask what types of underlayments are available.
DiamondDeck is a synthetic underlayment manufactured by CertainTeed. It is waterproof, enviromentaly friendly, and nearly impossible to tear. Where felt underlayments tear easily, DiamondDeck is strong enough to withstand serious abuse—it’s like a roof under your roof! Masters Roofing suggests DiamondDeck on all new roof installations. Not only does it give you peace of mind about the integrity of your roof, DiamondDeck is also very affordable; the cost difference between traditional underlayment and DiamondDeck is marginal.