How It Is Made
The building paper used for roofs is made from a virgin kraft paper. It is not actually felt. Asphalt is impregnated into the kraft paper, providing an excellent barrier for moisture. The kraft paper consists of long fibers that allows it to be lighter weight than felt. This property allows the kraft paper better mechanical properties.
How It Is Rated
Paper is grated through the use of a minute rating system. A piece of the paper is placed on water to float like a boat. There is a chemical within the paper that is sensitive to moisture. The sample is timed to see how long the chemical reaction takes. The longer it takes, the higher the rating.
Grade D felt paper is available in 10, 20, 30 and 60 minute ratings. The higher the rating, the heavier the paper. A 60 minute paper will weigh approximately 6 pounds per square. A 30 minute paper will weigh 3.3 pounds per square. The 20 minute paper will weigh approximately 3.3 pounds per square.
Grade D roofing felt is lighter than regular roofing felt. The cost is more economical. Application is easier due to the lighter rolls. The Grade D roofing felt density is less than regular roofing felt, which allows for an ease in cutting.
Grade D felt paper is thinner than normal roofing felt. This causes the Grade D paper to be less resistant to moisture. Roofing felt will provide a thicker, heavier and better resistance to water at a higher cost.