 In fact, #15 mats can weigh from 7.5 to 12.5 pounds/sq depending on the manufacturer and the standard to which felt is made (i.e., CGSB, ASTM D227 Standard Specification for Coal-tar saturated Organic Felt Used in Roofing and Waterproofing, ASTM D4990, Standard Specification for Coal Tar Glass Felt Used in Roofing and Waterproofing, or none).Thirty-pound (30#) felt is now #30 felt, and usually weighs 16 to 27 pounds per square.Tar paper is more accurately a Grade D building paper (the Grade D designation derives from a federal specification in the United States), which is widely used in the West.