One contractor’s bid includes No. 15 underlayment and another contractor says he only uses No. 30 because its the best. Who is right?
For asphalt shingles, National Roofing Contractors Association (NRCA) recommends a single layer of No. 15 asphalt-saturated underlayment be used with roofs having slopes of 4:12 (18 degrees) or greater. For roof slopes between 3:12 (14 degrees) to 4:12 (18 degrees), NRCA recommends a minimum of two layers of No. 15 underlayment. If you are installing a heavier-weight shingle with a projected long service life, using No. 30 underlayment instead of No. 15 would be appropriate.