Are two layers of No. 15 underlayment the same as one layer of No. 30 underlayment?
No. Two layers of underlayment are referred to as a double-layer of underlayment and there is a 19 inch overlap between layers. One layer of underlayment is called a single layer of underlayment and there is a two inches of overlap between the layers. NRCA recommends a double layer for roof decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees)