lb vs 30 lb felt, when do I use which?
On standard 4 in 12 or greater sloped roofs the UBC (Uniform Building Code) for California requirement is a single layer of 15 lb ASTM rated felt underlayment. If the slope is less than 4 in 12 but greater than 2 in 12, a double layer of 15 lb is required. The felt shall be installed shingle fashion starting from the bottom edge with an 18 strip being covered with a 36 strip then sub-sequent strips installed going up the roof at 18 increments. We find that 30 lb felt is an unnecessary expense when installing over plywood sheathing, but over 1 x 6 or any other board sheathing, although not required we recommend a 30 lb to help smooth and protect the more imperfect surface.
