15 lb. covers twice the area of 30 lb. felt. Since they came out with the new stuff, don't remember what it's called, I haven't used roofing felt. The other stuff takes SO many fewer fasteners, is light weight, doesn't tear anywhere nearly as easily (actually almost impossible to tear), gives much better traction for walking on, and doesn't make tools and clothes dirty. It is some more expensive, but so worth it.