Generally speaking, roofing felt is a felt paper that is soaked in asphalt and other water resistant compounds to produce a membrane. Of course, it is a bit more technical than that, but this is the basic idea behind how felt works. Felt is commonly sold in either 15# or 30# rolls measuring 36 inches wide. Long ago, a 15# role actually weighed 15 pounds, but over the years, that has changed.
Synthetic underlayment was developed as a felt replacement. By weaving together polypropylene and polymer, a water resistant and vapor resistant underlayment was created. This new underlayment is lighter and stronger than its predecessor.
To decide which underlayment is the optimum choice for your project, you first have to understand the actual uses of underlayment and what they can and cannot do:
Not every home needs the best roof underlayment available on the market and not every budget can afford it. A standard asphalt shingle installation does not require the same vapor protection that might be needed for a metal or slate roof, but a steep roof installation might be safer with the better walking surface provided by some synthetic underlayment.
One of the main barriers to using synthetic underlayment for roofing contractors is the cost is higher per square foot than traditional felt.