Synthetic felt is impermeable to water. Therefore making it waterproof.
When a roof has been felted in, and the shingles have not been installed, the roof is considered to be dried in. The felt is not a guarantee that the roof will not leak in the event of rain, but it is possible to leave a roof felted in with either a 15 lb felt 30 lb felt or a synthetic underlayment as the protection for many months.
Will Underlayment stop the rain? Yes, Underlayment, if it is installed correctly, will stop the rain. But we do not recommend this because it is possible for the Underlayment to blow off of the roof.
Synthetic felt is impermeable to water and can be used as a tarp for a roof.