Synthetic felt is fabulous for the field of a roof, meaning the straight portions.  It is not very useful for the valleys.  This is an area where we want to use Ice and water shield instead of synthetic felt.

This felt is lapping over the hips and ridges.  The lines of this felt are very straight.  Plastic cap nails are used to hold the synthetic felt onto the roof.

Here is a video explaining how to install synthetic felt on a roof.

It is clear that the best roofing felt is a synthetic underlayment.  This is because of the ripstop performance of the synthetic.  Traditional felts like 15 lb and 30 lb felts can tear much easier as they are tar paper.

A 15 lb felt will have a chance to be torn with 15 kg of sheer applied.  This can and usually occurs when workers walk on a roof with the felt applied.  This is why insurance companies have typically “Upgraded” the felt for steeper roofs to a 30 lb felt.  It requires approximately 30 lb of sheer to tear a 30 lb felt.  The felt is intended to be a second line of defense for the roof, but it is also a surface for the workers to walk on.  So the fact that 15 lb felt can tear easier on a roof than a 30 lb felt makes the heavier 30 lb felt a better underlayment to use on steeper roofs.  

Synthetic underlayment typically comes in 1000 square feet quantities.  15 lb and 30 lb felt come in 400 square feet and 200 square feet r0les.

Litespeed Construction still suggests getting synthetic felt over 30 lb felt for all roofs.

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.

Synthetic underlayment is a very thin material.  It is much thinner than 15 lb felt.  The standard felt, or tar papers, can come up to 90 lb felt type.  This is very thick tar paper or underlayment.  Much thicker than synthetic underlayment.