The short answer
Synthetic underlayment is stronger, lighter, lays flatter and resists water far longer than felt if the roof is left exposed. The cost difference across a whole roof is small. Felt is still used to shave a quote, and it is one of the first things to check when comparing bids.
Side by side
| Factor | 15 or 30 lb felt | Synthetic |
|---|---|---|
| Tear resistance | Low, tears at fasteners | High |
| Weight | Heavy rolls | Roughly a quarter the weight |
| Exposure tolerance | Hours to days | Weeks, product dependent |
| Wrinkling | Absorbs moisture and buckles | Lays flat |
| Walkability | Slippery when damp | Textured, safer |
| Cost across a roof | Lower by a few hundred | Slightly higher |
Choose 15 or 30 lb felt if
- Budget is genuinely the deciding factor and the roof will be dried in and covered the same day
- A specific manufacturer system calls for it
Choose Synthetic if
- You want the roof protected if weather interrupts the job
- The roof is steep, where tear resistance and footing both matter
- You want it to lay flat so the shingle plane is true
- You are installing in a wetter season
What this looks like in the Hill Country
Texas weather makes this more than academic. A front can move in mid job, and a roof dried in with synthetic handles a delay far better than one covered in felt that has already absorbed moisture and buckled. Synthetic is standard on our installs and we would push back on any quote here that specifies felt to hit a number.
