Short answer
Choose synthetic when the roof will sit exposed for a while, when wind or foot traffic would tear felt, or when you want lighter rolls that cover more. Choose felt for a small roof that gets covered the same day, or where its vapor permeability matters. Use self-adhered membrane at eaves, valleys and low slopes. Whatever you pick, follow the roofing maker’s list of approved underlayments.
The three types
Roof underlayment types compared
| Asphalt felt | Synthetic | Self-adhered | |
|---|---|---|---|
| Made of | Felt mat saturated with asphalt | Woven or spun polymer | Rubberized asphalt on a film or mat |
| Roll | 36 in wide, 72 to 144 ft | About 4 x 250 ft | Usually 36 in wide |
| Exposure | Short: cover it quickly | About 4 to 12 months | Per maker, often limited |
| Fastening | Staples or nails where allowed; cap nails better | Cap nails or cap staples | Sticks to the deck; seals around nails |
| Vapor | Permeable; changes when wet | Most under 1 perm (a vapor barrier) | Vapor barrier |
| Cost per sq ft | About 5¢ for #15 | About 11 to 15¢ | Highest |
Sources: Fine Homebuilding for sizes, exposure and cost; DECRA for vapor and ventilation. Cost figures are the article’s estimates and change with the market.
Problems with synthetic underlayment, and the fixes
- Leaks at fasteners. Plain staples and roofing nails through synthetic can let water wick in. Use the cap nails or cap staples the maker specifies (Fine Homebuilding).
- Trapped moisture. Most synthetics barely pass vapor, so a roof deck over an unvented space can stay damp. DECRA says makers recommend them over vented attics or vented cathedral ceilings (DECRA). Vapor permeable synthetics exist but cost more.
- Slippery surface. Some products are slick when wet or dusty. Most offer better traction than felt, but treat any fresh underlayment with care and use fall protection.
- Not approved by the roofing maker. Some roofing warranties list approved underlayments. Check before you buy.
Problems with felt
- Wrinkles when it gets wet, which can telegraph through thin roofing.
- Tears in wind and under boots.
- Short exposure. Plan to cover it the same day or soon after.
What to buy
-
01Tough
Synthetic underlayment
Best for: exposure, wind, larger roofs over vented attics
- Roll
- about 4 x 250 ft
- Fasten
- cap nails or staples
-
02Cheap
#15 or #30 felt
Best for: small roofs covered the same day; under roll roofing
- Grades
- Type I / Type II
- Width
- 36 in
- Details in the roofing felt guide.
-
03Leak zones
Self-adhered ice and water membrane
Best for: eaves, valleys, low slopes, around penetrations
- Type
- rubberized asphalt
- Install
- peel and stick
- See ice and water shield.
Questions people ask
What are the main types of roof underlayment?
Asphalt saturated felt (#15 and #30), non-bitumen synthetic (woven or spun polymer sheets), and rubberized asphalt or polymer modified bitumen self-adhered membranes, often called ice and water shield. DECRA describes the same three groups.
What are the problems with synthetic roof underlayment?
The common ones: it costs more than felt; it needs cap nails or cap staples, because plain staples or nails can let water wick in; some products are slippery when wet; and most synthetics are vapor barriers, so DECRA says makers recommend them only over ventilated attics or vented ceilings. Check that the product is approved by your roofing maker.
Is synthetic underlayment better than felt?
It is tougher, lighter per square foot and can stay exposed much longer, from about four to 12 months by Fine Homebuilding’s summary. Felt is cheaper, vapor permeable, and tolerates staples. Better depends on the job.
Can synthetic underlayment go under roll roofing?
Check the roll roofing maker’s instructions. The GAF and TAMKO roll roofing instructions we read name felt or a breather type underlayment. When the wrapper names a type, use it.
See synthetic roof underlayment on Amazon
Sources
Last reviewed October 12, 2026. Products and listings change; check the data sheet and wrapper before you buy, follow the maker’s instructions, and check your local building code.