Short answer
#15 felt is the lighter, everyday underlayment; #30 is heavier and tougher. The numbers are labels, not weights: today #15 runs about 7.5 to 12.5 lb per square and #30 about 16 to 27 lb, and the standard calls them ASTM D226 Type I and Type II. Use #15 under most roll roofing and shingles; pick #30 for a roof that will sit exposed for a while or take more foot traffic, and check what your roofing maker and local code require.
#15 vs #30 at a glance
Roofing felt grades
| #15 (Type I) | #30 (Type II) | |
|---|---|---|
| Weight today (per square) | about 7.5 to 12.5 lb | about 16 to 27 lb |
| Feel | Thin, light, easy to handle | Thicker, stiffer, tears less |
| Common use | Under shingles and roll roofing | Longer exposure, steeper or busier roofs |
| Downside | Tears and wrinkles more easily | Heavier rolls, cost more |
Weights from DECRA; the Type I and Type II names come from ASTM D226, as IIBEC explains. Because weights vary so much within each grade, compare the actual weight on the label if toughness matters.
Options worth buying
-
01Everyday
#15 roofing felt
Best for: under roll roofing and shingles that go on right away
- Grade
- ASTM D226 Type I
- Width
- 36 in
- Fasten
- cap nails or as code allows
- Cheap and widely stocked.
- Covers more per roll than #30 because it is thinner.
-
02Tougher
#30 roofing felt
Best for: roofs that stay exposed a few days, or take foot traffic
- Grade
- ASTM D226 Type II
- Width
- 36 in
- Weight
- about 16 to 27 lb/sq
- Holds up better to wind and boots while you work.
- Heavier to carry up a ladder; plan for that.
-
03Alternative
Synthetic underlayment instead
Best for: long exposure, wind, or big roofs
- Roll
- about 4 x 250 ft
- Exposure
- 4 to 12 months
- Fasten
- cap nails or staples
- Fine Homebuilding notes synthetics are lighter per square foot covered and highly tear resistant (Fine Homebuilding).
- Most are vapor barriers; see synthetic vs felt.
Felt for a shed roof
For a shed, felt is usually the underlayment under mineral roll roofing or shingles, not the finished roof. Bare felt is not made to face years of sun and rain. If you want the cheapest roof that will last, cover the felt with mineral surface roll roofing. The shed roofing guide walks through the options.
What else to buy
What you need
Step by step: how to install roofing felt.
Questions people ask
What is the difference between 15 lb and 30 lb roofing felt?
Thickness and toughness. The names once matched the weight of 100 square feet of felt, but DECRA notes that link no longer holds: #15 now weighs about 7.5 to 12.5 lb per square and #30 about 16 to 27 lb. In ASTM D226 they are Type I and Type II. Thirty pound felt resists tearing and wrinkling better.
Is roofing felt the same as tar paper?
In everyday use, yes: people call asphalt saturated roofing felt tar paper. Historically some products were coal tar based. Our felt vs tar paper page explains the history and what to buy today.
Do I need felt under roll roofing?
TAMKO recommends asphalt saturated felt over the entire deck under its roll roofing, and GAF says underlayment has many benefits and is required by some code bodies. On a shed it is cheap insurance; check your local code.
How long can roofing felt stay exposed?
Not long. Felt is meant to be covered soon after it is installed; it wrinkles when wet and can tear in wind. If the roof will sit uncovered for weeks, synthetic underlayment, which makers rate for months of exposure, is the better choice.
How much does a roll of felt cover?
It depends on the product. Felt comes in 3 foot wide rolls, typically 72 to 144 feet long according to Fine Homebuilding, so roll coverage ranges from about two to four squares before laps. Check the label.
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.