Roof underlayment: felt or synthetic, one layer or two, and the code's rules
Underlayment is the layer between the roof deck and the shingles. On a shingle roof of 4:12 or steeper the code calls for one layer laid shingle fashion from the eave, lapped 2 inches, with 4 inch end laps offset 6 feet; from 2:12 to 4:12, two layers, starting with a 19 inch strip at the eave and 36 inch sheets lapped 19 inches (IRC Table R905.1.1(2)). It has to carry a label showing the standard it meets.
Anthony Hicks30+ years in construction. Updated November 5, 2026.
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What underlayment does
Underlayment covers the roof deck before the shingles go on: a second layer against water that gets past the shingles, and cover for the deck while the roof is being laid. The guide to the parts of a roof shows it between the shingles and the decking.
The kinds
| Underlayment | What it is |
|---|---|
| Asphalt-saturated felt | Felt paper soaked in asphalt |
| Synthetic | A woven or spun plastic sheet |
| Self-adhered | A polymer-modified bitumen sheet that sticks to the deck |
The code lists the standards underlayment has to meet, such as ASTM D226, D4869 and D6757 for felts and D1970 for self-adhered sheets, and calls for a label showing the standard (IRC R905.1.1). A self-adhered sheet meeting ASTM D1970 can be used in place of the usual underlayment when it goes on to both makers’ instructions (R905.1.1, exception 1).
One layer or two
Shingles go only on roofs of 2:12 or steeper (IRC R905.2.2), and the slope sets the underlayment (Table R905.1.1(2)):
| Slope | Underlayment |
|---|---|
| 4:12 and steeper | One layer, shingle fashion from the eave, lapped 2 inches; end laps 4 inches, offset 6 feet |
| 2:12 up to 4:12 | Two layers: a 19 inch strip at the eave, then 36 inch sheets each lapped 19 inches over the last; end laps 4 inches, offset 6 feet |
| Where wind design is required | Two layers, as for the low slopes |
The roof pitch guide shows how to read the slope.
At the edges
- Eaves: underlayment goes over the drip edge (IRC R905.2.8.5).
- Rakes: it goes under the drip edge. The drip edge guide covers both.
- Ice country: where the code’s map shows ice forming along the eaves, an ice barrier goes at the eaves from the lowest edge to at least 24 inches inside the outside wall line (R905.1.2).
How much underlayment a roof needs
One layer covers the roof’s area plus its laps; two layers on a low slope take about twice that. The roofing calculator gives the roof’s area in squares, which is where the rolls are figured from.
Common questions
Is synthetic underlayment better than felt?
Either is allowed when it meets the standard on its label and goes on to the code’s laps and the shingle maker’s instructions.
How many layers of underlayment does a roof need?
One on shingle roofs of 4:12 and steeper, two from 2:12 up to 4:12 and where wind design is required (IRC Table R905.1.1(2)).
Is underlayment on a roof estimate?
Yes, a line of its own priced by the square, with an ice barrier where the code calls for one.