Underlayment is priced by the roll, specified by the square, and quoted as part of the roof. Pulling it apart matters because the three products differ by roughly ten times in price per square foot, and the reasons to pay more are specific rather than general.
Coverage and price
A square is 100 ft² of roof.
| Product | Roll covers | Squares per roll | Typical roll price | Per square |
|---|---|---|---|---|
| #15 felt | 400 ft² | 4 | $20–35 | $5–9 |
| #30 felt | 200 ft² | 2 | $25–40 | $13–20 |
| Synthetic | 1,000 ft² | 10 | $95–180 | $10–18 |
| Ice & water shield | 200 ft² | 2 | $95–170 | $48–85 |
Prices are typical US supply-house figures, not quotes. What does not vary is the coverage — that is printed on the roll and it is how you convert a roof area into an order.
Rolls overlap. Felt laps 2 inches on the horizontal seams, 4 inches on the verticals; synthetic is usually 4 and 6. Add 10–15% to the roof area before dividing by roll coverage, more on a cut-up roof.
Worked example
A 24-square roof, synthetic in the field, ice barrier at the eaves:
- Ice barrier: eaves total 80 linear ft, one 3 ft course = 240 ft² → 2 rolls, ~$250
- Field: 24 sq − 2.4 sq covered by the barrier = 21.6 sq, +12% = 24.2 sq = 2,420 ft² → 3 rolls, ~$400
- Underlayment total: ~$650 on a roof that will cost $12,000–20,000
Between three and five per cent of the job.
Where the money is worth paying
Ice and water shield at the eaves — not a preference. The IRC requires an ice barrier from the eave edge to a point 24 inches inside the exterior wall line in regions with a history of ice damming. That is a code requirement, not an upgrade, and it is the one place where the expensive product is mandatory. See ice and water shield requirements.
Synthetic over felt in the field — the argument is not water resistance on a finished roof, because the shingles do that. It is what happens in the gap:
- Felt wrinkles when it gets wet and telegraphs through the shingles
- Felt tears in wind; a roof left dried-in over a weekend can lose its felt
- Synthetic weighs about a quarter as much per square, so fewer trips up the ladder
- Most synthetics are rated for 90–180 days of exposure; felt for days
If the roof goes on the same day, felt does the job it always did. If there is any chance of the deck sitting dried-in through weather, synthetic is the cheaper product.
#30 over #15 — a middle option that mostly exists for low-slope work, where the code requires a doubled underlayment layer between 2:12 and 4:12 pitch.
What to ask a roofer
- Which underlayment, by product name, and how many squares
- How far up from the eave the ice barrier runs, measured from the wall line, not the edge
- What happens if the roof is dried-in and the weather turns
The last one is the question that tells you whether the felt line on the quote is a real decision or a default.
Low-slope changes the requirement
Between 2:12 and 4:12 the IRC requires a doubled underlayment layer under asphalt shingles — two plies, lapped 19 inches, effectively giving continuous double coverage over the whole plane. Below 2:12, asphalt shingles are not permitted at all and the roof needs a membrane.
That doubling changes the order sharply:
| Pitch | Underlayment needed for 10 squares |
|---|---|
| 4:12 and above | 10 squares, single layer |
| 2:12 – 4:12 | 20 squares, double layer at 19" lap |
| Below 2:12 | Not shingles — membrane |
A porch roof or a low addition tied into a steeper main roof is the usual place this bites, because it is quoted as part of the same job at the same rate per square.
Fasteners
Underlayment fastening is specified separately from shingle fastening and it is where synthetic and felt differ most in practice.
- Felt is normally stapled or nailed with roofing nails, one every 12–18 inches on the laps
- Synthetic almost always requires plastic cap nails or cap staples — a 1 inch plastic washer under the head
The cap is not optional on synthetic. A bare staple through a woven synthetic sheet pulls through under wind load, which is why a roof dried-in with synthetic and stapled like felt can end up with the underlayment shredded off overnight. Most synthetic manufacturers void the exposure warranty if caps were not used.
Cap nails run about $25–40 per 3,000-count box, and a box covers several squares. It is a small line that gets left off a materials list.
Where felt is still the right answer
Synthetic is better in almost every measurable way and it is still not always the choice:
- Same-day dry-in on a small simple roof — felt costs a third as much and the exposure advantage never gets used
- Repairs and tie-ins — patching synthetic into a felt roof means two materials with different lap rules meeting at the join
- Where the shingle warranty names it — a few manufacturers still specify felt for particular systems, and departing from the named underlayment can affect the shingle warranty
That last one is worth checking rather than assuming. The underlayment is cheap; the shingle warranty is not.