Roofing underlayment calculator

Rolls are not a standard size. A roll of #15 felt covers four squares, the same length of #30 covers two, and a synthetic roll covers ten — so "how many rolls" has no answer until you say which product. Enter the roof and the roll on your pallet.

Roof area The area the roof covers seen from above, including overhangs. If you have already measured the sloped surface, set the pitch below to "already the sloped area". Roof pitch (rise per 12 in of run) Square feet printed on the roll sq ft #15 felt 432 sq ft (36 in × 144 ft). #30 felt 216 sq ft (36 in × 72 ft). Most synthetics 1,000 sq ft (48 in × 250 ft). #30 also comes as a 108 sq ft split roll and a 180 sq ft jumbo. Roll width Felt is 36 in. Synthetic is usually 48 in. This is what decides how much of the roll the lap eats. Lap between courses IRC R905.1.1 sets a 2 in minimum on slopes of 4:12 and steeper. Synthetics print their own lap line, usually 4–6 in. On a 2:12 to 4:12 roof the code wants double coverage — enter 19 on a 36 in roll and the roll count doubles by itself. Extra for waste % 10% covers end laps, the offcut at each rake and the strip wasted at valleys. Go to 15% on a cut-up roof.

Rolls to buy 4 rolls

What one roll actually covers 408 sq ft Gross roll area minus the strip lost to the lap.

Area to be covered 1,230 sq ft Pitch and waste applied.

Lost to the lap on each roll 24 sq ft

Roof size in squares 11.18 squares

Three products, three different rolls

"How many rolls of underlayment" is unanswerable until you say which roll, because the standard sizes are not close to each other:

Product	Roll	Gross area	Sold as
#15 felt	36 in × 144 ft	432 sq ft	4 squares
#30 felt	36 in × 72 ft	216 sq ft	2 squares
Synthetic	48 in × 250 ft	1,000 sq ft	10 squares

Those felt figures are Tarco's published data for asphalt-saturated organic felt; the 48 in × 250 ft synthetic roll is what GAF FeltBuster, Grip-Rite and Menards' VB Shield all come on. A synthetic roll covers five times what a #30 roll covers. Swap the product without redoing the arithmetic and you are either short by a factor of five or standing over eight spare rolls.

#30 exists because it is heavier and tears less, which matters on a long exposure or a windy site — but the roll is half the length, not the same length in a thicker felt. That is the detail people miss at the counter.

The lap comes off the roll, not out of the roof

Underlayment is lapped like shingles, so part of every course is buried. The printed square footage is the gross roll area, and what you get on the deck is less:

coverage = roll area × (roll width − lap) ÷ roll width

A 432 sq ft roll of #15 at the code minimum 2 in lap covers 408 sq ft. A 1,000 sq ft synthetic roll at its printed 4 in lap line covers 917 sq ft. That is 6–8% you will not get back, and it sits on top of your waste factor, not inside it.

Then there is slope. IRC R905.1.1 allows a single layer with 2 in laps at 4:12 and steeper. Below 4:12 and down to 2:12 the code requires double coverage, which on a 36 in roll means lapping each course 19 in so only 17 in shows. Enter 19 in the lap box and watch the roll count roughly double — that is not a safety margin, it is the code, and it is the single biggest miss on a low-slope porch or addition.

What you buy alongside it

Rolls are the cheap part. Before you climb up you also need:

Cap nails or cap staples. Plain roofing nails tear through synthetic in wind, and most synthetic warranties require caps.
Ice-and-water membrane at the eaves in cold climates — a separate, self-adhering product, run from the edge to at least 24 in inside the warm wall line. It replaces underlayment in that band rather than adding to it.
Drip edge, which goes under the underlayment at the rakes and over it at the eaves. Order it at the same time as the felt so you install them in the right sequence — see the drip edge calculator.

What the lap model includes, and what it leaves to the waste factor

The only lap in the arithmetic is the side lap — the strip of each course buried by the one above, taken off the width of the roll. Two other losses are just as real and are not modelled. End laps, where one run of felt meets the next along the same course, are 4 in of code offset 6 ft apart. And every rake and every valley throws away part of a full-width course. Both sit inside the waste percentage rather than being calculated, which is why 10% is a floor on a plain gable and short on a cut-up roof.

The double-coverage case is an approximation too, deliberately. A 19 in lap on a 36 in roll reproduces the exposure the code wants below 4:12 — 17 in showing instead of 34 — so the roll count comes out right. What it does not add is the half-width starter course at the eave that the code method opens with. And note that 19 in belongs to a 36 in roll: on a 48 in synthetic, double coverage means a lap of more than half that width, and you have to work it out and type it in.

The roll count also does not take the ice-and-water membrane back out of the roof. That band at the eaves — run to at least 24 in inside the warm wall line, so commonly a course or two of 36 in membrane — replaces the underlayment there rather than sitting under it, but the area divided here is the whole roof. On a cold-climate re-roof that is most of a roll of felt you will never unroll, and it is deliberate: it sits with the waste allowance rather than being netted off, because how far the membrane reaches depends on your eave overhang and your climate zone, neither of which this page asks for.

Finally, this counts rolls. It does not check that the product is allowed where you are putting it. Minimum slope, how long a synthetic may stay exposed before it has to be covered, and whether your shingle manufacturer accepts that underlayment inside its warranty at all are on the product's own instructions, and they differ by brand in ways no roll count can express.

Questions people ask

How many square feet is a roll of roofing felt?

A #15 roll is 36 in × 144 ft, or 432 sq ft — four squares. A #30 roll is 36 in × 72 ft, or 216 sq ft — two squares. Both are 36 in wide, so #30 is the same width in half the length, not a thicker version of the same roll.

How many rolls of underlayment for a 1,500 sq ft roof?

At a 2 in lap, one #15 roll covers 408 sq ft, so 1,500 sq ft plus 10% waste needs 5 rolls. The same roof takes 9 rolls of #30, or 2 rolls of synthetic. Always work from the roll on your pallet, not from a generic figure.

Is synthetic underlayment better than felt?

For most re-roofs, yes — it is lighter, does not wrinkle when it gets damp, grips better underfoot and tolerates weeks of exposure rather than days. Felt still wins on price and it is what some low-slope self-sealing details assume, so read the shingle manufacturer's instructions before switching.

How much should underlayment overlap?

At least 2 in between courses and 4 in at end laps, with end laps offset 6 ft, on slopes of 4:12 and steeper. Synthetics print their own lap line, usually 4–6 in — follow the line, because the fastening pattern is printed to match it.

Do I need two layers of underlayment on a low-slope roof?

Yes, between 2:12 and 4:12. The code method is a 19 in starter strip at the eave then 36 in courses lapped 19 in, which leaves only 17 in exposed and roughly doubles the number of rolls. Below 2:12 asphalt shingles are not permitted at all.

Last reviewed 2026-07-31