How many squares does a roll of underlayment cover?

Roughly: synthetic about 10 squares per roll, #15 felt about 4 squares, and #30 felt about 2 squares. Always confirm the coverage printed on your specific product.

Is synthetic underlayment better than felt?

For most jobs, yes — it is lighter, stronger, lies flatter, resists UV and covers far more per roll, so you handle fewer rolls. Felt is cheaper up front and still perfectly serviceable on simple roofs.

How many rolls of underlayment for a 2,236 sq ft roof?

At 24.6 squares (with 10% waste): about 3 rolls of synthetic (10 sq/roll) or 13 rolls of #30 felt (2 sq/roll). Use the underlayment calculator with your roll coverage.

Do I need ice-and-water shield too?

In cold climates, yes — a self-adhering membrane at eaves, valleys and penetrations. That is a separate product added by the linear foot, not part of the field underlayment roll count. It seals around fasteners and blocks water backed up by ice dams, so many roofs use ice-and-water at the vulnerable edges and synthetic across the field.

Roofing underlayment: felt vs synthetic, and how many rolls

Underlayment is the water-shedding layer between deck and shingles. The type you choose changes how many rolls you buy — here is the difference, and the arithmetic.

By Francesco Zinghinì

What underlayment does

Underlayment is the sheet material rolled over the roof deck before the shingles go on. It is a secondary water barrier that sheds any water getting past the shingles, protects the deck from weather during installation, and provides a smoother, safer surface to work on. It is required under asphalt shingles by building code and by most manufacturer warranties — skip it and you can void the warranty. The two common families are asphalt-saturated felt and synthetic, and they cover very different areas per roll, which is exactly what changes your order quantity.

Felt: the traditional choice

Felt (tar paper) comes in two weights. #15 felt is lighter and covers roughly 4 squares per roll; #30 felt is heavier, more tear-resistant, and covers only about 2 squares per roll. Felt is inexpensive and time-proven, but it has drawbacks: it is heavier to carry up a ladder, it can wrinkle and buckle when it gets damp (telegraphing lines through the shingles), it tears more easily in wind during installation, and it degrades faster if left exposed to sun before the shingles go on. Because #30 covers half of what #15 does, choosing the heavier felt roughly doubles your roll count.

Synthetic: the modern default

Synthetic underlayment is a woven polymer sheet. It is far lighter, much stronger, lies flat, resists wrinkling and UV far better, and typically covers about 10 squares per roll — so you handle far fewer, larger rolls. It usually costs more per roll but covers more area and installs faster, and many products can be left exposed for weeks if the shingles are delayed by weather. For most re-roofs today it is the default choice. Exact coverage varies by brand, so the underlayment calculator lets you enter the roll coverage printed on your specific product rather than assuming a number.

How many rolls: the formula

Rolls follow from squares-with-waste and the roll coverage:

rolls = ceil( squares_with_waste ÷ coverage_per_roll )

Use the same square count and waste as your shingles so the layers match up. Worked example on our 24.6-square order (22.36 squares plus 10% waste): with synthetic at 10 squares/roll, 24.6 ÷ 10 = 2.46, rounding up to 3 rolls. With #30 felt at 2 squares/roll, 24.6 ÷ 2 = 12.3, rounding up to 13 rolls — more than four times as many rolls to buy and haul up. That difference in handling alone pushes many DIYers toward synthetic.

A word on overlap

The coverage figures on the wrapper already assume the standard head and side laps (usually a couple of inches of overlap between courses). If you install with wider-than-standard laps for extra protection, your effective coverage per roll drops a little — round up and buy an extra roll on a big or complex roof so you're not caught short at the ridge.

Ice-and-water membrane at the edges

In cold climates, code and good practice call for a self-adhering ice-and-water membrane at eaves, in valleys and around penetrations — not full-field underlayment. This rubberized sheet seals around fasteners and blocks water backed up by ice dams. It is a separate product bought by the roll for those specific runs, not part of the field underlayment count above, so add it based on your eave and valley lengths. Many roofs use ice-and-water at the vulnerable edges and synthetic across the field.

Code, warranties and exposure limits

Underlayment is not optional. Building codes require a layer under asphalt shingles, and shingle manufacturers require it — often a specific type or their own branded product — for the material warranty to stay valid. Installing bargain underlayment under a premium shingle can quietly void the coverage you paid for, so read the shingle warranty's fine print before you choose. Exposure time matters too: felt should be covered by shingles quickly, because sun and damp degrade and wrinkle it within days, while many synthetics are rated for weeks or even months of UV exposure — a real advantage if weather stalls the job mid-roof. Temperature and traction also differ: some synthetics get slick when wet, so the better products add a walkable textured surface. None of this changes the roll-count arithmetic, but it should steer which product you count — match the underlayment to the shingle warranty and to how long it may sit exposed.

Choosing, and a safety note

If budget is tight and the roof is simple, felt still works and has decades of track record. For fewer trips up the ladder, better tear resistance during a multi-day job, and longer safe exposure if weather delays the shingles, synthetic wins for most homeowners. Whatever you pick, confirm the exact coverage on the wrapper — these are planning estimates, and working on a roof is dangerous, so measure from the ground or a plan and consider a licensed roofer for the install.