Best Roof Underlayment: Synthetic vs Felt vs Self-Adhered

Compared by type

Pros and cons describe each type from its specifications; this is not a hands-on product test. As an Amazon Associate we earn from qualifying purchases.

Synthetic (woven or spun polypropylene)

Best for: The whole field of a shingle or metal roof. Check: UV exposure rating in days and an ICC-ES evaluation report.

Pros

• Much lighter per square than felt, with far larger rolls (often 10 squares)
• High tear strength — survives wind and foot traffic during the job
• Does not wrinkle or absorb water like felt
• Many are rated to sit uncovered for weeks or months

Cons

• Usually needs plastic-cap fasteners, not bare staples
• Some surfaces are slippery when wet or dusty — check the walking-surface description
• Not self-sealing around nails

Self-adhered membrane (ice and water shield)

Best for: Eaves, valleys, around chimneys, skylights and low-slope sections. Check: Granulated vs smooth face, and the temperature rating.

Pros

• Bonds to the deck and seals around nail shanks
• Stops water backed up behind ice dams
• Required at the eaves by code in many cold climates

Cons

• Costs several times more per square than synthetic
• Cannot be repositioned once the release film is off
• Vapour-impermeable — covering a whole roof needs a well-ventilated attic

Asphalt-saturated felt (#15 / #30)

Best for: Tight budgets and roofs where the shingle warranty names felt. Check: ASTM D226 or D4869 type on the label, and squares per roll.

Pros

• Lowest cost per square
• Familiar to every roofer and inspector
• Easy to cut and nail

Cons

• Heavy — a #30 roll typically covers about 2 squares
• Tears in wind and wrinkles when it gets wet
• Short safe exposure time before shingles go on

Cap nails and cap staples

Best for: Fastening synthetic underlayment to the deck. Check: Cap diameter and shank length the underlayment maker specifies.

Pros

• The wide plastic cap holds synthetic sheets without tearing
• Coil versions run in a cap nailer for speed
• Specified by most synthetic manufacturers

Cons

• An extra purchase on top of the roll
• Hand-driven caps are slow on a full roof

What to look for on the listing

• Code evidence. Felt should state ASTM D226 or D4869. Synthetic products do not have one ASTM standard, so look for an ICC-ES evaluation report (ESR) number — that is what an inspector checks.
• UV exposure rating. It is the number of days the sheet may sit uncovered. If the shingles will not go on for a week, buy a product rated for longer than your worst-case delay.
• Coverage per roll. Synthetic rolls commonly cover 10 squares (1,000 sq ft); #15 felt about 4 squares; #30 felt about 2. Price per square, not per roll.
• Walking surface. Synthetic sheets vary a lot in traction. On anything steeper than 6/12 treat every underlayment as slippery and work from roof jacks.
• Fastener instructions. Read how the manufacturer wants it fastened — cap type, spacing at laps and in the field — before buying nails.
• Shingle warranty. Enhanced warranties from the big shingle brands often require their own underlayment. Check before mixing brands.

How many rolls you need

Rolls are ordered against the sloped roof area, not the footprint. Multiply the footprint by the pitch factor, add waste for laps and cuts, then divide by the roll coverage. The underlayment calculator does exactly this; the example below uses the same function.

Ice and water membrane at the eaves is measured separately in linear feet of eave; the ice and water shield calculator sizes it. On slopes from 2/12 up to 4/12, the residential code calls for a double layer of underlayment under asphalt shingles, which roughly doubles the field quantity.

Which type goes where

Safety notes

• Underlayment is at its most slippery on a dewy morning and in dust from the tear-off. Sweep the deck and wait for it to dry.
• Fall protection is required for work at height; on steep roofs, set roof jacks before rolling out the first course, not after.
• Do not leave a roll loose on a sloped deck. A 40 lb roll sliding off the eave is a real hazard below.

Frequently asked questions

Is synthetic underlayment better than felt?

For most re-roofs, yes: it is lighter, stronger, does not wrinkle when wet and can sit exposed longer. Felt is still acceptable where code allows it and costs less per square. Check whether your shingle warranty names a specific underlayment.

How many squares does a roll of synthetic underlayment cover?

Many synthetic rolls cover 10 squares (1,000 sq ft), but sizes vary, so read the coverage on the listing. Divide your sloped roof area plus waste by that figure to get rolls.

Can I use ice and water shield on the whole roof?

You can, and some coastal and high-wind builders do, but a fully sealed deck cannot dry into the attic, so attic ventilation must be right. Most roofs use membrane only at the eaves, valleys and penetrations.

What underlayment goes under a metal roof?

A synthetic or self-adhered underlayment rated for the higher temperatures under metal panels. Standard felt and some low-temperature membranes can soften or stick to the panels.

Do I need two layers of underlayment?

Under asphalt shingles on slopes from 2/12 up to less than 4/12, the residential code requires double coverage. On 4/12 and steeper, one layer meets code.