Synthetic vs Felt Underlayment: Tear Strength, Weight, Cost, and When Felt Still Wins

Synthetic underlayment is the default on 90 percent of roofs in 2027, and for good reason. But felt hasn''t disappeared, and there are still a handful of jobs where it''s the right spec. Here''s the real comparison.

The Numbers Side By Side

Property#15 Felt#30 FeltSynthetic (typical) Weight per square7 to 8 lb14 to 16 lb2 to 4 lb Roll coverage4 squares (400 sq ft)2 squares (200 sq ft)10 squares (1,000 sq ft) Roll cost$22 to $30$35 to $55$150 to $220 Cost per square$5.50 to $7.50$17.50 to $27.50$15 to $22 Tear strength (MD)10 to 15 lbf20 to 30 lbf60 to 150 lbf UV rating30 days60 days90 days to 12 months Wrinkle on hot days?YesYesNo Slippery when wet?YesYesDepends, most are grippy Rolls per truck load~100~60~200

Why Synthetic Wins Most Jobs

• 10x the coverage per roll: fewer trips to the truck, less time rolling out, less money to the supplier.
• Stays flat in sun: felt buckles on a hot deck, telegraphing wrinkles through the shingles for the life of the roof. Synthetic doesn''t.
• Safer to walk on: grippy surface on most products, even when wet.
• Longer dry-in window: most products rated 90 days UV, some up to 12 months. Felt starts to powder and tear at 45 days.
• Lighter to carry: a 10-square synthetic roll weighs 30 pounds; a 10-square equivalent in felt is 140 pounds.

Synthetic Brands Worth Buying

• InterWrap Titanium UDL-30: 30-lb tear strength, 12-month UV, grippy top. Heavy, premium. $180 to $220 per 10-square roll.
• InterWrap Titanium UDL-50: 50-lb tear strength, bulletproof. $240 per roll.
• RhinoRoof U20: lighter, 60-day UV, $130 to $160 per roll. Good for fast same-day loaded jobs.
• GAF FeltBuster: required for GAF system warranty. 6-month UV. $150 to $180 per roll.
• Owens Corning Deck Defense: pairs with Duration. $160 to $190.
• CertainTeed DiamondDeck: pairs with Landmark. $170 per roll.
• Malarkey Right Start UDL: pairs with Vista/Legacy. $160.
• Atlas Summit 60: 60-day UV, budget synthetic. $120 to $140.

Avoid the no-name big-box synthetic priced at $80 a roll. Those tear at a dog track, leak at the nail holes, and have 30-day UV. You save $60 now and lose $800 if it rains before dry-in.

When Felt Still Wins

1. Tile roofs in jurisdictions requiring #30 felt minimum. Some AHJs specifically require #30 asphalt-saturated felt under tile, not synthetic. Read local code.
2. Historic restorations. Some preservation specs require traditional materials, including #30 felt.
3. Extreme budget 3-tab reroof on a low-slope rental. $28 worth of #15 on a 10-square reroof beats $150 of synthetic when the margin is tight.
4. Repair patches. Cutting in a 6-foot patch in an existing felt field, match the felt.

Installation Differences

Fasteners

Felt gets nailed with roofing nails 8 inches on center at laps, every 16 inches in the field. Synthetic gets fastened with capped nails (plastic cap nails, Henry or ETS) at the spacing on the wrapper, typically 6 to 8 inches at laps and 12 inches in the field.

Laps

Felt: 2-inch horizontal lap, 4-inch vertical end lap (IRC R905.2.8.4).

Synthetic: follow wrapper. Most are 4-inch horizontal, 6-inch vertical. Wider laps since the material is slicker.

Rollout

Felt gets rolled out with the writing down. Synthetic gets rolled out with the printing / walking surface up. Brands print specific rollout direction on the material.

Common Failures

• Felt wrinkled under shingles: hot deck plus #15 felt. Shingles telegraph wrinkles permanently. Switch to synthetic or cooler-day install.
• Synthetic with smooth roofing nails instead of capped: torn nail holes leaking during dry-in. Always capped.
• Synthetic rolled direction inverted: slick side up, crew slips off the roof. Read the manufacturer''s print.
• Felt left exposed 60 days: powder by the time shingles load. Tear off and re-dry-in.

Cost on a Real Job

A 25-square reroof:

ProductRolls neededCost #15 felt7 rolls$175 #30 felt13 rolls$585 Titanium UDL-303 rolls$585 RhinoRoof U203 rolls$450 GAF FeltBuster3 rolls$510

Synthetic equals or beats #30 felt on cost, and dramatically outperforms it. #15 felt is the only real budget play, and you''re trading long-term performance for $400 savings.

Warranty Tie-In

GAF Golden Pledge, CertainTeed 4-Star, Owens Corning Platinum: all require manufacturer-brand synthetic underlayment. Using Titanium UDL under Timberline HDZ gets you the shingle-only warranty. Using FeltBuster gets you the full system warranty. The material is basically identical, the paperwork isn''t.

RoofKnockers Material Specs

In RoofKnockers the material picker locks to manufacturer-matched underlayment when you select a warranty tier so the spec matches what you''re selling. Features. See also the full underlayment guide and the ice and water shield guide.

FAQ

Is synthetic waterproof?

No, synthetic is water-resistant. It sheds water but it''s not a vapor barrier or waterproof membrane. Peel-and-stick is the actual waterproof layer at eaves and valleys.

Can I mix synthetic and felt on the same roof?

Physically yes, but it''s weird and it voids most system warranties. Stick with one product across the whole roof.

Does synthetic work under tile?

Some synthetics are rated for under-tile use (FT Synthetics, Boral TileSeal) but many standard synthetics aren''t. Check the datasheet. When in doubt, #30 felt or 40-mil self-adhered is the safe spec under tile.