Roofing Underlayment Options Explained: #15 Felt, #30 Felt, Synthetic, and Peel-and-Stick

Underlayment is the layer nobody sees and every failed roof comes back to. I''ve torn off 25-year-old shingles sitting on felt that was powder in my hand, and I''ve torn off 8-year-old synthetic that looked factory fresh. Here''s what each product does, what it costs, and when I''d spec it on a job.

The Four Categories That Matter

1. #15 asphalt-saturated felt (the old standard)
2. #30 asphalt-saturated felt (the heavier old standard)
3. Synthetic underlayment (Titanium UDL, RhinoRoof, GAF FeltBuster, Owens Corning Deck Defense)
4. Peel-and-stick / self-adhered (Grace Ice & Water Shield, GAF StormGuard, CertainTeed WinterGuard)

#15 Felt: Still Used, Barely

Weight: 7 to 8 pounds per square. Price: $22 to $30 per roll covering 4 squares. Tear strength: low, tears at the capped nail in 30 mph wind. UV exposure: 30 days max, often less.

Still code-compliant in most jurisdictions over 2:12 pitch. I use it when a homeowner is on the absolute tightest budget on a 3-tab roof they plan to sell within 5 years. That''s it. Any job over 6:12 or longer than a one-day install gets upgraded.

#30 Felt: The Middle Child

Weight: 14 to 16 pounds per square. Price: $35 to $55 per roll covering 2 squares. Tear strength: moderate. UV exposure: 60 days.

Heavier, tougher than #15, still an organic-asphalt product that wicks water and rots if exposed. Used under tile in a lot of markets because code often requires either #30 felt or a 40-mil self-adhered membrane under tile. I still spec #30 under concrete tile on budget jobs. Otherwise synthetic wins.

Synthetic Underlayment: The New Default

Weight: 2 to 4 pounds per square. Price: $150 to $220 per roll covering 10 squares. Tear strength: 5 to 10x felt. UV exposure: 90 days to 12 months depending on product.

Common brands crews trust:

• InterWrap Titanium UDL-30 and UDL-50: 12-month UV, heavy, grippy. UDL-50 is basically bulletproof.
• RhinoRoof U20: lighter, cheaper, 60-day UV. Good for fast reroof crews who''ll be loaded the same day.
• GAF FeltBuster: required on GAF System Plus warranty jobs. 6-month UV.
• Owens Corning Deck Defense: paired with Duration shingles for their warranty upgrades.
• Malarkey Right Start UDL: pairs with Vista and Legacy shingles for their warranty.

Synthetic is what goes on 90 percent of new architectural shingle installs in 2027. Walking safer, lighter to carry, wider roll means fewer laps, and the warranty paperwork lines up with the shingle manufacturer.

Peel-and-Stick (Self-Adhered Membrane)

Price: $90 to $160 per roll covering 2 squares. Temperature rated: usually 40F or higher for install, some products good to 25F.

This is your eave, valley, penetration, and low-slope rescue layer. It''s also code-required ice barrier in climate zones 4 and higher. See our ice and water shield guide for install rules. Brands I''ll actually put on a house: Grace Ice & Water Shield (the original), GAF StormGuard / WeatherWatch, CertainTeed WinterGuard, Owens Corning WeatherLock.

Which Goes Where

LocationSpec Eaves (climate zone 4+)Peel-and-stick, 24" past interior wall line Valleys36" wide peel-and-stick centered in valley Around penetrations (pipe boots, skylights)Peel-and-stick patch, 12" around Low slope (2:12 to 4:12)Double layer felt or full peel-and-stick Main field (4:12 and up)Synthetic Under tile (any slope)#30 felt or 40-mil self-adhered per code

Warranty Tie-In

If you''re selling a GAF Golden Pledge, CertainTeed 4-Star, or Owens Corning Platinum warranty, the underlayment has to be their brand. Mixing a Titanium UDL under GAF shingles still gets you the shingle-only warranty, but not the system warranty. Know which one you sold before the truck leaves the supply house.

How RoofKnockers Handles Material Specs

Inside RoofKnockers you can lock material specs per job so a homeowner''s contract shows synthetic UDL vs #15 felt without hand-editing. See features, or try it on a free account.

FAQ

Can I leave synthetic exposed all winter?

If it''s UDL-30 or UDL-50, yes, up to 6 to 12 months per the datasheet. Check the specific product. RhinoRoof U20 is only 60 days, which is not a dry-in over winter.

Does #15 felt violate any code?

Not by itself, but the 2018 IRC requires a minimum 3.5-inch headlap on felt and a capped-nail attachment pattern. A lot of older crews still hand-gun it with smooth nails, which is out of code.

Is double-layer felt the same as peel-and-stick on low slope?

No. Double-layer felt is a code minimum, but water will find any nail hole. On anything under 4:12 that a homeowner actually cares about (patio cover, addition), full peel-and-stick is the only answer.