Synthetic Underlayment vs Felt | Wannamaker

Most homeowners spend hours picking shingle colors and never give a single thought to what goes underneath them. That's a mistake. Underlayment is the secondary water barrier between your roof deck and the shingles — and in a market like San Antonio where we see 100°F+ summers, sudden hailstorms, and wind-driven rain, this layer can be the difference between a leak-free decade and an early roof replacement.

The two main options are synthetic underlayment and 15-lb asphalt-saturated felt (sometimes called tar paper). Let's break down each one without the sales pitch.

What 15-lb Felt Actually Is

Fifteen-pound felt is an organic or fiberglass mat saturated with asphalt. It's been used on roofs for over a century, and it works. The "15-lb" designation refers to its original weight per 100 square feet (the actual weight today is closer to 8–12 lbs due to manufacturing changes, but the name stuck).

Pros of 15-lb Felt

• Low material cost. A roll of 15-lb felt typically runs $15–$25 for 4 squares (400 sq ft). On a 2,000 sq ft roof, you're looking at roughly $75–$125 in material.
• Proven track record. It's been protecting roofs since before your grandparents were born. When installed correctly and covered quickly, it does the job.
• Breathability. Felt allows a small amount of moisture vapor to pass through, which can help prevent moisture trapping in certain roof assemblies.

Cons of 15-lb Felt

• Tears easily. Foot traffic during installation can rip felt, especially on steep pitches. Every tear is a potential weak point.
• UV and heat vulnerability. Left exposed for more than a day or two in San Antonio's summer sun, felt paper dries out, curls, and becomes brittle. We've peeled back shingles on tear-off jobs and found felt that crumbled like dried leaves.
• Absorbs water. Felt can soak up moisture and wrinkle, creating uneven surfaces under shingles. In a pop-up thunderstorm mid-install, this becomes a real problem.
• Heavier and slower to install. Rolls are bulkier and cover less area per roll, meaning more labor time on the roof.

What Synthetic Underlayment Is

Synthetic underlayment is a woven or non-woven polypropylene or polyethylene sheet. Brands like GAF FeltBuster, CertainTeed DiamondDeck, and Owens Corning ProArmor are common in the San Antonio market. It's lighter, stronger, and more UV-resistant than felt — but it costs more.

Pros of Synthetic Underlayment

• Tear resistance. Synthetic products are significantly harder to rip during installation, which means fewer compromised areas when the crew is walking the deck.
• UV stability. Most synthetics can handle 30–90 days of UV exposure (some up to 6 months) without degrading. If a project stalls — say, due to weather or a materials delay — your deck stays protected.
• Lightweight and fast. A single roll can cover 10 squares (1,000 sq ft) compared to felt's 4 squares. Less hauling, faster coverage, fewer seams.
• Doesn't absorb water. Synthetic sheds rain instead of soaking it in, so it lies flat even after a storm mid-installation.
• Better slip resistance. Most synthetics have a textured walking surface. On a 7/12 or steeper pitch — common in Stone Oak and Alamo Heights — this is a legitimate safety advantage for the crew.

Cons of Synthetic Underlayment

• Higher material cost. A roll of mid-grade synthetic runs $50–$90 for 10 squares. For a 2,000 sq ft roof, expect $100–$180 in material — roughly 50–80% more than felt.
• Vapor permeability varies. Some synthetic products are vapor-impermeable, which can trap moisture in certain roof assemblies. This matters more in humid climates, but San Antonio's humidity spikes in spring and fall are real.
• Not all synthetics are equal. Cheap synthetic underlayment exists, and it can delaminate or lose its coating prematurely. This isn't a product where you want the lowest bidder cutting corners.

What We Use and Why

At Wannamaker Roofing, we've standardized on synthetic underlayment for most residential installations. The cost difference on a typical San Antonio re-roof — usually $50–$100 more in material for a standard home — is negligible compared to the performance gains. When you factor in faster installation and fewer callbacks, synthetic actually costs less in total project terms.

There are exceptions. On certain flat roofing assemblies and some tile roofing systems, felt or a specialized self-adhered membrane may be the better choice due to vapor management and compatibility requirements. We match the underlayment to the system, not the other way around.

The "30-lb Felt" Question

Some contractors push 30-lb felt as a middle ground. It's thicker and more durable than 15-lb, but it still absorbs water, tears more easily than synthetic, and weighs significantly more. For asphalt shingle roofs, we don't see a compelling reason to use it over synthetic. Where 30-lb felt still has a role is under wood shakes and certain tile profiles where code or manufacturer specs call for it specifically.

Does Underlayment Choice Affect Your Warranty?

Yes — and this is the part most contractors don't explain well. Shingle manufacturers like GAF, Owens Corning, and CertainTeed each have system warranty programs that require specific underlayment products. If your contractor uses a non-approved underlayment, you might void the enhanced warranty without knowing it. When we install a GAF system, for example, we use GAF-approved synthetic underlayment so the full system warranty stays intact.

How to Tell What's on Your Roof Now

You can't see underlayment from the ground, and you shouldn't climb up to check. During a free roof inspection, we can often identify the underlayment type by examining exposed edges at drip lines or checking attic-side indicators. If your roof was installed before roughly 2015 in the San Antonio area, it's almost certainly felt. Roofs installed in the last five to eight years are a mix, depending on the contractor.

Bottom Line

Fifteen-pound felt still works. It's not a scam, and a contractor using it isn't automatically cutting corners — especially if the project is completed quickly and the product meets code. But for the San Antonio climate, where extreme heat, sudden storms, and hail are part of the deal, synthetic underlayment delivers meaningful advantages for a small cost increase. The real risk isn't which product you choose — it's hiring a contractor who doesn't explain the choice to you at all.