Roofing Underlayment Material Types: A Complete Guide

If you have ever looked at a roofing estimate and wondered what “synthetic underlayment” or “#30 felt” actually means, this guide is for you. We compare every common roof underlayment type side by side — roofing felt types, roofing paper types, synthetic sheets and self-adhered membranes — with typical costs, lifespans and where each one makes sense on an Austin home.

Underlayment is the layer you never see once the roof is finished, which is exactly why it gets overlooked. It sits between the wood deck and the shingles or metal panels, and it is the last line of defense when wind peels back a shingle, hail cracks a tab or rain is driven sideways under the edge of the roof. Choosing the right underlayment material is one of the quiet decisions that separates a roof that lasts 25 years from one that starts leaking at year 12.

Layers of a Roof: What the Inside of a Roof Is Made Of

Before comparing types of roof underlayment, it helps to know where underlayment fits. A typical pitched residential roof in Central Texas is built in layers, from the inside out:

1. Framing (rafters or trusses)

   The wooden skeleton that carries the roof’s weight. Most Austin-area homes built since the 1980s use engineered trusses; older homes in neighborhoods like Hyde Park or Travis Heights often have site-built rafters.

2. Insulation and attic ventilation

   Below the deck sits the attic, with insulation on the ceiling (or sprayed foam under the deck) and a system of intake vents at the eaves and exhaust vents near the ridge that move hot air out. Read more about why that airflow matters in our guide to attic ventilation upgrade benefits.

3. Roof decking (sheathing)

   Plywood or OSB panels nailed to the framing. This is the surface everything else is fastened to. Older homes may have plank (board) decking with gaps between boards.

4. Drip edge

   Metal edging along the eaves and rakes that directs water off the roof and into the gutters, protecting the edge of the deck and fascia.

5. Underlayment

   The water-resistant sheet covering the whole deck — felt, synthetic, or self-adhered membrane. Leak barriers are added at valleys, penetrations and other vulnerable spots.

6. Starter strip, shingles or panels, flashing

   The visible roof covering, plus metal flashing at walls, chimneys and pipe boots, and finally ridge cap or ridge vent at the top.

So when people ask “what is the inside of a roof made out of?”, the answer is mostly wood (framing and decking), with underlayment as the critical membrane between that wood and the weather.

Why Roofing Underlayment Matters

Shingles shed water; they are not watertight on their own. Water gets underneath them at laps, nail holes, flashing joints and anywhere wind lifts a tab. Underlayment catches that water and channels it down to the eaves before it reaches the wood. It does several other jobs too:

• Temporary weather protection during installation, between tear-off and shingles going down.
• Secondary water barrier when shingles are damaged or blown off — the difference between a few wet shingles and a soaked ceiling after a spring storm.
• Separation layer that keeps resins in the decking from reacting with asphalt shingles and lets the roof covering move slightly with heat.
• Code compliance and warranty eligibility: the International Residential Code requires underlayment beneath asphalt shingles, and shingle manufacturers specify approved underlayments for their system warranties.
• Fire rating: some roof assemblies rely on a specific underlayment to achieve their listed fire classification.

Roof Underlayment Types Compared

Here is how the main roofing underlayment options stack up. Costs are typical U.S. material-only planning ranges per roofing square (100 sq ft of roof) and vary by brand, supplier and market; lifespans assume the underlayment is covered by a properly installed, ventilated roof.

Installed labor for underlayment is usually bundled into the per-square price of the whole roof, so homeowners rarely see it as a separate line. If you want to estimate how many rolls a roof needs, our roofing square and bundle calculator converts your footprint and pitch into squares and underlayment rolls.

Asphalt-Saturated Felt Underlayment (Roofing Felt Types)

Felt — also called roofing paper or tar paper — was the standard underlayment for most of the last century. It is made by saturating a paper, organic or fiberglass mat with asphalt. Felt is sold by weight class, and those classes are what people mean when they search for roofing felt types or roofing paper types.

#15 vs #30 Roofing Felt

The numbers once referred to weight per square; modern felts are lighter than the names suggest, but the relationship holds. #15 felt is thinner and easier to tear. A typical roll covers roughly four squares. #30 felt is about twice as thick, holds up better to foot traffic and wind during installation, and lays flatter on steep slopes; a roll typically covers about two squares. Many felts are manufactured to ASTM D226 (Type I corresponds to #15, Type II to #30) or ASTM D4869.

In Austin’s climate, felt’s biggest weakness is that it absorbs moisture during a summer pop-up storm and then bakes in 100°F sun. It is still used — and still meets code — but it is now the exception rather than the rule on residential shingle roofs.

Synthetic Roofing Underlayment

Synthetic underlayment is made from woven or spun polypropylene or polyethylene, usually with a laminated or coated surface that sheds water and a textured top that gives installers grip. It has largely replaced felt as the shingle underlayment of choice across Texas.

Why roofers prefer it:

• Strength: many times the tear resistance of felt, so it survives wind gusts and foot traffic during the job.
• Weight: a 10-square roll of synthetic often weighs less than a 4-square roll of #15 felt.
• UV tolerance: many products are rated for several months of exposure, which matters if a storm interrupts a project.
• No wrinkling: it lies flat when wet, giving a cleaner shingle finish.
• Walkability: slip-resistant surfaces make the roof safer for the crew.

The trade-off is that most synthetic underlayments are vapor retarders — they block moisture vapor more than felt does. That is not a problem on a properly vented attic, but it makes good intake and exhaust ventilation even more important. Synthetic is also mechanically fastened with cap nails or plastic caps, and the fastening pattern specified by the manufacturer must be followed.

On a typical composition shingle roof, Austin Roofing installs synthetic underlayment over the field of the deck and adds self-adhered membrane in the valleys and around penetrations.

Rubberized Asphalt Self-Adhering Underlayment

Rubberized asphalt membranes — often sold as “ice and water shield” — combine polymer-modified asphalt with a polyester or polyethylene top surface and a sticky back protected by a release film. Once the film is removed, the membrane bonds directly to the deck.

The key benefit is that it self-seals around nail shanks. When a fastener punches through, the rubberized asphalt grips it and stops water from following the nail into the deck. That makes it ideal for:

• Valleys, where water from two roof planes concentrates
• Around chimneys, skylights, pipe boots and roof-to-wall transitions
• Eaves and rakes exposed to wind-driven rain
• Low-slope sections (typically 2:12 to 4:12) where water moves slowly

In northern states, code requires it at the eaves to stop ice dams. Central Texas does not get ice dams in any meaningful way, so here the membrane earns its keep against horizontal rain during spring squall lines and tropical remnants. For a deeper look at pricing, see our guide on ice and water shield installation cost.

Peel-and-Stick Synthetic Underlayment

Peel-and-stick synthetic is a newer hybrid: a strong synthetic top sheet with a butyl or modified-asphalt adhesive layer. It installs without nails through the field (or with far fewer), creating what many manufacturers call a sealed roof deck.

The appeal is simple: if a hailstorm or 70 mph gust strips shingles off, a fully adhered underlayment keeps the house dry until repairs are made. Insurance-backed programs such as the FORTIFIED Roof standard promote sealed roof decks for exactly this reason.

Downsides are cost and ventilation sensitivity. A fully adhered deck acts as a vapor barrier, so attic ventilation has to be designed correctly, and future tear-offs are harder because the membrane bonds to the wood. Still, for homeowners in hail-prone parts of Williamson and Travis County who plan to stay for decades, it is worth pricing.

Underlayment for Metal Roofing Systems (High-Temp)

Metal panels get very hot in direct Texas sun — the air space and deck beneath a dark standing seam roof can reach temperatures that would soften ordinary asphalt products. That is why most metal panel manufacturers require a high-temperature underlayment, typically rated to around 240–260°F or more.

Options include high-temp self-adhered membranes and high-temp synthetic sheets. Some standing seam systems also use a slip sheet so panels can expand and contract without abrading the underlayment. Felt is generally a poor match for metal: it can stick to panels, degrade under heat and trap moisture against the steel.

Stone-coated steel shingles and tiles also need an underlayment rated for the system. If you are considering metal, our Austin metal roofing team can walk you through what each manufacturer specifies.

Shingle Underlayment Types: What Goes Under Asphalt Shingles?

Most shingle manufacturers approve a short list of underlayments for their roof systems, and using matching components is often a requirement for enhanced warranties. On a modern asphalt shingle roof you will typically see:

• Synthetic underlayment across the open field of the roof
• Self-adhered leak barrier in valleys, around penetrations and sometimes along eaves and rakes
• Optional full-deck peel-and-stick for a sealed deck

Felt remains acceptable under code, but on a roof expected to last 25–30 years, the small savings rarely justify it.

Breathable vs Non-Breathable Underlayment

“Breathability” refers to vapor permeability — how much water vapor can pass through the sheet. Felt is moderately permeable. Most synthetics and all self-adhered membranes are low-perm or effectively vapor-impermeable. A few specialty synthetics are engineered to be breathable.

In a vented attic, moisture leaves through the ventilation system, so low-perm underlayment works well. The risk comes with unvented or poorly ventilated assemblies: moisture from the house can condense on the underside of the deck, leading to mold, delamination or soft OSB. In Central Texas, humid spring and fall weather makes this a real concern. The practical rule: match the underlayment to the ventilation, not the other way around.

Climate and Building Code Considerations in Austin and Texas

Most Texas cities, including Austin, adopt a version of the International Residential Code with local amendments. The IRC requires underlayment under asphalt shingles and specifies approved standards for felt and synthetic products, plus extra layers on low-slope roofs. Code editions and amendments change, so your contractor and local building department are the authority on what applies to your address.

Climate factors that shape underlayment choices here:

Coastal Texas counties have separate windstorm certification rules through the Texas Department of Insurance that call for specific underlayment methods; those do not apply in Travis, Williamson or Hays County, but some insurers offer discounts for enhanced roofing details anywhere in the state.

Underlayment Cost Differences and Long-Term Value

Upgrading from felt to synthetic typically adds only a small amount per square — on an average Austin home with 20 to 30 squares of roof, the difference is usually a few hundred dollars. Adding self-adhered membrane in valleys and around penetrations adds a bit more. A full peel-and-stick sealed deck costs considerably more, often adding $1,000 to several thousand dollars depending on roof size and product.

Weigh that against the cost of a single interior leak — wet insulation, stained drywall, possible mold remediation — and the math usually favors the better underlayment. It also matters for re-roofing timelines: the roof you install today should not need its underlayment replaced for decades. To see how underlayment fits into the overall budget, see our cost of asphalt shingles per square guide.

Choosing the Right Roofing Underlayment

A simple way to decide:

• Standard asphalt shingle roof: synthetic underlayment plus self-adhered membrane in valleys and around penetrations.
• Hail- and wind-exposed home you plan to keep long-term: consider a full peel-and-stick sealed deck.
• Low-slope sections (2:12–4:12): self-adhered membrane or double-layer underlayment per code.
• Metal or stone-coated steel: high-temperature underlayment approved by the panel manufacturer.
• Tight budget or outbuilding: #30 felt is still a code-compliant option.

Ask any roofer you interview which underlayment brand they plan to use, whether it is approved by the shingle manufacturer, and where they will add leak barrier. A clear answer is a good sign.

Final Thoughts

Underlayment is invisible once the roof is finished, but it determines how forgiving your roof is when something goes wrong. For most Central Texas homes, synthetic underlayment with self-adhered membrane at the trouble spots offers the best mix of cost and protection. If you are planning a roof replacement in Austin and want a straight answer on which underlayment fits your home, Austin Roofing offers free inspections and no-pressure quotes — contact us to schedule one.

Written by the Austin Roofing team. We inspect, repair and replace roofs across Austin and Central Texas every week, and we write these guides to answer the questions homeowners ask us on the roof. Questions about your own roof? Call +1 (956) 465-6045 for a free inspection.