Synthetic Underlayment vs. Felt Under Shingles: A Cleburne Reroof Comparison

Peel the shingles back on an older Cleburne house and you usually find a black, paper-like sheet stuck to the decking. Peel them back on a roof done in the last few years and the sheet is more likely a gray or blue woven fabric printed with a brand name. That layer is the underlayment, and synthetic underlayment vs felt is the choice it comes down to on almost every Johnson County reroof. Neither is visible once the job is done, which is why it is worth understanding before the quote is signed.

Underlayment is the backup layer between the shingles and the deck

Roof underlayment is a sheet installed over the wood decking before any shingles go down, because shingles alone are not a watertight membrane. The Asphalt Roofing Manufacturers Association's technical bulletin on steep-slope underlayments opens with a flat rule: asphalt shingles should not be installed directly on a wood deck. ARMA lists its jobs as a secondary barrier against wind-driven rain, keeping the deck dry during installation, forming a required part of a fire-classified roof assembly, reducing the picture-framing of uneven deck panels, and separating shingles from resins in wood sheathing.

In Cleburne the rain job is the one homeowners feel. Spring storms drive rain sideways under shingle edges, and a hail-cracked shingle stops shedding water long before anyone notices from the yard. Whatever gets past it reaches the underlayment, which decides whether the water rolls off at the eave or soaks into the plywood.

Asphalt felt is the 15-lb and 30-lb paper roofers used for decades

Asphalt-saturated felt is a mat of organic fibers soaked in asphalt, sold in rolls under the ASTM D226 and D4869 standards. The ARMA bulletin notes that felts come in weights such as No. 15 and No. 30, which is where the familiar 15-lb and 30-lb names come from; the No. 30 product is the heavier, thicker sheet. ARMA also points out that felts allow some vapor to pass and absorb some moisture, and that perforated felt is not suitable as shingle underlayment at all.

Felt has real points in its favor. It is cheap, it has the longest track record of any underlayment, and being semi-permeable it lets some vapor move through rather than trapping it against the deck. On a straightforward roof with good decking it does the job the code asks of it.

Its weaknesses show up during installation more than after. A Journal of Light Construction comparison of synthetic underlayments reports that felts wrinkle and buckle more severely than synthetics as temperatures change, and that synthetics are more tear- and puncture-resistant. Felt that wrinkles can leave ridges that show through the finished shingles if it is covered without being flattened or replaced.

Synthetic underlayment swaps saturated paper for a woven polymer sheet

Synthetic underlayment is a polymer fabric, usually polypropylene or polyethylene, built to the ASTM D8257 standard. ARMA describes synthetics as lighter than saturated felts and able to cover more roof area per roll, and the JLC piece adds that the rolls are generally wider and longer as well.

Synthetic sheets also resist tearing when a crew walks them or wind gets under a loose edge. JLC notes that virtually all synthetics carry a UV-resistant coating, and that roofs dried in with synthetic and fastened with cap nails or cap staples have waited six to twelve months for the finish roofing. The maker's printed exposure limit is what governs any given product.

Synthetics have trade-offs too. They cost more per square than felt, and JLC reports that the labor savings are significant but do not fully cover the higher material price. Most synthetics are impermeable to both water and vapor, which makes attic ventilation matter more, not less. General-purpose synthetics also carry the shortest product warranties, while premium lines carry longer ones.

Felt and synthetic side by side on a Texas roof

The two materials differ less in what they do once buried under shingles than in how they behave on the way there:

Texas heat touches most of these. A July tear-off in Johnson County can leave a deck under full sun for hours, and a pop-up storm can soak whatever is exposed before the shingles catch up. A sheet that stays flat and shrugs off a shower makes that window less risky, which is a practical argument for synthetic on summer jobs rather than a claim that felt fails once covered.

What code and the shingle maker require underneath

The building code sets the floor and the shingle maker's instructions sit on top. Cleburne adopted the 2021 International Residential Code effective December 1, 2022, so that edition is the reference for a permitted reroof in the city. Writing in NRCA's Professional Roofing, Mark Graham's column on underlayment requirements in the 2021 codes notes that synthetic roof underlayments do not specifically comply with the 2021 IBC and IRC, and that NRCA proposals approved for the 2024 editions bring ASTM D8257 synthetics into the code text.

In practice a synthetic on a Cleburne job should be a product the shingle manufacturer approves, named in the quote. ARMA closes on the same advice: check local codes and the shingle maker's requirements, because not every product is approved in every area.

Low slopes need a double layer

Pitch changes the rule. ARMA notes that for slopes from 2:12 up to 4:12, codes generally require a double layer of mechanically fastened underlayment or a single layer of self-adhering membrane. Graham describes the two-layer method as a 19-inch starter strip along the eave followed by full-width courses overlapping 19 inches. Porch tie-ins and rear additions on older Cleburne houses are where these pitches hide.

Ice-and-water membrane belongs where water concentrates

Self-adhering membrane, often called ice-and-water shield, is a rubberized asphalt sheet with a sticky back built to ASTM D1970. ARMA explains that it seals around nail shanks, so water that backs up under the shingles is unlikely to get through even where fasteners pierce it, and calls it excellent secondary protection on ridges, hips and valleys. North Texas seldom holds the snow that builds ice dams, so in Cleburne the membrane earns its place where runoff concentrates: valleys, wall and chimney transitions, and low-slope sections.

Roofing Cleburne's replacement page describes its own approach in those terms: synthetic across the field and a self-adhering membrane through the valleys. Covering the entire deck in membrane is a different decision. ARMA notes that these membranes act as vapor retarders, so ventilation matters more when they cover the whole roof. The attic ventilation post on this site covers why a sealed deck over a starved attic is a poor pairing.

Where the choice changes the outcome on a Cleburne reroof

On a steep roof with sound decking, shingled the same day, the gap between 30-lb felt and a decent synthetic is modest. It matters in a few situations:

Whatever the material, the written scope should say which product goes on, the standard it meets, where each type runs, and how it is fastened. Roofing Cleburne states that its roof replacement scopes name the underlayment type and where each type runs before work starts, and a free roof inspection can grade the decking and pitch that drive the choice in the first place.

Underlayment Questions From Cleburne Homeowners

Is synthetic underlayment better than 30-lb felt?

Synthetic is stronger, lighter and more tolerant of sun and rain during installation, which makes it the more forgiving choice on most reroofs. Felt still meets code when installed correctly and costs less. The better question is which product the shingle maker approves and how long the deck will sit exposed.

Do I need ice-and-water shield over the whole roof in Texas?

Usually not. Ice dams are uncommon around Cleburne, so the membrane is typically used where water concentrates, such as valleys, wall and chimney transitions and low-slope sections. Covering the whole deck turns it into a vapor retarder, so it should only happen alongside a ventilation plan.

How long can underlayment stay uncovered?

It depends on the product. Felt is meant to be shingled promptly, while many synthetics carry printed exposure ratings measured in months when fastened as directed. The manufacturer's label is the limit to follow, not a rule of thumb.

Does a low-slope porch roof need something different?

Yes. Slopes from 2:12 up to 4:12 generally need two layers of mechanically fastened underlayment or a single self-adhering membrane, and below 2:12 asphalt shingles are not the right roof covering at all. Check the pitch of any additions separately.