Walk past a reroof in Sherman on tear-off day and you see the part of a roof nobody photographs: bare plywood or OSB decking, a crew rolling out sheets across it, and a pile of old shingles in the dumpster. Those sheets are the roof underlayment, and the choice between synthetic vs felt underlayment is one of the few material decisions on a shingle roof that a homeowner rarely hears about until it is already installed.
Both products do the same basic job. They differ in what they are made of, how they handle heat, moisture and sun before the shingles cover them, and which test standards they are built to. Here is each type, a side-by-side comparison, and what the code and Grayson County weather mean for your next roof.
What Roof Underlayment Does Under Asphalt Shingles
Roof underlayment is the sheet material fastened over the roof deck before shingles go on, and it serves as a second line of defense against water. Shingles shed most rain, but wind-driven water gets pushed up under tabs, around nails and along edges, and underlayment is what keeps that water off the wood.
It also protects the deck during the job itself. Between tear-off and the last bundle of shingles, the underlayment is the only thing standing between an open deck and a passing shower, and on a larger or more complex roof that window can stretch across several days. On a shingle roof it is a code requirement, so the real questions are which product, how many layers, and how it is lapped and fastened.
Felt Underlayment: Asphalt-Saturated Paper
Felt underlayment is a mat of paper or other organic fiber saturated with asphalt, the material many older roofs were built over. It is usually sold by weight class, and InterNACHI's guide to underlayment types notes it comes in two common thicknesses, 15-pound and 30-pound, with the heavier sheet being much thicker, stiffer and more resistant to damage while shingles are installed.
Felt's limits come from that organic core. InterNACHI describes it as water-resistant but not waterproof, and notes that water absorption can make it wrinkle as the sheet expands. Sunlight speeds its breakdown, and heat drives off the volatile compounds that keep it supple, so felt left uncovered too long dries and becomes brittle. Those weaknesses show up mainly when a job drags on or the sheet gets wet before the shingles go down.
Synthetic Underlayment: Woven and Spun Polymer Sheets
Synthetic underlayment is a sheet made from polymers such as polypropylene or polyethylene, often woven or spun-bonded and coated, rather than asphalt-soaked paper. Writing in the NRCA's Professional Roofing magazine, an Owens Corning technical lead explains that synthetics were designed for greater tensile and tear strength than felt, which absorbs moisture, can turn wavy on the deck and tears easily around fasteners.
The same article traces how the testing caught up. For years synthetics were qualified under standards written for asphalt felt, which allowed only a handful of tests. ASTM D8257, a specification written for mechanically attached polymeric underlayment on steep-slope roofs, adds tests for fastener pull-through, ultraviolet resistance and hydrostatic resistance, and the task group behind it spent eight years developing it.
Two practical traits matter on a job site. Because synthetics do not absorb moisture, they lie flat instead of wrinkling, and InterNACHI notes many can be left exposed for six months to a year depending on the manufacturer. Most are also lighter per roll and have a textured, non-skid walking surface, though any roof surface gets slick when it is wet or dusty.
Synthetic vs Felt Underlayment Compared
The two types line up as follows on the points that matter for a North Texas shingle roof. Exposure limits vary by brand, so the product's data sheet governs.
| Factor | Felt (asphalt-saturated) | Synthetic (polymer) |
|---|---|---|
| Base material | Organic mat soaked in asphalt | Woven or spun polymer sheet |
| Common test standards | ASTM D226 or D4869 | ASTM D8257 or a manufacturer evaluation report |
| Moisture | Absorbs water and can wrinkle | Does not absorb water and stays flat |
| Sun exposure before shingles | Short; UV and heat speed breakdown | Often months, per manufacturer |
| Tear resistance | Tears around fasteners more easily | Designed for higher tear strength |
| Weight to carry up a ladder | Heavier, especially 30-pound | Lighter per roll |
| Walking surface | Can scuff and soften in heat | Usually textured and non-skid |
Read the table as tendencies, not a verdict. A careful felt installation beats a sloppy synthetic one, because underlayment fails at its laps, edges and fasteners first.
What the Building Code Says About Underlayment
The International Residential Code, which Texas cities commonly adopt with local amendments, lists the products allowed under asphalt shingles by ASTM standard. According to an NRCA technical column on what the 2021 IRC requires for underlayment, the listed standards are felt-based specifications such as ASTM D226, ASTM D4869 and ASTM D6757, and synthetics are not specifically addressed in the 2021 codes. The same column notes that underlayments meeting ASTM D8257 were expected to comply with the 2024 IRC.
In practice, synthetics have been installed under earlier code editions on the strength of manufacturer evaluation reports, and the 2021 NRCA article mentions ICC-ES acceptance criteria written for that purpose. That is why a quote that says "synthetic underlayment" with no product name is incomplete: the brand and its evaluation or ASTM listing are what a building official and a shingle manufacturer look for.
Slope changes the rules too. The NRCA column explains that on low-slope shingle roofs starting at 2:12, the 2021 IRC requires two layers of underlayment, with successive sheets overlapped by 19 inches and end laps of at least 4 inches. Porch roofs and shallow additions are where that requirement most often comes up.
Shingle manufacturers add rules of their own. Their installation instructions name acceptable underlayments, and a system warranty can depend on following them, so confirm the underlayment on your quote is one the shingle maker accepts. The City of Sherman's building inspections office can tell you which code edition and amendments apply to a reroof permit at your address.
How Grayson County Weather Changes the Choice
Grayson County's climate favors an underlayment that tolerates heat and surprise rain. The Texas State Historical Association's Grayson County entry puts the average July high at 96 degrees and annual precipitation at about 37 inches. April and May bring North Texas its heaviest severe weather, along the US 75 corridor from Denison to Van Alstyne included.
Heat is felt's main problem here. A dark sheet on an unshaded July deck softens, scuffs under boots and loses the compounds that keep it pliable. If a summer job pauses for a day or two, synthetic's longer exposure window and flat-lying surface remove most of that worry.
Spring is a different test. A shower that rolls in mid-job hits whatever is on the deck, and felt that soaks up water can ripple, and those ripples can show through new shingles as visible waves. Synthetics shed that rain and stay flat.
Neither type is the right material for the most vulnerable spots. Valleys, the area around chimneys and pipe penetrations, and in some cases the eaves call for self-adhering membrane, often called ice-and-water shield, which seals around nails. The roof replacement scope at Roofing Sherman Pros uses synthetic underlayment across the field and self-adhering membrane in valleys and around penetrations.
When Felt Still Makes Sense
Felt remains a code-recognized, workable product, and there are situations where it is a reasonable choice. A small repair that matches an existing felt roof, a job where shingles go on the same day the deck is papered, or a shingle system whose manufacturer specifically names a felt product are all cases where 30-pound felt does the job.
If your contractor recommends felt, ask why, which weight, and how long it will sit exposed. A clear answer to those questions is worth more than the material label on the line item.
What should not change is the installation detail. Whichever sheet goes down, the laps run shingle-fashion so water flows over each seam rather than into it, fasteners follow the manufacturer's spacing, and the underlayment sits over the drip edge at the eaves and under it at the rakes so runoff cannot get behind the metal.
What to Put in Writing Before the Tear-Off
The decision that protects you is not felt or synthetic on its own; it is getting the underlayment specified on paper before the old roof comes off. Ask the quote to name the product and brand, the standard or evaluation report it meets, and the number of layers on each slope, including any porch or addition below 4:12. Ask where self-adhering membrane goes, how long the deck may stay papered before shingles cover it, and whether the shingle manufacturer's warranty accepts that underlayment.
In Sherman's July heat or a stormy April, synthetic is usually the more forgiving sheet, but a written spec and careful laps matter more than the label. If your roof is nearing replacement, Roofing Sherman Pros' replacement service lays that scope out up front, and a free roof inspection is a sensible first step if you are not sure it is time.