The day before our roof replacement, the crew dropped off a pallet of shingles and a few big rolls of something gray. I almost didn’t look. I’m glad I did. Those rolls were the underlayment, and the label told me more than the quote had.
What Underlayment Does
Underlayment is the layer between the wood deck and the shingles. It sheds water that gets past the shingles. It also protects the deck during the job, before the shingles go on.
There are two common types. Felt is the older choice, made of paper or fibers soaked in asphalt. Synthetic underlayment is a newer sheet made mostly of polymers.
The Label Tells You the Standard
This is the part I didn’t know. Building codes require felt and self-adhering underlayment to carry a label that shows which standard it meets. Different standards cover different products:
- ASTM D226 and ASTM D4869 cover asphalt-saturated organic felt.
- ASTM D8257 covers mechanically attached polymeric, or synthetic, underlayment.
- ASTM D1970 covers self-adhering membranes, like the ones often used in valleys.
The synthetic standard is fairly new. The 2024 edition of the model code lists it directly. Inside Houston city limits, the residential code is based on the 2021 edition, which took effect in January 2024. Under that edition, synthetics are used as an approved alternative to felt, backed by product evaluation reports.
Either way, Professional Roofing magazine noted that D8257 products must be plainly marked with the maker’s name, the brand name, and the D8257 number. So if a roll is synthetic, you should be able to see that on it.
Before our job, I’d read a local roofer’s comparison of underlayment options. It covers how felt and synthetic hold up in Houston weather. That helped me know what to look for on the label.
Synthetic Underlayment vs Felt in Houston
Our afternoon storms can roll in fast. If rain hits after the old roof is torn off, the underlayment is all that’s protecting the deck. Each synthetic product has its own limit for how long it can sit uncovered. That number comes from the maker, so ask your roofer what it is.
Felt still meets code, and it has a long track record. InterNACHI lists felt paper at 15 to 30 years. The right choice depends on your roof, your budget, and your shingle warranty.
Low-Slope Areas Need Extra Care
Some homes have a low-slope section over a porch or patio. The Asphalt Roofing Manufacturers Association says codes call for extra protection on slopes from 2:12 up to 4:12. That means a double layer of nailed-down underlayment or a single layer of self-adhering underlayment. Ask how your roofer plans to handle those spots.
Spray Foam Attics Change the Answer
If your attic has spray foam on the underside of the roof deck, tell your roofer before you pick an underlayment. Most synthetic underlayments block water vapor. Building science writers note that most of their makers don’t allow them over unvented roofs, and spray foam under the deck usually makes a roof unvented. Felt or a vapor-permeable synthetic may be the better fit.
Questions I’d Ask Again
- What underlayment is in my quote, by brand and type?
- Which ASTM standard does it meet?
- How long can it stay uncovered?
- What goes in the valleys and around chimneys?
- Does my attic setup limit which underlayment you can use?
- Does the underlayment affect my shingle warranty?
One more tip: take a photo of the label before the shingles go on. If you ever need to file a warranty claim, it helps to know exactly what’s under your roof.
If you want to see what the synthetic standard covers, ASTM D8257 is listed on ASTM International’s site. You don’t need to buy it. Just knowing the number helps you read the label.