TL;DR
The best roof underlayment for Texas weather is synthetic across the whole deck, with a self-adhering membrane in the valleys, along the eaves, and around every penetration. Synthetic handles the heat and UV that dry out and crack felt, and the peel-and-stick seals around nails where wind-driven rain gets pushed in. Ice and water shield isn’t required by code here, but the membrane still earns its place. On a new roof, get synthetic in writing and check where the membrane goes.
The best roof underlayment for Texas weather is a synthetic layer across the whole deck with a self-adhering membrane in the spots that leak first. That pairing shrugs off the heat, the UV, and the wind-driven rain we get here, while cheap felt dries out and tears loose long before your shingles wear out. On a tear-off you can usually read what was under the old roof by how it comes up, and felt almost never comes up in one piece. Here’s what actually holds up, where the good stuff goes, and how to spot it on an estimate before you sign.
What’s the best roof underlayment for Texas weather?
Synthetic underlayment across the full deck, plus a peel-and-stick membrane in the valleys, along the eaves, and around every roof penetration. Synthetic takes the heat and won’t shred during a windy install, and the self-adhering membrane seals tight around the nails, right where storms push water in. Felt still meets code, but on a hot roof it’s the first layer to give out.
Think of it as two jobs. The synthetic sheds water and protects the deck everywhere, and the membrane locks down the few spots that see the most water. Most of our residential roofing systems get built that way for a reason, and it isn’t the sales pitch. It’s what we’ve had to redo when someone else cut the corner.
Synthetic vs. felt: what holds up in San Antonio heat?
Synthetic, and it isn’t close in our climate. Felt is an asphalt product, so months of attic heat dry the oils out of it until it gets brittle, curls at the laps, and cracks. Synthetic is a woven plastic that holds its shape past 200 degrees and stays put. When we pull an old roof, the felt often comes up in dried, shattered pieces, and you can see where water had already found the seams.
That heat is the whole problem. Our summers bake past 100 degrees for weeks at a stretch, and attic temperatures run well above the outside air. The same heat that makes asphalt shingles curl is cooking the felt underneath them even faster, because it never sees a breeze. It isn’t just our opinion either: side by side, synthetic underlayment outlasts felt on tear strength and UV resistance, the two things a hot roof punishes most.
| What matters | #15 / #30 felt | Synthetic |
|---|---|---|
| Heat and UV | Dries, cracks, gets brittle | Holds up, rated past 200°F |
| Tearing on install | Tears in wind, wrinkles | Very hard to tear |
| If it gets rained on | Absorbs water, can wrinkle | Sheds water, stays flat |
| Weight and coverage | Heavier, shorter rolls | Lighter, more per roll |
| Real-world lifespan | The first thing to fail | Outlasts most shingles |
Felt costs less up front, and that’s the only round it wins. Spread over a roof that’s supposed to last 20 or more years, the few hundred dollars you save picks the layer most likely to let water reach your deck.
Does a San Antonio roof actually need ice and water shield?
Not by code, and this trips people up. The building code only requires an ice barrier in climates where January averages 25 degrees or colder, which we clearly aren’t. So the name is misleading here. We don’t get ice dams. What we get is wind-driven rain, and that’s the real reason the membrane belongs on your roof.
A self-adhering membrane isn’t about ice for us, it’s insurance against water getting shoved uphill. In a hard storm, wind drives rain sideways and up under the shingle edges, past where gravity would carry it. The peel-and-stick seals around every nail it’s fastened through, so even when water reaches the deck it has nowhere to go. That’s protection worth having whether the code asks for it or not.
Where does self-adhering membrane actually go?
In the spots that leak first: the valleys, the eaves, the low-slope sections, and around anything that pokes through the roof. Water concentrates in valleys and creeps back at the edges, and every vent, skylight, and chimney is a hole in your roof that depends on a good seal.
You don’t membrane the whole deck, and you don’t need to. That’s what the synthetic field layer is for. The membrane is the reinforcement in the few places where a plain sheet of underlayment isn’t enough, and skipping it there is one of the quiet ways a bargain roof turns into a callback.
Can the right underlayment help with summer heat?
Yes, a reflective radiant-barrier underlayment can knock down attic temperatures and take some load off your AC. It has a foil face that bounces radiant heat back out instead of letting it soak into the attic, and in our summers that adds up. It works best paired with a metal roof, where the metal reflects the sun and the barrier stops what’s left from radiating down.
It’s not magic and it won’t replace good attic insulation and ventilation. But on a west-facing slope that bakes all afternoon, the difference in attic temperature is something you can feel, and the roof deck runs cooler too, which is easier on whatever’s on top of it.
How do you know your roofer isn’t speccing cheap felt?
Read the estimate, because this is where the cheapest bid hides. Two quotes can look close on shingle brand and price while one quietly specs #15 felt and no membrane. That gap is real money the other guy isn’t spending on your roof, and you won’t see it once the shingles are on.
Ask three things. What underlayment goes on the field, felt or synthetic? Where does the peel-and-stick membrane go? And is it in writing on the estimate? A roofer who’s confident in the build will answer without dancing. If the answers are vague, or the line item just says “underlayment,” you’ve found where the price came down.
Get the underlayment right before the shingles go on
You only see underlayment for one day, the day it goes down, and then it’s buried for 20 years. That’s exactly why it’s worth getting right the first time. Synthetic across the deck, a self-adhering membrane in the valleys and edges and around the penetrations, and a radiant barrier if heat’s your fight. Get those calls right and the layer you never think about quietly does its job through every storm.
BH Roofing is a GAF Master Elite roofer serving greater San Antonio, and every job starts with a free 27-point roof inspection so you know what’s really up there before anyone quotes a build. Schedule your free inspection or call (210) 267-9029.
