Back to Blog Materials Roof Underlayment: The Invisible Layer That Makes or Breaks Your Roof
Mark WhiteAugust 28, 20255 min read If shingles are your roof's armor, underlayment is the bulletproof vest underneath. You can't see it, you probably don't think about it, and cheap roofers sometimes skip it or use the bare minimum. That's a mistake that won't show up for a few years, and then it shows up all at once.
Here's what's happening under your shingles and why it matters.
What Underlayment Does
Underlayment is a water-resistant (or waterproof) layer installed directly on your roof deck before shingles go on. Its job is to provide a secondary barrier against water intrusion when shingles fail to keep water out, which happens more than you'd think during wind-driven rain, around penetrations, and in valleys.
Think of it as your roof's backup plan. The shingles handle 95% of the weather. The underlayment handles the other 5%, which is the 5% that causes water damage.
Felt vs. Synthetic: The Choice That Matters
Traditional felt paper (15# or 30#) was the standard for decades. It works, but it's heavy, tears easily during installation, wrinkles when wet, and deteriorates faster in Texas heat. If your roof deck is exposed to rain during installation (common during San Antonio's unpredictable storms), felt absorbs water and can wrinkle permanently under the shingles.
Synthetic underlayment is lighter, stronger, lies flat, resists tearing, and handles moisture exposure without degrading. It costs about $200-$400 more on a typical home. At Legacy, we use synthetic underlayment on every residential roof, no exceptions. The performance difference is too significant to cheap out on.
Schedule a Free Roof Inspection
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Book Your Free Inspection →Ice and Water Shield: Critical for San Antonio
Ice and water shield is a self-adhering, fully waterproof membrane used in leak-prone areas: valleys, around penetrations (chimneys, vents, skylights), along eaves, and anywhere water concentrates. Unlike regular underlayment, ice and water shield bonds directly to the deck, even if a nail penetrates it, the membrane seals around the nail.
We don't get ice dams in San Antonio, but we absolutely get wind-driven rain. Ice and water shield in valleys and around penetrations is non-negotiable in our climate.
When comparing roofing estimates, check the underlayment specs. If a competitor's quote says '15# felt paper' or doesn't mention underlayment at all, that's your sign to look closer.
Got a roofing question? Something keeping you up at night? (Besides that mysterious dripping sound?) Email me personally at mwhite@legacytx.com. I read and answer every single one.
MW Mark White
Owner, Legacy Home Improvements · San Antonio since 1999
Mark has been in the roofing business for over 25 years. He personally inspects homes, works with insurance adjusters, and answers every email at mwhite@legacytx.com.
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