Is Synthetic Roof Underlayment Waterproof?
“Your shingles face the storm, but your underlayment takes the first hit.”
Introduction:
When choosing a roof underlayment, homeowners and contractors often ask: Is synthetic roof underlayment waterproof? It’s a fair question—after all, this layer sits just below your shingles and plays a key role in keeping moisture out. In this post, we break down what "waterproof" really means in roofing, how synthetic underlayments compare to traditional felt, and whether they truly keep your deck dry.
What Is Synthetic Roof Underlayment?
Synthetic underlayment is a lightweight, engineered roofing membrane made from woven or spun polypropylene or polyethylene. It’s designed to resist tearing, offer superior traction during installation, and protect the roof deck from water infiltration before and after shingles are installed.
So… Is It Waterproof?
Short answer: Yes, but with a caveat.
Synthetic underlayment is highly water-resistant and can withstand rain, snow, and standing water for a limited time before shingles are installed. Most premium synthetic underlayments have:
Hydrophobic surfaces that shed water
Non-absorbent fibers that don’t swell or break down
Seam overlaps that channel water off the roof
UV-resistance to maintain integrity for up to 90–180 days exposed
But: Synthetic underlayment is not a fully sealed membrane like peel-and-stick rubberized asphalt. It's designed to be part of a layered roofing system—not the sole waterproofing barrier.
Synthetic vs. Felt Underlayment
FeatureSynthetic UnderlaymentAsphalt FeltWater ResistanceExcellent (shed water)Moderate (can absorb water)Tear ResistanceHigh (walkable & durable)Low (tears easily when wet)Weight & HandlingLight & easy to installHeavy & slippery when wetExposure Duration60–180 days (brand dependent)7–30 days typical
Where Waterproofing Truly Matters
Before Shingles Are Installed: Synthetic underlayment offers critical temporary water protection if the roof gets exposed to rain.
Long-Term Protection: Once covered with shingles, the underlayment serves as backup—not the first line of defense.
Ice & Water Zones: For valleys, eaves, and ice-dam areas, rubberized peel-and-stick underlayments are recommended for true waterproof sealing.
Conclusion:
Yes—synthetic roof underlayment is effectively waterproof for practical purposes. It resists water, sheds rain, and keeps the roof deck dry during construction. However, it’s not meant to be a standalone waterproofing solution. Pair it with proper shingles, flashing, and (in key areas) ice and water shield for a truly weatherproof roofing system.
