Synthetic Underlayment vs. Felt Paper
For decades, roofers rolled out asphalt-saturated felt (commonly called #15 or #30 tar paper) as the layer between the wood deck and the shingles. Synthetic underlayment is the engineered successor, made from tightly woven plastic fibers rather than paper soaked in asphalt.
- Stronger: It resists tearing and foot traffic far better than felt, which is brittle and prone to ripping in cold weather.
- Lighter: A roll covers far more area at a fraction of the weight, so it goes down faster with fewer seams.
- Water-resistant, not water-absorbing: Felt soaks up moisture and can wrinkle or rot; synthetic sheds water and stays dimensionally stable.
- UV-tolerant: Most synthetics can be left exposed for weeks during a project without breaking down, unlike felt.
Why It Matters in the Spokane / Inland Northwest Climate
Our region puts roofs through a brutal cycle: heavy snow loads, repeated freeze-thaw swings, ice dams along the eaves, and gusty winds off the Palouse and through the mountain valleys. Synthetic underlayment is built for exactly these stresses.
- Freeze-thaw: Felt becomes brittle and cracks when temperatures drop below freezing; synthetic stays flexible, so the secondary barrier holds when an ice dam forces meltwater backward under the shingles.
- Snow and moisture: Because it does not absorb water, synthetic will not wrinkle or wick moisture into the deck during the long wet shoulder seasons of fall and spring.
- Wind: Its tear resistance and tighter fastening pattern mean it stays put during installation and provides a more reliable backup layer if a shingle is ever lifted in a windstorm.
It works alongside, not instead of, a peel-and-stick ice-and-water shield at the eaves, valleys, and penetrations where ice dams and leaks are most likely.
Where Synthetic Underlayment Fits in a Roof System
Underlayment is the quiet second line of defense. Shingles take the brunt of the weather, but when wind-driven rain, blowing snow, or ice-dam backup gets past them, the underlayment is what keeps water off the wood deck and out of your attic.
On a full roof replacement, we typically combine self-adhered ice-and-water membrane in the vulnerable zones with synthetic underlayment across the broad field of the roof. As a GAF Master Elite contractor, DG Contracting installs manufacturer-matched components so the whole system qualifies for the strongest available warranty coverage.
Signs Your Underlayment May Be Failing
Underlayment is hidden, so homeowners rarely see it directly. Watch instead for the symptoms of a compromised secondary barrier:
- Water stains on ceilings or in the attic after a snowmelt or heavy rain.
- Persistent ice dams that leak at the eaves each winter.
- An aging roof (typically built with old felt) that has already lost or curled shingles.
- Decking that feels soft or shows moisture when inspected from the attic.
If you are seeing these signs, a roof assessment can confirm whether the underlayment and deck are still protecting your home. We offer a free estimate across Spokane and North Idaho.