How Underlayment Works in Your Roof System
Your shingles are the first line of defense against rain and snow, but they are not waterproof on their own. Underlayment is rolled out over the wood roof deck before shingles go on, creating a continuous protective sheet that catches any water that slips past the shingles.
There are three common types you may hear about:
- Asphalt-saturated felt (the traditional black "tar paper"), rated by weight as 15-lb or 30-lb.
- Synthetic underlayment, a lightweight woven polymer sheet that resists tearing and lies flatter.
- Self-adhering ice-and-water membrane, a rubberized peel-and-stick sheet that seals tightly around nails and is used in the most vulnerable areas.
Why Underlayment Matters in the Spokane and Inland Northwest Climate
Our region puts roofs through a punishing cycle: heavy winter snow loads, repeated freeze-thaw swings, and ice dams that form when snow melts at the warm ridge and refreezes at the cold eaves. When an ice dam backs water up under the shingles, the underlayment is often the only thing standing between that meltwater and your attic, insulation, and ceilings.
Self-adhering ice-and-water membrane is especially important here. Building codes and best practices call for it along eaves, in valleys, and around penetrations like chimneys and vents, because those are the exact spots where Inland Northwest ice and wind-driven moisture cause the most leaks.
Choosing the Right Underlayment for Your Home
For most Spokane-area homes, a quality synthetic underlayment across the main roof field plus ice-and-water membrane in the high-risk zones offers the best protection against our snow and freeze-thaw conditions. Felt can still work but tends to wrinkle, tear in high wind, and absorb moisture if left exposed.
Because underlayment is hidden once shingles are installed, the only practical time to upgrade it is during a full roof replacement. As a GAF Master Elite contractor, we install underlayment as part of a complete, warrantied roofing system rather than treating it as an afterthought.
Signs Your Underlayment May Be Failing
You usually cannot see underlayment directly, but warning signs of failure include:
- Water stains on attic sheathing or upper-floor ceilings after snowmelt or heavy rain.
- Recurring leaks near valleys, eaves, chimneys, or skylights.
- An older roof (often 20-plus years) that used thin felt now nearing the end of its life.
Catching these early can prevent rot in the roof deck and framing. If you are seeing stains, schedule a free estimate and we will inspect the system before damage spreads.