If you live in the Pacific Northwest, especially in areas like King County, Snohomish County, your roof endures months of rain, wind, and moisture every year. And while shingles or metal panels are your roof’s visible shield, it’s the underlayment beneath that serves as your home’s true last line of defense.
In this guide, we’ll explain what roof underlayment is, why it matters more in our wet climate, and how the right materials can dramatically improve the durability of your roofing system.
What Is Roof Underlayment?
Roof underlayment is a protective layer installed between the roof deck and the final roofing material (like shingles or metal). It’s designed to provide a secondary water-resistant barrier in case your top layer fails, lifts, or leaks.
There are typically three main types of underlayment used in the Pacific Northwest:
- Asphalt-saturated felt (tar paper) – A traditional option that provides basic protection.
- Synthetic underlayment – More durable and moisture-resistant; now widely preferred in modern roofing.
- Self-adhering membrane (ice & water shield) – Offers extra protection in vulnerable areas like valleys, edges, and penetrations.
Why Roof Underlayment Is Critical in the Pacific Northwest
1. We Get a Lot of Rain
King and Snohomish counties average 35–50 inches of rainfall per year, and underlayment helps protect against leaks that shingles alone can’t always stop. Especially when wind-driven rain hits flashing or lifts shingles, underlayment keeps water from entering your attic.
2. Moss, Debris, and Tree Cover Increase Risk
Leaves and pine needles often collect on roofs in forested neighborhoods like Woodinville, Bothell, and Lynnwood. This organic matter traps moisture and accelerates deterioration, but quality underlayment provides a layer of security.
3. Moisture Backup from Clogged Gutters
Improper drainage causes water to back up under shingles, especially on lower roof pitches. A waterproof membrane in critical areas helps prevent rot and mold inside the roof structure.
Where High-Performance Underlayment Makes a Difference
- Valleys and eaves (where runoff concentrates)
- Roof penetrations (vents, chimneys, skylights)
- Low-slope areas prone to pooling water
- Metal roofing systems that may allow condensation under panels
A roof without high-quality underlayment in these areas is at much higher risk for interior damage, especially during the Pacific Northwest’s long wet season.
Recommended Underlayment Schedule
| Roof Type | Underlayment Recommendation |
| Asphalt Shingles | Synthetic + ice & water shield at eaves/valleys |
| Metal Roofing | Synthetic or breathable underlayment, plus eave protection |
| Low-Slope Roofs | Full-coverage waterproof membrane |
Does Underlayment Impact Warranties?
Yes. Most roofing manufacturers require specific types of underlayment for warranty coverage to remain valid. Using incorrect or low-quality materials could void your warranty, even with a brand-new roof.
Bottom Line: Don’t Skip the Layer That Keeps Your Home Dry
Underlayment may not be visible, but it plays a critical role in roof performance — especially in a damp region like the Pacific Northwest. If you’re planning a roof replacement or repair in King or Snohomish County, talk with your contractor about the underlayment being used.
At Blue and Blue Roofing, we use premium synthetic and waterproof underlayment systems to match each roof’s specific needs and climate exposure. Contact us today to schedule a consultation or learn more about how to protect your home from the inside out.
