What Is Roof Underlayment, and Why Does It Matter Here in Skagit and Whatcom County?

Updated August 2026 · Alpine Exteriors · WA contractor license ALPINI751OW

The Direct Answer

Roof underlayment is the water-resistant layer installed directly on the roof deck, underneath your shingles, metal panels, or other visible roofing material. It is the roof's backup barrier — the layer that keeps water out if wind-driven rain, ice, or a damaged shingle lets moisture past the primary surface.

Most homeowners never see their underlayment because it disappears under the finished roof during installation, but it is doing real work every wet week of the year in this part of Washington. On a re-roof, the underlayment is arguably as important as the shingles themselves, because it is the material actually touching your roof deck and your home's structure.

The Detail Behind It

Underlayment has been part of roofing for well over a century, originally as asphalt-saturated felt paper — what most people still call "tar paper" or "15-lb felt." Felt is inexpensive and still code-compliant in most applications, but it tears more easily during installation, absorbs water if it's rained on before the roof covering goes down, and can wrinkle over time, which sometimes telegraphs through asphalt shingles as a slightly uneven roof plane.

Synthetic underlayment has largely replaced felt on the roofs we install and repair. It's woven polypropylene or polyester rather than saturated paper, so it's lighter, far more tear-resistant underfoot, and much less likely to absorb water and wrinkle if a work day gets rained out mid-install — which happens more than roofers would like to admit around here. Synthetic products also tend to have a longer UL-rated exposure window, meaning the underlayment itself can sit exposed to weather for a longer stretch before the finish roofing goes on, which matters when a job spans more than a day or two.

There's a third category worth knowing: self-adhered ice-and-water membrane, a rubberized asphalt product that bonds directly to the deck and seals around fastener penetrations. It's typically used only in specific vulnerable zones — valleys, eaves, around chimneys and skylights, and roof-to-wall transitions — rather than across the whole roof, both because it's more expensive per square and because it isn't a vapor-permeable material, so overusing it can trap moisture in the roof assembly.

Underlayment also plays a secondary role most people don't think about: it's the last line of defense during the gap between tear-off and the new roof going on. A roof that's been stripped to bare decking is vulnerable, and a properly lapped and fastened underlayment is what keeps an interrupted job from turning into a ceiling leak overnight.

What Makes This Region Different

Underlayment choice and detailing matter more here than in a lot of the country, mainly because of how much rain this region gets and how long it stays wet, not because of any single dramatic weather event. Skagit and Whatcom County don't see hurricanes or the wind-mitigation concerns that drive roofing codes in places like Florida — the pressures here are slower and more cumulative.

• Sustained winter rain: The marine climate from Mount Vernon and Burlington out through Anacortes and La Conner brings long stretches of steady, low-intensity rain rather than short intense storms. That means underlayment has to perform for months at a time, not survive one event — lap direction, fastener pattern, and how carefully the deck is prepped all matter more when a roof stays wet for days on end.
• Moss and shaded roof sections: Homes tucked under fir and cedar canopy in Sedro-Woolley, the foothills, and parts of Bellingham hold moisture longer on north-facing and shaded slopes. Moss growth doesn't just look bad — it lifts shingle edges and holds water against the roof surface, and a compromised or aging underlayment underneath has less margin for error when that happens.
• Wind off the Skagit flats and the Strait: Open farmland around Mount Vernon, Burlington, and the flats near the Skagit River, plus exposure off the Strait of Juan de Fuca near Anacortes, produces steady wind-driven rain rather than sudden violent gusts. That's exactly the condition where a weak underlayment or poor lap technique shows up first, at eaves and valleys where wind pushes water sideways and uphill under the shingle line.
• Salt air on the islands: Camano Island and the Anacortes and Fidalgo waterfront deal with airborne salt that accelerates corrosion on exposed metal fasteners and flashing. It doesn't degrade the underlayment fabric itself, but it does mean the metal components that interact with it — nails, drip edge, valley metal — need to be corrosion-resistant, or the underlayment ends up doing more of the work than it should.
• Occasional ice and snow in the foothills: Areas east toward Sedro-Woolley and the higher elevations around the Cascade foothills see occasional freeze events and light snow that lowland Bellingham or Stanwood rarely get. Where that risk exists, ice-and-water membrane at the eaves is the right call, sized to extend past the interior wall line so it actually protects against ice-dam backup, not just decorative coverage at the roof edge.

The Washington State Energy Code also shapes how underlayment fits into the whole roof assembly, even though the code itself is aimed at insulation and air-sealing rather than underlayment specifically. Adequate roof ventilation is required to control condensation from the warm, moist interior air that's common in well-insulated Northwest homes, and a vapor-permeable underlayment works with that ventilation rather than against it. Using the wrong underlayment — something that's effectively a vapor barrier where the assembly needs to breathe — can trap moisture in the sheathing over a wet winter, which is a slower and less obvious problem than a leak but can be just as costly to fix.

What the Homeowner Should Actually Do

If your roof is over 15 to 20 years old, or you've noticed granules collecting in gutters, curling shingle edges, or damp spots on interior ceilings after a heavy rain, it's worth having someone actually look at the deck and underlayment condition, not just the shingles from the ground. Underlayment failure is often invisible until water has already found its way through, so a proactive look during a re-roof estimate or after a wind event is cheaper than waiting for a stain to appear on the ceiling.

A few honest things to know before you sign a contract:

• Felt underlayment is not automatically wrong — it's still code-compliant and cheaper, and on a budget re-roof with a competent installer and good weather timing, it can perform fine for its expected service life. The tradeoff is less forgiveness if the job gets rained on mid-install or if the crew rushes the lap detailing.
• Synthetic underlayment costs more per square than felt — often somewhere in the range of an extra $10 to $30 per roofing square depending on the product line — but the extra durability and tear resistance often pays for itself in fewer callback issues, especially on steeper or more complex roof planes common on older Skagit Valley farmhouses and craftsman-style homes.
• Ice-and-water membrane at eaves and valleys is worth insisting on for shaded, moss-prone roofs and for anything in the foothill zones with occasional ice risk — it's a relatively small added cost concentrated exactly where failures start.
• Ask what underlayment product is actually being quoted, not just "underlayment included." Contractors sometimes list it as a line item without naming the product, and felt versus synthetic versus ice-and-water membrane are genuinely different materials with different performance, not interchangeable line items.
• Don't skip re-decking discussions. If the sheathing underneath is soft, delaminated, or has visible rot, no underlayment — however good — will hold up long term on a compromised deck.

Alpine Exteriors walks homeowners through these choices during every roofing estimate, because the underlayment decision is one that gets buried under the visible shingle color and style choices, even though it affects how the whole roof performs over the wet season. We'll tell you plainly when felt is a reasonable choice for the budget and when it isn't worth the savings for your specific roof.

FAQ

Can I just have new shingles installed over my existing underlayment?

Generally no, at least not on a full tear-off re-roof — once the old shingles come off, the underlayment underneath is exposed and inspected, and it's almost always replaced as part of the job since it's been through however many years of weather the old roof endured. Overlay roofs, meaning new shingles installed directly over old ones without a tear-off, skip this step entirely, which is one of several reasons overlays are a shorter-term fix rather than a full reset of the roof system.

Does synthetic underlayment eliminate the need for ice-and-water membrane at the eaves?

No, they serve different purposes. Synthetic underlayment sheds general moisture across the whole roof field, while ice-and-water membrane self-seals around fasteners and handles the specific risk of ice damming or wind-driven water backing up under the shingle line at eaves and valleys — most well-detailed roofs in this region use both, in the zones where each performs best.

Will a better underlayment stop moss from growing on my roof?

Not directly — moss grows on the shingle surface and in shaded, damp areas regardless of what's underneath, and underlayment quality doesn't change how much moss establishes. What good underlayment does is limit the damage if moss holds moisture against the deck long enough to lift shingle edges, which is why regular moss removal and roof ventilation matter alongside the underlayment choice, not instead of it.

How long does roof underlayment actually last?

It depends heavily on the product and how well it was protected during installation and by the roof covering above it — felt underlayment is generally considered the shorter-lived option, while synthetic products are built to outlast the shingles they're installed under in most cases. In practice, underlayment condition should be evaluated whenever the shingles are removed for any reason, rather than tracked on its own separate replacement schedule.

Get a Straight Answer on What Your Roof Needs

If you're planning a re-roof or just want an honest read on what's happening under your current shingles, Alpine Exteriors serves homeowners across Mount Vernon, Anacortes, Burlington, Sedro-Woolley, La Conner, Stanwood, Camano Island, and Bellingham, and we're happy to walk your roof and explain what we find in plain terms. Alpine Exteriors holds Washington contractor license ALPINI751OW — reach out for a roof inspection or re-roof estimate, and we'll tell you what your underlayment situation actually calls for, not just what's easiest to sell.