What Is Ice and Water Shield? A Guide for Skagit, Island and Whatcom County Roofs

Updated August 2026 · Alpine Exteriors · WA contractor license ALPINI751OW

The Short Answer

Ice and water shield is a self-adhering, waterproof membrane installed directly on the roof deck beneath the visible roofing material, most commonly at the eaves, valleys, and around penetrations like chimneys and skylights. It seals tightly around every nail that passes through it, creating a backup barrier in the exact spots where water is most likely to find its way underneath shingles or metal panels and into the house.

The Detail Behind It

A standard roof relies on layers working together: the shingles or panels shed the bulk of the water, a synthetic or felt underlayment covers the whole deck as a general moisture barrier, and flashing directs water away from transitions. Ice and water shield is a third, more aggressive layer used only where the risk of water intrusion is highest. Unlike standard underlayment, which is simply laid down and fastened, ice and water shield has a rubberized asphalt or synthetic adhesive backing that bonds to the plywood or OSB deck and self-seals around fastener penetrations. That self-sealing property is the whole point — even if wind-driven rain or meltwater works its way past the shingles above, it hits a membrane with no clear path downward through nail holes.

The name comes from its original purpose in colder climates: stopping water from ice dams. An ice dam forms when heat escaping through the attic melts snow on the upper roof, that meltwater runs down to the colder overhanging eave, refreezes, and builds a ridge of ice that backs water up under the shingles. Ice and water shield at the eaves gives that backed-up water nowhere to go but back out, instead of into the attic insulation and ceiling drywall. But the membrane does the same job for any water forced backward under the roofing by wind or debris, which is the more common scenario in this part of Washington.

Roofers typically run the membrane from the edge of the eave to a point roughly two feet inside the exterior wall line, far enough to cover the area directly above living space, and they run full-width strips down every valley, since valleys concentrate more runoff per square foot than any other part of the roof. Low-slope sections, dormer cheeks, skylight curbs, and chimney saddles usually get it too, because those are the spots where flashing alone has the least margin for error.

What Makes This Region Different

Skagit, Island and Whatcom counties don't deal with the deep, sustained freeze-thaw cycling that makes ice damming a near-certainty in the Rockies or the upper Midwest. In most winters, a roof in Mount Vernon, Burlington, or Anacortes will see more rain than snow, and true ice dams are the exception rather than the rule at sea level. That said, a few local conditions still make ice and water shield worth taking seriously here, just for slightly different reasons than in a colder state:

• Heavy, sustained winter rain. The wet season here runs long, and it's the volume and duration of rain — not any single storm — that eventually finds every weak point in an aging roof. A membrane at the eaves and valleys buys real margin over years of near-constant moisture exposure.
• Wind off the Skagit flats and the Strait. Open farmland around Mount Vernon, Burlington and La Conner funnels wind with little to break it, and homes on Camano Island, in Anacortes, or facing the Strait of Juan de Fuca take wind-driven rain that pushes water sideways and upward under shingle tabs rather than letting gravity do the work of shedding it. That's exactly the failure mode ice and water shield is built to catch.
• Moss on shade-heavy roofs. Douglas fir and cedar canopy over much of Skagit and Whatcom County keeps north-facing roof slopes damp and shaded for months at a time, which is ideal moss habitat. Moss holds moisture against the roofing material and can lift shingle edges over time, which increases the odds that water gets underneath — another reason the eave and valley membrane matters even where classic ice damming isn't the driver.
• Salt air on the islands and waterfront. Camano Island, La Conner, and the Anacortes shoreline see salt-laden air that accelerates the wear of fasteners and metal flashing faster than it would inland. As flashing and fastener seals age faster, the backup membrane underneath becomes more valuable, not less.
• Occasional ice and snow in the foothills. Homes up toward Sedro-Woolley and the higher ground east of it do see real snow and the occasional hard freeze most winters, and that's where classic ice-dam behavior at the eaves is most likely to actually show up. A roof in the foothills is a stronger candidate for full eave coverage than one down on the flats.

What doesn't apply here is worth saying plainly: this isn't hurricane country, and ice and water shield in this region isn't doing the job of a wind-uplift or storm-surge defense the way it might in a Gulf Coast or Southeast installation. Its job locally is quieter — managing chronic wet weather, wind-driven rain, and the occasional cold snap, not a named storm event.

What the Homeowner Should Actually Do

You don't need to cover an entire roof deck in ice and water shield, and a contractor who proposes that for a standard asphalt shingle roof on the flats is probably overbuilding the job and the price along with it. What's worth asking for, and worth confirming is actually happening, is coverage at the places that matter: the eaves (extended further if your home is up toward the foothills or has a documented history of ice buildup), every valley, and around every roof penetration — skylights, chimneys, plumbing vents, and any dormer.

• If you're re-roofing, ask your contractor to walk the specific eaves, valleys, and penetrations on your roof and explain where they plan to install membrane and why, rather than accepting a generic answer.
• If your home is shaded and prone to moss, treat and physically remove moss regularly — the membrane is a backup, not a substitute for keeping the roof surface itself in good condition.
• Keep gutters clear, especially under fir and cedar canopy. Clogged gutters back water up at the eave in almost the same way an ice dam does, and a clear gutter reduces the load on every other layer of the roof system.
• If you're on Camano Island, in Anacortes, or anywhere with direct exposure to the Strait, ask specifically about fastener and flashing material, since salt air shortens the service life of standard galvanized components faster than it does inland.
• Budget for it as part of a reroof rather than as an afterthought. Adding ice and water shield during a full roof replacement is a modest incremental cost — commonly a few hundred to around a thousand dollars depending on roof size and how many valleys and penetrations there are — while retrofitting it later usually means tearing into finished roofing, which costs considerably more.

The honest drawback is that ice and water shield isn't a cure-all: it won't fix a roof that's already failing structurally, it won't stop moss from growing, and it does nothing for a leak caused by bad flashing detail work rather than water finding a nail hole. It's one layer in a system, and it only performs as well as the flashing, underlayment, and shingle work installed around it. Alpine Exteriors treats it as a standard part of a properly detailed reroof in this region, not an upsell, because the places it protects are the places we see the most callbacks on roofs that skipped it.

FAQ

Does every roof in Skagit, Island or Whatcom County need ice and water shield?

Not the entire roof deck, but nearly every roof benefits from it at the eaves, valleys, and penetrations, since those are the spots where wind-driven rain and any freeze-thaw activity concentrate. How far up the eave it extends is a reasonable thing to vary by elevation and exposure — a home near Sedro-Woolley's higher ground or with a documented ice history warrants more coverage than a sheltered lot down on the flats.

Will ice and water shield stop moss from growing on my roof?

No. Moss grows on the surface of the roofing material because of shade and moisture, and a membrane installed underneath the shingles has no effect on that. What ice and water shield does is reduce the consequences if moss-lifted shingles let water underneath — it's a backup for water intrusion, not a moss treatment, and moss still needs to be managed separately.

How much does adding ice and water shield cost during a reroof?

It varies with roof size, roof pitch, and how many valleys and penetrations the roof has, but as part of a full reroof it's typically a modest incremental cost — commonly in the range of a few hundred to around a thousand dollars — rather than a major line item. Adding it later, after the roofing is already installed, generally costs more because it requires removing finished material to get to the deck.

Is ice and water shield required by Washington building code?

Washington's adopted residential building code requires an ice barrier membrane at eaves in areas with a history of ice-dam-related water damage, and most roofing contractors in this region install it at eaves and valleys as standard practice regardless of the specific code trigger for a given jurisdiction, simply because it's inexpensive insurance against the wettest and windiest parts of the year. A local contractor can tell you what applies for your specific city and roof design.

Get a Straight Answer for Your Roof

If you're planning a reroof or dealing with a leak that keeps coming back in the same spot, Alpine Exteriors can walk your roof, show you exactly where a membrane like this makes sense for your home's exposure and elevation, and give you a clear scope before any work starts. Alpine Exteriors is a licensed Washington contractor (ALPINI751OW) serving Mount Vernon, Anacortes, Burlington, Sedro-Woolley, La Conner, Stanwood, Camano Island and Bellingham — reach out for a roof evaluation and a straightforward, written estimate.