Understanding Roof Underlayment and Ice-and-Water Shield

Most homeowners judge a roof by what they can see: the shingles or metal panels on top. But the material that does the quiet, unglamorous work of keeping water out of your house is often the layer you never look at. Underlayment sits between the roof deck and the finish surface, and on a rainy island roof it can be the difference between a dry attic and a slow, hidden leak. In San Juan County, where roughly 40 inches of rain falls every year and November storms drive water sideways, understanding these layers helps you spend your roofing budget where it actually counts.

What Underlayment Actually Does

Underlayment is the sheet material installed directly over your roof deck (the plywood or OSB sheathing) before shingles or panels go on. Its job is to be a secondary water barrier. Wind-driven rain routinely pushes moisture underneath shingles and up around fasteners. Ice dams, moss buildup on shaded north slopes, and simple aging all create paths for water. When that happens, underlayment is what stops the water from reaching your wood deck and the living space below.

Think of it as insurance. Your primary roof surface handles 99 percent of the weather. Underlayment handles the leaks your primary surface will eventually let through.

The Three Main Types

Roofers generally work with three categories of underlayment, and the right choice depends on slope, budget, and exposure.

Asphalt-saturated felt is the traditional "tar paper," sold in 15-pound and 30-pound weights. It is inexpensive and time-tested, but it can wrinkle when it gets wet and tears more easily during installation.
Synthetic underlayment is a woven polypropylene or polyethylene sheet. It is lighter, stronger, resists tearing, and holds up far better if it sits exposed for a few days before the roof surface goes on — a real advantage when ferry schedules delay material or crews.
Self-adhering ice-and-water membrane is a peel-and-stick rubberized-asphalt sheet that bonds directly to the deck and seals around nails. This is the premium layer, reserved for the most vulnerable areas of the roof.

Ice-and-Water Shield: Where It Belongs

Despite the name, ice-and-water membrane matters on the islands far less for ice than for the sheer volume of wind-driven rain. This membrane creates a fully sealed, self-healing barrier — when a fastener punctures it, the rubberized asphalt grips the shank and closes around it.

You do not need it across the entire roof on most homes. It belongs in the trouble spots:

Valleys, where two roof planes meet and channel large volumes of water.
Eaves, the lower edge where water collects and where ice dams can form during a cold snap.
Around penetrations like chimneys, skylights, and vent pipes.
Low-slope transitions, where a steep roof meets a shallower section.

On coastal and exposed west-side homes — where wind can run roughly 20 percent stronger than nearby Anacortes — many builders extend membrane coverage further up rakes and eaves for extra protection.

Comparing Your Options

The table below is a general guide to help you weigh cost against performance. Figures are estimates, not a quote, and real pricing depends on roof size, slope, and access.

Underlayment Type Relative Cost Best Use Lifespan Range 15/30-lb felt Lowest Budget re-roofs, steep slopes 12–20 years Synthetic sheet Moderate Full-roof primary layer 25–50 years Ice-and-water membrane Highest Valleys, eaves, penetrations 30–50 years Full-coverage membrane Premium Low-slope, high-exposure roofs 30–50 years

A common island approach pairs a quality synthetic across the field of the roof with ice-and-water membrane in every valley, eave, and around every penetration. That combination balances cost and protection well for the local climate.

Why This Matters More Near Saltwater

Salt air is corrosive, and homes within sight of the water take a harder beating on fasteners and metal flashings. When those components loosen or corrode over decades, water finds the gaps. A properly sealed membrane layer under the flashings buys you time and reduces the odds of a leak reaching your deck before you catch it. Homeowners searching for https://cashjtjp132.hexaforgey.com/posts/how-rainwater-catchment-roofs-work-on-the-islands should first ask any prospective installer exactly which underlayment goes where, because the answer reveals how well the crew understands coastal conditions.

Questions Worth Asking

Before signing off on a re-roof, confirm these details in writing:

Which underlayment covers the main field of the roof?
How far up the eaves and valleys does the ice-and-water membrane extend?
Are all penetrations getting membrane, not just tape or sealant?
What fastener type is being used to hold the underlayment (corrosion-resistant matters near saltwater)?

The Bottom Line

Underlayment is not where you want to cut corners on a wet, windy island. The finish surface gets the attention, but the hidden layers determine whether your roof merely looks good or actually stays dry through a decade of November storms. A modest upgrade from bargain felt to a quality synthetic, plus targeted ice-and-water membrane in the vulnerable spots, is one of the highest-value decisions you can make during a re-roof. It costs a fraction of the total project and protects everything above and below it.

When you review a roofing estimate, do not just compare shingle brands and panel colors. Read the underlayment line items. That is where a knowledgeable coastal roofer separates a roof built to survive island weather from one built to look fine on installation day.

About the Author

Marcus Reilly is a Pacific Northwest roofing consultant who has spent two decades inspecting and specifying roof assemblies for wet, wind-exposed coastal homes. He writes about building envelopes and moisture control for homeowners who want to understand what happens beneath the surface of their roofs.