Ice & Water Shield: Why San Juan Island Roofs Need It

When rain blows sideways off the strait and pounds a roof for days, plain underlayment isn’t always enough. That’s where ice and water shield earns its keep. It’s the one layer of a San Juan Islands roof that is truly waterproof, not just water-resistant. Here’s what it is, where it goes, and why it matters more out here than the mainland thinks.

Ice and water shield is a peel-and-stick, rubberized-asphalt waterproof membrane. It bonds right to the roof deck at valleys, eaves, and around penetrations like chimneys and skylights. In the San Juan Islands’ 40 inches of yearly rain and strong west-side wind, it seals the roof’s most leak-prone spots, even when water is pushed uphill under the shingles.

What exactly is ice and water shield?

Ice and water shield is a rubberized-asphalt membrane with a peel-off backing. Roofers roll it right onto the wood roof deck, where it bonds into one solid, fully waterproof layer. Its main trick is that it seals around fasteners. When a nail or screw goes through the sheet, the soft asphalt grips the shank so tightly that water can’t run down it into the deck. Plain felt and synthetic underlayment can’t do that. They shed water off the top but leave nail holes open. The membrane is also thicker and more bendable than felt. So it hugs corners, skylight curbs, and flashing without wrinkling. Because it’s truly waterproof, not just water-resistant, it’s saved for the parts of a roof where leaks really start, not spread across the whole deck.

Felt vs. synthetic vs. ice and water shield

Every re-roof needs some kind of underlayment between the roof deck and the shingles or panels. There are three common types, and a good island roof usually uses two of them together. Here’s how they compare on an island job:

Felt is the old standard. It’s cheap, but it wrinkles when it gets wet and only resists water. Synthetic underlayment is the modern layer: tougher, lighter, lies flat, and holds up far better through a wet island winter and a windy install day. Ice and water shield is the waterproofing specialist. You don’t wrap a whole roof in it. That wastes money and, on some roofs, can trap moisture. You put it right where water gathers or gets pushed uphill against the flow of the roof.

Where should ice and water shield be installed?

The membrane goes wherever water pools, funnels, or gets pushed backward. On a San Juan Islands home, that means:

1. Valleys. Where two roof planes meet, all that 40 inches of rain funnels into a fast-moving channel. Valleys are the most common leak point. So a full-width run of ice and water shield goes down first, before any flashing.
2. Eaves. The lower edge of each roof plane is where wind-driven rain can push water back up under the first courses. It’s also where backed-up water sits. Membrane here protects the deck and the top of the wall below.
3. Around penetrations. Chimneys, skylights, plumbing vents, and dormers are holes in the roof, and every hole is a leak waiting to happen. The membrane wraps these before the metal flashing goes on.
4. Rake edges and low-slope sections. On exposed west-side roofs, and on any pitch shallow enough that water drains slowly, the extra waterproofing pays off.
5. Sidewalls and dead valleys. Anywhere a roof meets a wall, or a spot where water can hang up instead of running clear.

On steep, well-drained areas, synthetic underlayment does the job just fine. The membrane’s cost and labor go only where they matter. That keeps the roof watertight and fairly priced.

What happens if a roof is built without it?

Corners get cut, especially on quick or cheap re-roofs. When ice and water shield is left out of the valleys and eaves, the roof can look perfect for a year or two. Then a hard November storm drives rain uphill. Water finds a nail hole or an open valley seam and runs down into the deck and ceiling. Most leaks our roof repair crews trace start in exactly these spots: a valley or a skylight curb that never got proper waterproofing under the visible roofing. Fixing it later usually means tearing back finished roofing to reach the bare deck. So it costs far more than doing it right the first time.

Do you need ice and water shield in the Pacific Northwest?

Yes — but not for the reason the name suggests. The product was named for ice dams, which are a bigger deal in colder, snowier places. Here in the San Juan Islands, hard freezes and heavy snow are rare. Our threat is water: about 40 inches of rain a year, heaviest in November, often coming in nearly sideways on west-side wind that gusts about 20% stronger than Anacortes. That wind-driven rain is just what gets pushed uphill under shingles and into valleys. And it’s just what a waterproof membrane is built to stop. Add the steady damp that feeds moss on shaded north slopes under the Douglas firs, and the case for sealing your roof’s weak points is stronger here than in a drier place. It’s a small line item against the full cost of a new roof in the San Juan Islands. And it’s one of the first things we spec, whether you’re leaning toward metal or asphalt shingles.

How ice and water shield fits a proper re-roof system

Underlayment isn’t a stand-alone product. It’s part of a layered system, and the layers have to go on together to work and to be covered by warranty. On a quality roof replacement, the order from the deck up looks roughly like this:

1. Ice and water shield at valleys, eaves, and penetrations.
2. Synthetic underlayment across the rest of the deck, lapped over the membrane.
3. Flashing at every edge, valley, and penetration.
4. Shingles or standing-seam panels as the finished surface.
5. Ridge venting and, where moss is a concern, zinc or copper strips near the ridge.

Makers of both shingles and standing-seam metal roofing tie their strongest material warranties to a full-system install. That almost always includes ice-and-water membrane in the high-risk areas. Leave it out, and you can void the coverage you paid for. Do it right, and the waterproofing layer stays hidden for the life of the roof — which is the point. That’s why we list underlayment on our written estimates. We’re a licensed, bonded, and insured WA roofing contractor, and we back every re-roof with a workmanship warranty. We’re glad to show you exactly where the membrane goes on your roof before we start.

Get a straight answer on your roof

Not sure if your roof has proper waterproofing in the valleys and eaves? Or if a nagging leak traces back to missing membrane? A free on-island inspection will tell you, with honest repair-vs-replace advice and no pressure. Contact us or call (360) 205-1462 for a free inspection and written estimate anywhere on San Juan, Orcas, Lopez, or Shaw.