Flashing & Underlayment: What's Really Protecting Your Roof

The Part of the Roof Nobody Notices Until It Fails

Ask most homeowners what their roof is made of, and they'll say "shingles." That's fair — shingles are what you see from the street. But shingles are really just the outer shell. The actual waterproofing system underneath is a combination of underlayment and flashing, and in a climate like Bellingham's, that hidden layer matters just as much as anything on top of it.

Whatcom County sits right on the water, which means our roofs deal with a specific combination of stresses: salt-laden air off Bellingham Bay, wind-driven rain that doesn't fall straight down, and a moss season that can stretch for most of the year on shaded, north-facing slopes. Shingles are designed to shed water in normal conditions. Flashing and underlayment are what keep the roof dry when conditions aren't normal — which, around here, is often.

What Underlayment Actually Does

Underlayment is the layer of material installed directly on the roof deck, before shingles go down. Think of it as the roof's backup plan. If wind lifts a shingle edge, if a nail hole works itself loose over time, or if wind-driven rain gets pushed sideways under the shingle tabs, the underlayment is what stands between that moisture and your plywood decking.

There are a few common types:

• Felt (asphalt-saturated paper) — the traditional option, still code-compliant, but it tears more easily and absorbs moisture over time.
• Synthetic underlayment — a woven, water-resistant sheet that holds up better to foot traffic during installation and resists tearing in wind.
• Self-adhering (peel-and-stick) membrane — used in high-risk areas like eaves, valleys, and around penetrations, where it bonds directly to the deck and seals around nail penetrations.

In a driving-rain climate, we pay particular attention to how underlayment is lapped and fastened, not just what brand it is. A high-quality synthetic installed with sloppy overlaps performs worse than a basic felt installed correctly. Installation discipline matters more than the marketing on the package.

What Flashing Actually Does

Flashing is the metal (or sometimes rubber) material installed at every place where the roof plane changes direction or meets something else — a chimney, a wall, a skylight, a vent pipe, a valley where two roof slopes meet. These transition points are where a roof is most likely to leak, because shingles alone can't seal an angle change or a gap around a penetration.

Common flashing types include:

• Step flashing — individual pieces woven into each shingle course where a roof meets a sidewall, so water is directed out and away at every layer.
• Valley flashing — a wider metal channel where two roof slopes meet, carrying a heavier volume of water than any other part of the roof.
• Chimney and skylight flashing — a combination of base flashing and counter-flashing designed to shed water around a fixed object without relying on caulk alone.
• Drip edge — the metal strip along eaves and rakes that keeps water from wicking back underneath the shingle edge and into the fascia.

Caulk and roofing cement have their place, but they degrade with UV exposure and temperature cycling. When flashing is designed and installed correctly, it works by shape and gravity — directing water down and away — rather than depending on a sealant bead to hold back moisture indefinitely. That's the standard we build to: flashing should still be doing its job long after any caulk applied around it has dried out and cracked.

Why This Matters More on the Coast

Salt air accelerates corrosion on unprotected or lower-grade metal flashing. Combine that with driving rain that hits roofs at an angle instead of straight down, and you get water testing every lap, seam, and fastener a roof has — not just the flat field of shingles. Add a long moss season on the shaded sides of steep-roofed homes common around Bellingham, and you have organic growth that can hold moisture against flashing and underlayment for weeks at a time if it's not managed.

None of this means a Bellingham roof needs to be over-engineered. It means the details — the metal gauge and finish, the underlayment type at eaves and valleys, the way step flashing is woven instead of just slid in behind a wall — deserve real attention rather than being treated as an afterthought to the shingle choice.

What to Ask About on Your Next Roof

Whether you're getting a repair estimate or planning a full replacement, it's worth asking specifically about the flashing and underlayment plan, not just the shingle brand and color. A few questions worth asking any contractor:

1. What underlayment is being used, and where does self-adhering membrane get installed versus standard underlayment?
2. Is step flashing woven into new shingle courses, or reused from the old roof?
3. What's the plan for valleys — open metal valley, or woven shingle valley?
4. How is the roof-to-wall and chimney flashing being handled, and what metal is being used?

A contractor who can answer these clearly, without hesitation, is telling you something about how they build a roof — not just how they sell one.

If you'd like a straightforward look at your own roof's flashing and underlayment condition, Bellingham Roofing Company offers free, no-pressure estimates — just fill out the form below and we'll take a look.