How often should flashing be inspected on a home in this area?

We generally recommend a visual check once a year, ideally in early fall before the heavy rains set in, plus a look after any major windstorm. Homes near Bellingham Bay with more salt exposure benefit from closer attention to flashing corrosion specifically.

What should I ask a contractor to make sure they're not cutting corners on underlayment?

Ask them to name the exact underlayment product they plan to use and where self-adhering ice and water shield will be applied versus standard synthetic. A contractor who can't answer specifically, or who talks only about shingle brand, hasn't put much thought into the layers underneath.

Is synthetic underlayment worth the extra cost over traditional felt?

In a climate with as much sustained rain as Whatcom County gets, we think it generally is, since it resists moisture and tearing better and tends to outlast felt. That said, felt is still a legitimate, lower-cost option for some budgets and roof types, and we'll lay out the trade-offs honestly rather than push one option.

Does the type of shingle I choose affect how flashing and underlayment are installed?

Somewhat — steeper-profile shingles and heavier architectural shingles can affect how flashing overlaps are detailed, and low-slope sections may need broader ice and water shield coverage regardless of shingle type. The core flashing and underlayment principles stay the same across most asphalt shingle types.

Does moss on my roof mean the underlayment is already compromised?

Not necessarily — moss on the surface is common in our climate and doesn't automatically mean water has reached the underlayment. But heavy, long-term moss growth increases the odds of moisture working under shingle edges over time, which is why we check underlayment condition, not just moss, during an inspection.

Flashing & Underlayment: What's Really Protecting Your Roof

The Part of Your Roof You Never See Is the Part Doing the Real Work

When most homeowners think about their roof, they picture shingles. Shingles are what you see from the street, and they're what most roofing quotes emphasize. But shingles are really just the outer shell. The materials underneath and around them — the underlayment and the flashing — are what actually keep water out of your attic, walls, and framing. A roof can have brand-new, expensive shingles and still leak badly if the flashing and underlayment underneath were installed poorly or skipped altogether.

In Whatcom County, this distinction matters more than in drier climates. Bellingham sits between the Salish Sea and the Cascade foothills, which means we get long stretches of driving rain pushed sideways by wind off the water, salt-laden air near the bay, and a moss season that runs most of the year thanks to our shade and moisture. Roofing systems here have to handle water coming from angles that shingles alone were never designed to block. That's where flashing and underlayment do their job.

Underlayment: The Roof's Backup Layer

Underlayment is the material installed directly on the roof deck, before shingles go down. Its job is simple but critical: if water ever gets past the shingles — through wind-driven rain, ice, a lifted shingle edge, or a small gap — the underlayment is the layer that stops it from reaching the plywood deck and the structure below.

The Main Types

Asphalt-saturated felt (felt paper): The traditional option. It's inexpensive and has been used for decades, but it absorbs moisture, can wrinkle, and tears more easily during installation, especially in wet weather — which is a real concern on a job site in Bellingham for a good chunk of the year.
Synthetic underlayment: A woven or non-woven polymer sheet. It's lighter, stronger, more tear-resistant, and doesn't absorb water the way felt does. It also holds up better to foot traffic during installation and to UV exposure if a job gets delayed by weather, which happens.
Self-adhering ice and water shield: A rubberized asphalt membrane that seals around fastener penetrations. It's typically used in vulnerable zones — eaves, valleys, around chimneys and skylights — rather than across the whole roof, though some homeowners choose full coverage for extra protection.

Our standard is synthetic underlayment as the general field material, with self-adhering membrane reinforced at eaves, valleys, and penetrations. Felt still has a place on some budget-conscious jobs, but given how much rain this region sees, we're honest with clients that synthetic performs more consistently over the life of the roof.

Flashing: Where Most Roof Leaks Actually Start

If underlayment is the backup plan, flashing is the front line at every place your roof isn't just a flat, uninterrupted plane — chimneys, skylights, walls, valleys, and roof edges. Metal flashing (usually galvanized steel or aluminum) directs water away from these transition points, which are structurally the weakest spots for water intrusion because shingles alone can't seal a corner or a joint.

Common Flashing Types on Bellingham Homes

Step flashing: Individual L-shaped metal pieces woven between shingle courses where a roof meets a wall. Done correctly, each piece overlaps the next so water is always directed downward and outward.
Counter flashing: Installed above step flashing, usually cut into masonry or siding, to cover and protect the top edge of the step flashing from behind.
Valley flashing: Runs along the internal angle where two roof slopes meet — one of the highest-volume water paths on any roof, and one of the first places we check when diagnosing a leak.
Drip edge: Metal trim along the eaves and rakes that directs water off the roof edge and into the gutter instead of behind the fascia.
Chimney and skylight flashing: A combination of step, counter, and sometimes saddle flashing built to shed water around a penetration rather than rely on caulk or sealant alone.

Flashing failures are rarely dramatic. They tend to show up as a slow stain on a ceiling, a musty smell in an attic, or soft trim near a chimney — months or years after the flashing was installed wrong or started to corrode. That slow-leak pattern is exactly why flashing deserves more attention during a roof inspection than it usually gets.

Comparing Underlayment Options

Material	Moisture Resistance	Typical Lifespan	Best Use
Asphalt felt (15 or 30 lb)	Fair — absorbs some moisture	10-20 years	Budget re-roofs, drier climates
Synthetic underlayment	Good — water-resistant, doesn't wick	20-30+ years	Full-roof coverage in wet regions like Whatcom County
Self-adhering ice & water shield	Excellent — fully sealed membrane	Life of roof	Eaves, valleys, penetrations, low-slope sections

Why This Matters More in Bellingham Than in a Lot of Places

Three things about our local climate put extra stress on flashing and underlayment specifically:

Driving Rain

Wind off Bellingham Bay and the Strait of Georgia often pushes rain sideways rather than straight down. That means water can get up under shingle edges and hit flashing joints at angles that a purely vertical-rain roof design wouldn't have to deal with. This is a big part of why flashing overlap and underlayment coverage at transitions matter so much here.

Salt Air

Homes closer to the water deal with airborne salt that accelerates corrosion on unprotected or lower-grade metal flashing. Galvanized steel and aluminum flashing hold up, but fasteners, seams, and lower-quality metal can corrode faster near the coast than they would further inland in Whatcom County.

Moss Season

Bellingham's tree cover, shade, and near-constant fall-through-spring moisture make moss a near-year-round issue. Moss holds water against the shingle surface and can work its way under shingle edges and around flashing, keeping those areas damp far longer than an open, sunny roof would stay. Roofs with poor moss control often show flashing corrosion and underlayment degradation years before they should.

Common Failure Points We See on Local Roofs

Chimney flashing sealed with caulk instead of properly integrated step and counter flashing
Valleys where shingles were laid without adequate underlayment reinforcement underneath
Drip edge missing or installed under the underlayment instead of over it at the eaves
Felt underlayment that's brittle, torn, or wrinkled from age and moisture exposure
Skylight flashing kits installed without step flashing woven into the surrounding courses
Nail penetrations near ridge and hip areas with no self-adhering membrane backup

Repair vs. Replace: What Drives the Decision

Not every flashing or underlayment issue means a full re-roof. Isolated flashing failures — a corroded chimney flashing, a failed valley, a section of drip edge — can often be repaired without disturbing the rest of the roof. Underlayment problems are different: because it sits under the shingles, addressing it usually means removing and reinstalling shingles in that area at minimum, or a full re-roof if the underlayment has failed broadly.

Factor	Favors Repair	Favors Replacement
Location of failure	Isolated (one valley, one chimney)	Widespread across multiple areas
Roof age	Roof is under 10-15 years old	Roof is near or past expected lifespan
Shingle condition	Shingles still in good shape	Shingles curling, cracking, or brittle
Underlayment access	Only local removal needed	Full deck exposure already required
Moss/moisture history	Recently cleaned, well-ventilated	Chronic moss, poor attic ventilation

A Checklist Before You Sign Off on Any Roofing Work

Whether you're getting a full re-roof or a targeted repair, these are worth asking about directly:

What specific underlayment material is being used, and is it rated for our climate?
Will valleys, eaves, and penetrations get self-adhering ice and water shield, not just standard underlayment?
What type of flashing metal is being installed, and is it appropriate for coastal/salt-air exposure?
Is step flashing being woven into the shingle courses, or is the plan to rely on sealant alone?
How is drip edge being installed relative to the underlayment at the eaves and rakes?
Does the estimate distinguish between materials and labor for flashing versus general roofing work?
Is there a written warranty that covers workmanship on flashing and underlayment specifically, not just shingles?

What a Proper Inspection Actually Looks At

A roof inspection that only looks at shingle condition from the ground is incomplete. A thorough inspection checks flashing at every chimney, skylight, and wall transition for gaps, rust, or lifted edges; examines valleys for granule loss and metal exposure; and, where accessible, checks the attic for staining, damp insulation, or daylight around penetrations — all signs that water is getting past the surface layer, whether or not it's visible on the roof itself yet.

Getting an Honest Look at Your Roof

If you're dealing with a slow leak, planning a re-roof, or just want to know what's actually underneath your shingles before you make a decision, we're happy to take a look and walk you through what we find in plain terms. If you'd like a free, no-pressure estimate, the form below is a good place to start.