What's Under Your Shingles Matters Most

The Shingles Get the Credit, But They're Not Doing the Real Work

Walk down any street in Bellingham and you'll see roofs described by their shingles — architectural, cedar-look, three-tab, metal. Homeowners compare colors and warranties and call it a day. But the components that actually keep water out of a house are the ones nobody sees once the job is done: the underlayment beneath the shingles and the flashing at every joint, edge, and penetration. Shingles shed most of the water most of the time. Flashing and underlayment are what handle the water that shingles miss — wind-driven rain forced sideways under a tab, meltwater backing up behind an ice dam, condensation working its way down from a poorly vented attic. In Whatcom County, where driving rain off the Salish Sea and a long, wet moss season are just part of the calendar, these hidden layers matter as much as the roof covering itself.

This page walks through what underlayment and flashing actually do, where they tend to fail, and what a properly built roof looks like from the deck up. The goal isn't to sell you on any one brand — it's to help you ask better questions, whether you're getting a full re-roof, a repair, or just trying to understand why a contractor's quote looks different from the one down the street.

Underlayment: The Roof's Backup Plan

Underlayment is the water-resistant barrier installed directly on the roof deck, before any shingles go down. Its job is simple: if water gets past the shingles — through wind lift, ice damming, or a failed seal — the underlayment is what keeps it off the plywood or OSB sheathing underneath. Sheathing that stays dry lasts decades. Sheathing that gets wet repeatedly starts to delaminate, sag, and eventually rot, which turns a shingle problem into a structural one.

Felt vs. Synthetic Underlayment

Traditional asphalt-saturated felt (sometimes called "tar paper") has been used for generations and still works when installed correctly. But it absorbs water, tears more easily during installation, and can wrinkle under a roof left exposed during a wet stretch — a real concern here, where a dry installation window isn't guaranteed. Synthetic underlayment, made from woven polypropylene or polyethylene, has largely replaced felt on quality installs because it's lighter, stronger underfoot, resists tearing, and doesn't absorb moisture the way felt does. It also tends to hold up better if a storm rolls through mid-project and the deck is exposed for a day or two longer than planned.

Ice and Water Shield (Self-Adhered Membrane)

In the highest-risk areas of a roof — eaves, valleys, around chimneys and skylights — many installers use a self-adhered rubberized membrane instead of standard underlayment. This material sticks directly to the deck and seals around any nail penetrations, so it doesn't just shed water, it stops it even if the surface is punctured. Building codes in cold-winter regions often require it at the eaves to guard against ice-dam backup; in Bellingham's milder but very wet climate, it's less about ice and more about giving the most vulnerable parts of the roof a second line of defense against sustained, driving rain.

Flashing: Where Roofs Actually Fail

If you talk to anyone who's spent years chasing roof leaks, the pattern is consistent: it's almost never the open field of shingles that fails first. It's the transitions — where the roof meets a wall, a chimney, a skylight, or another roof plane. Those transitions are handled by flashing: formed metal (or sometimes rubber/plastic composite) that directs water around a joint instead of letting it soak in.

The Common Failure Points

• Valleys — where two roof slopes meet and water volume concentrates
• Chimneys — step flashing up the sides and a cricket or saddle behind to divert water
• Sidewalls and headwalls — where a roof meets a vertical wall, such as a dormer
• Skylights and roof windows — factory flashing kits still need to be integrated correctly with the surrounding underlayment
• Pipe boots and vent penetrations — rubber collars that degrade with UV and age faster than the metal around them
• Drip edge — the metal strip at eaves and rakes that keeps water from wicking back under the shingles into the fascia

Flashing failures are rarely dramatic. They usually show up as a slow stain on a ceiling, a musty smell in an attic, or soft wood around a chimney — small signs that, if ignored, turn into expensive sheathing and framing repairs.

Materials: Aluminum, Galvanized Steel, and Copper

Most residential flashing is either painted aluminum or galvanized steel, both of which perform well for decades when properly installed and not in direct contact with dissimilar metals that can cause corrosion. Copper is the premium option — it develops a protective patina, resists corrosion extremely well, and is often chosen for chimneys or architectural details where longevity and appearance both matter. Copper costs more upfront, but it's a one-time investment in areas that are otherwise difficult and expensive to re-flash later. Given Bellingham's proximity to the water, we pay close attention to salt-air exposure when recommending metal type and fastener choice, since coastal air accelerates corrosion on lower-grade or improperly coated materials.

Why Bellingham's Climate Puts Extra Stress on These Layers

Whatcom County isn't a climate that punishes a roof with extreme heat or hail the way some regions are. Instead, it wears a roof down through persistence: salt-laden air off Bellingham Bay and the Salish Sea that slowly corrodes unprotected metal, driving rain that comes in sideways during winter storms and tests every flashing detail rather than just the open shingle field, and a moss season that stretches for much of the year in our damp, shaded neighborhoods. Moss doesn't just look bad — it holds moisture against the shingles and can work its way under tabs and around flashing edges, keeping those areas wet far longer than they'd otherwise stay. None of these factors are dramatic on their own, but combined, they make correct underlayment and flashing detailing more important here than in drier, calmer climates where a roof can get away with more shortcuts.

Signs Your Flashing or Underlayment May Already Be Compromised

• Water stains on ceilings or upper walls, especially near chimneys or where roof planes meet
• Visible rust, gaps, or lifted edges on metal flashing around chimneys, skylights, or vent pipes
• Daylight visible through the roof deck from inside the attic
• Soft or spongy sheathing felt underfoot during an attic inspection
• Granules collecting in gutters near valleys, which can indicate accelerated wear at a stress point
• Persistent moss buildup concentrated at valleys, chimney backs, or wall intersections
• Musty or damp odor in the attic that doesn't clear with ventilation

Any one of these on its own isn't necessarily an emergency, but they're worth a professional look before the next wet season, not after.

Comparing Underlayment Options

Underlayment Type	Water Resistance	Durability During Install	Typical Use
Asphalt felt (15# or 30#)	Moderate — absorbs some moisture	Tears more easily, wrinkles if exposed	Budget installs, still code-compliant in most cases
Synthetic underlayment	High — sheds water, doesn't absorb	Strong, tear-resistant underfoot	Most quality re-roofs as the standard field layer
Self-adhered ice & water membrane	Very high — seals around fasteners	Sticks to deck, very durable	Eaves, valleys, chimneys, skylights, other high-risk zones

None of these options is "wrong" in every situation — cost, roof pitch, and the specific risk areas of a given roof all factor into what makes sense. What matters most is that the highest-risk zones get the highest level of protection, even if the rest of the field uses a more standard product.

What a Properly Built Roof System Looks Like

Our Standard Approach

We treat flashing and underlayment as the part of the job that determines whether a roof performs for its full expected lifespan or starts leaking years early. That means self-adhered membrane at eaves, valleys, and around penetrations; new flashing at every chimney, wall, and skylight rather than reusing old, bent, or corroded pieces; and drip edge installed correctly at both eaves and rakes so water can't wick back under the shingles into the fascia board. We don't consider it acceptable to leave old flashing in place just because it "looks fine" from the ground — flashing is inexpensive relative to the rest of a roofing job, and reusing it is one of the more common shortcuts that leads to early leaks.

Ventilation Plays a Role Too

Underlayment and flashing keep water from getting in from the outside, but proper attic ventilation keeps moisture from building up from the inside. A roof can have excellent flashing and still develop rot if warm, moist attic air has nowhere to escape and condenses against the underside of the deck. We look at ventilation as part of the same conversation, not a separate issue, especially on older Bellingham homes where insulation and venting may not match current best practice.

Maintenance That Actually Protects These Layers

Underlayment is largely maintenance-free once installed correctly — its job is to sit there and do nothing unless it's needed. Flashing and the shingles around it, however, benefit from periodic attention:

• Keep gutters and valleys clear of debris so water doesn't pool against flashing edges
• Address moss buildup before it spreads across large sections of the roof, particularly on north-facing or shaded slopes common in Bellingham's tree-heavy neighborhoods
• Have chimney and skylight flashing checked every few years, since these are the joints most likely to move slightly or corrode over time
• After any major windstorm, a quick visual check (from the ground, or by a professional) for lifted shingles or bent flashing is worth the ten minutes

Getting an Honest Look at What's Underneath

You can't fully evaluate flashing and underlayment from the ground or from photos — it takes someone up on the roof, and sometimes a look in the attic, to see how the layers underneath your shingles are actually holding up. If you're dealing with a stain, planning ahead for a re-roof, or just want a straight answer about what condition your roof is really in, we're happy to take a look. We offer free, no-pressure estimates and inspections, and we'll tell you honestly what we find — including if the answer is "you're fine for now."