Birch Bay Siding Building Decks for the Sumas Area's Actual Weather
Homes in the Sumas area near Birch Bay deal with a specific combination of conditions that most decking material and most installation crews were never designed around. You've got salt-laden air drifting in off the water, long stretches of driving rain through fall and winter, and a moss season that can run half the year on anything that doesn't drain and dry properly. A deck built here has to handle all three at once, year after year, without turning into a maintenance project every spring.
We build and replace decks throughout the Birch Bay area, including Sumas, and we size the material and the substructure to that reality rather than to a generic spec sheet. This page covers what composite decking actually needs to hold up here, what a correct installation looks like underneath the surface, and how our process works from first estimate to finished deck.
Why Composite Decking Makes Sense in This Climate
Composite decking is a blend of wood fiber and plastic, formed into boards that don't behave like solid lumber. That matters a lot in a climate defined by moisture. Wood decking swells when it's wet, shrinks when it dries, and that constant movement opens gaps, loosens fasteners, and creates the rough, splintering surface you see on older decks around Whatcom County. Composite boards don't absorb water the way wood does, so they don't cup, crown, or split from repeated wet-dry cycles the same way.
That doesn't mean composite decking is maintenance-free — no exterior product is, in this climate. It means the maintenance shifts from structural repair (replacing rotted boards, resetting popped nails) to surface care (keeping moss and organic buildup off the deck). That's a trade most homeowners here are glad to make once they've dealt with a wood deck through a couple of wet winters.
Capped vs. Uncapped Boards
Most composite decking on the market today falls into two categories:
- Capped composite — the board has a plastic shell wrapped around the wood-composite core. This is what most manufacturers, including brands like Trex and TimberTech, sell as their standard product now. The cap resists moisture intrusion, staining, and moss growth much better than the core material alone.
- Uncapped composite — an older style, or a budget-tier product, where the wood-plastic blend is exposed on all sides. It's more prone to surface moisture absorption, fading, and moss or mildew taking hold in the grain texture.
In a climate with this much sustained moisture and a real moss season, we recommend capped composite for nearly every project here. The price difference is real, but so is the difference in how the deck looks and performs after five winters instead of one.
What a Correct Composite Deck Installation Involves
The board itself is only part of the story. Most composite deck problems we get called out to inspect — squeaking, sagging, boards that have gone spongy at the ends, moss colonizing the gaps between boards — trace back to what's underneath, not the decking material itself.
Substructure and Framing
Composite boards are heavier than wood and behave differently under load, so joist spacing has to match the manufacturer's spec for that specific product — typically tighter than what you'd use for a solid wood deck, and often tighter still at stair treads, picture-frame borders, and diagonal patterns. We also use joist tape or flashing on top of the framing itself, which protects the wood joists from the moisture that runs off and pools around fasteners over time. This is a step that's easy to skip and impossible to see once the decking is down — but it's the difference between a substructure that lasts and one that's rotting quietly underneath a deck that still looks fine on top.
Ventilation and Drainage
A deck that sits low to the ground, or has poor airflow underneath, holds moisture longer after every rain. That extended damp period is exactly what moss and algae need to establish themselves in the gaps between boards and along the edges. Proper joist height, drainage slope, and airflow underneath the deck all reduce how long the structure stays wet after a storm — which directly reduces moss pressure over the life of the deck.
Fasteners and Fastening Systems
Composite boards expand and contract with temperature more than they do with moisture, but they still need room to move. Hidden fastener systems, proper gapping between boards, and corrosion-resistant hardware all matter more here than in a drier inland climate, where salt air isn't accelerating corrosion on anything exposed or under-protected.
Our Process, Start to Finish
- On-site assessment. We look at the existing structure (if there is one), grading and drainage around the deck footprint, sun and shade exposure, and how close the site is to salt air exposure and prevailing wind and rain direction.
- Honest estimate. You get a clear breakdown of material options, substructure work needed, and a realistic price range — no lowball number that turns into change orders halfway through.
- Substructure build or repair. Framing, flashing, and joist protection go in to spec before a single composite board is installed.
- Decking installation. Boards, fastening system, and any picture-framing or stair details are installed to the manufacturer's spacing and fastening requirements — which is also what keeps the product warranty valid.
- Final walkthrough. We go over care and cleaning specific to this climate before we consider the job done.
Composite vs. Other Decking Materials
There's no single "best" decking material for every homeowner — budget, maintenance tolerance, and how long you plan to stay in the home all factor in. Here's how the main options compare for a coastal Whatcom County property:
| Material | Moisture Behavior | Moss/Algae Resistance | Ongoing Maintenance |
|---|---|---|---|
| Capped composite | Doesn't swell or absorb water like wood | Good — smooth cap sheds moisture and resists staining | Periodic cleaning; no staining or sealing |
| Uncapped composite | Some surface absorption over time | Fair — textured surface can hold moisture in grain | Occasional cleaning; more prone to surface staining |
| Pressure-treated wood | Swells, shrinks, and checks with wet-dry cycles | Poor without regular sealing | Annual sealing/staining, board replacement over time |
| PVC decking | Fully synthetic, no wood fiber to absorb moisture | Very good | Low; higher upfront material cost |
Composite decking sits in the middle in most cases — better moisture and moss performance than wood, at a lower material cost than full PVC. That's why it's become the default choice for most of the deck replacements we do in this area.
Living With a Composite Deck Here: Realistic Maintenance
Even the best-built composite deck in a climate like this needs some seasonal attention. It's far less than a wood deck requires, but "maintenance-free" oversells it. Here's what actually keeps a composite deck looking and performing well through the wet season:
- Sweep off fallen leaves and debris regularly in fall — trapped organic matter is what feeds moss growth on any deck surface.
- Rinse the deck periodically with a garden hose or low-pressure wash to clear salt residue and grime before it builds up.
- Clean visible moss or algae with a composite-safe cleaner and a soft-bristle brush — avoid wire brushes and high-pressure washing that can damage the cap surface.
- Keep gutters and downspouts near the deck clear so runoff isn't dumping extra water onto the deck surface or framing.
- Check that furniture, planters, and rugs aren't trapping moisture against the board surface for extended periods.
- Inspect fasteners, railings, and stair connections once a year for looseness — salt air can affect exposed hardware faster than an inland deck would show wear.
Mistakes We See on Decks Built Without This Climate in Mind
A lot of the deck work we get called out for isn't a failure of composite decking as a product — it's a deck that was built to a generic spec instead of this specific climate. Common issues include joists without any flashing or tape protection, decking installed with insufficient gapping so boards buckle when they expand, drainage that wasn't graded away from the house, and boards run too close to grade or vegetation, which traps moisture and accelerates moss growth on the underside and edges. None of these are visible on install day. They show up two or three winters later, and by then the fix is more expensive than getting it right the first time.
Why Hire a Crew That Already Works This Area
Decking specs written for a general national market assume average conditions. Sumas and the greater Birch Bay area don't have average conditions — the salt exposure, the rainfall totals, and the moss pressure here are all higher than what a lot of installation guides account for. A crew that works this specific stretch of Whatcom County knows to tighten joist spacing beyond the bare minimum, knows which framing details actually matter in salt air, and knows what a deck here looks like after five winters versus after one. That's the difference between a deck that needs attention in year three and one that's still doing its job in year fifteen.
If you're planning a new composite deck or replacing one that hasn't held up to the weather here, we're glad to come take a look. We offer free, no-pressure estimates — use the form below to get in touch and we'll walk through what your property actually needs.