National deck maintenance guides are written for climates that get summer dry seasons. Seattle doesn't. With 38 inches of annual rainfall concentrated between October and April, decking materials that would last a decade between interventions in Denver or Atlanta need attention every year in King County. This guide covers the actual maintenance schedule for each material type — not the marketing version, the honest one.

Cedar Deck Maintenance

Cedar requires the most active maintenance of any residential decking material in the Pacific Northwest, and the maintenance window is narrow: late March to mid-April, after the winter rains taper off and before summer humidity drops enough to let the wood dry fully.

**Annual inspection:** Walk every board. You're looking for checking (cracks running parallel to the grain), cupping (edges higher than the center), and any end grain that's become soft or fibrous under thumb pressure. These are the early signs of moisture damage. Probe suspect boards with a screwdriver tip — soft wood that yields easily means the fiber structure is breaking down. One or two soft boards can be replaced; more than 20–25% of the surface is a resurfacing conversation.

**Power washing:** Use a pressure washer at 1,200–1,500 PSI maximum on cedar. Higher pressure damages the wood surface and opens grain structure that holds more moisture. Wash along the grain, not across it.

**Staining and sealing:** Cedar on a sun-exposed south-facing lot needs a penetrating oil stain or sealer every two to three years. Cedar on a shaded north-facing lot — under tree canopy, under a roof overhang, or anywhere that stays damp into June — needs it every 12 to 18 months. Apply stain to dry wood only; staining wet or damp cedar traps moisture inside and accelerates decay. Waiting until late April or early May, after a week of dry weather, gives you the best penetration.

**Fastener inspection:** Galvanic corrosion at fastener heads causes surface staining and eventually fastener failure. If you're seeing rust staining radiating from screw heads, the fasteners are corroding. Replacement is straightforward on the surface but indicates the same fasteners in the framing should be checked.

Typical annual maintenance cost for a 400 sq ft cedar deck in King County: **$800–$1,800** depending on whether staining is required that year and whether any boards need replacement.

Pressure-Treated Wood Maintenance

PT framing — the joists, beams, and posts beneath most residential decks regardless of surface material — follows a similar inspection schedule to cedar but tolerates longer intervals between surface treatments. The critical inspection points are the ledger connection and post bases.

**Ledger connection:** Inspect the flashing and caulk line where the deck attaches to the house every spring. Flashing that has pulled away or caulk that has cracked allows water to migrate behind the ledger and into the house framing — the most common source of structural moisture damage on residential decks. If you see staining on the house siding above or below the ledger, investigate immediately.

**Post bases:** Any post base sitting directly on concrete without a standoff bracket holds moisture at the end grain — the fastest point of rot entry. Look for rust streaking on hardware and soft wood at post bottoms. Post base replacement is straightforward when caught early; replacing a rotted post requires temporarily supporting the structure above.

Composite Deck Maintenance (Standard)

Standard composite and entry-level capped composite — Trex Select, TimberTech Terrain, Fiberon Symmetry — requires significantly less intervention than wood but is not zero-maintenance in Seattle's climate.

**Annual cleaning:** April power washing at 1,500 PSI maximum, using a fan tip (not a pencil tip). For moss and mildew spots, use a composite-safe deck cleaner and a soft-bristle brush. Do not use a wire brush — it scratches the cap layer and creates micro-grooves that hold more organic material. Bleach will fade some composite colors over time; check your manufacturer's care guidelines before using it.

**What to inspect:** Look at fastener heads for corrosion staining and check the framing beneath — composite surfaces protect the boards themselves but don't prevent moisture from reaching the substructure. A composite deck on a rotting frame is still a problem. The surface looks fine until it isn't.

No staining, no sealing, no annual oiling required.

Capped Composite Maintenance (Premium Lines)

Premium capped composite — Trex Transcend, TimberTech Legacy, Fiberon Paramount — has a denser, harder cap layer that sheds organic growth more readily than entry-level products. Maintenance is the same protocol but the interval between deep cleanings is typically longer in practice.

Moss and mildew accumulation that requires scrubbing on standard composite often wipes off with a hose on premium capped products after the same winter exposure. The 25-year stain and fade warranty on these products reflects real material performance — in our experience, they look substantially better at year 15 than entry-level composite does at year 8 in PNW conditions.

PVC Deck Maintenance (AZEK, TimberTech PVC)

Cellular PVC is the lowest-maintenance decking material available for Seattle's climate. Its maintenance schedule is genuinely simple: an annual rinse with a garden hose, and a mild soap scrub for any surface discoloration.

Moss does not colonize PVC — the material has zero organic content, and moss requires organic substrate to establish. This makes PVC the correct specification for north-facing lots, heavily shaded properties, and any deck where annual staining or scrubbing is not something the homeowner is committed to. No staining, no sealing, no checking, no cupping — the physical properties of the material simply don't degrade in the ways wood and some composites do.

The trade-off: PVC surfaces run warmer underfoot in direct sun than composite or wood. On south-facing sun-exposed lots with significant afternoon solar exposure, this is worth discussing at the estimate.

When to Call a Contractor vs. DIY

Power washing, mild soap cleaning, and replacing one or two obviously failed surface boards are reasonable DIY tasks. Board-for-board replacement requires basic carpentry skill and the ability to match the existing fastener pattern.

Call a contractor when: any railing post has lateral movement, you've found soft wood anywhere in the framing (not just the surface), you're seeing ledger separation or movement, or the inspection reveals more than a few isolated board failures. Structural issues on decks don't announce themselves clearly in advance — a post that seems slightly soft in April can be genuinely compromised by the following winter.

Signs Your Deck Is Past Maintenance and Needs Replacement

A deck that has reached the end of its useful life shows specific signs that routine maintenance can't address: soft spots in the framing that probe yielding, any lateral movement in the structure when pushed, visible separation between the ledger and the house, and repeated surface board failure despite recent maintenance. When the frame is compromised, resurfacing treats the symptom without addressing the problem.

If your inspection raises structural concerns — or if you'd rather have a professional set of eyes on it before committing to a maintenance season — our team does deck assessments throughout King County. See our [deck repair services](/deck-repair) or [contact us for a free estimate](/contact).