Deck Maintenance in Metro Vancouver: Moss, Algae and the Winter Checklist

Why decks here get green and slippery

Algae, moss and mildew need three things: moisture, shade and food. Our winters provide the first, the trees and north walls provide the second, and the leaves, needles and pollen that settle between boards provide the third. Debris packed into board gaps also holds water against the wood, which is how joists rot from the top down while the surface still looks passable. Most maintenance is simply removing food and letting things dry.

Spring: the deep clean, by material

Cedar and pressure-treated wood. Start with warm, soapy water and a stiff-bristle brush. For mildew and green film, the Western Red Cedar Lumber Association recommends an oxygen-based bleach solution — not chlorine bleach, which damages the wood fibre and lightens it unevenly. Rinse thoroughly. Power washing is the fastest way to ruin a cedar surface; the WRCLA says plainly that power washers "can seriously damage cedar deck surfaces." If you must, use a fan tip at the lowest pressure that works, keep moving, and never let the stream dwell. Once the deck is clean and dry, test the finish: if water soaks in instead of beading, it's time to re-coat, typically every two to three years in our climate and sooner on sun-exposed boards.

Composite. Soap, water and a soft brush, working with the grain of the embossing. Mildew on composite is feeding on the dirt and pollen sitting on the board, so removing the film removes the problem. Pressure washers are allowed on current-generation composites within limits — Trex, for example, specifies no more than 3,100 psi, a fan tip, and no closer than 8 inches for its Transcend, Enhance and Select lines, and says not to pressure-wash its early-generation boards at all. Exceeding the manufacturer's limit can void the warranty; check yours. Avoid rubber-backed mats, which can discolour composite.

Vinyl membrane (Duradek). The membrane is waterproofing first and flooring second, so the rules are stricter. Duradek recommends cleaning about four times a year with its own vinyl cleaner, warm water and a broom, then flushing thoroughly. Never use solvents, waxes, abrasives or steel wool — they strip the pattern and the protective top layer. If you power wash, keep it to 1,500 psi maximum, 12 inches from the surface, and stay away from seams, railing posts and caulking. Only non-rubber-backed mats at doors (rubber and PVC don't get along).

Summer: let it breathe

• Move planters, benches and mats periodically so the deck beneath them dries — the WRCLA's top maintenance tip for cedar, and good advice for every surface.
• Put a non-rubber mat under the barbecue; grease stains are the hardest thing to get out of any decking.
• Trim back shrubs and branches that keep a section in permanent shade.

Fall: the most important month

• Clear leaves and needles from between boards with a putty knife or a stiff brush; don't let them winter there.
• Clean gutters and make sure downspouts don't discharge onto the deck or at a post base.
• On membrane decks, clear drains and scuppers and do Duradek's recommended annual caulking inspection around posts, corners, vents and drains — building movement breaks seals, and a failed bead is how water gets under a good membrane.
• Re-seal wood now if you skipped spring; a coat going into winter earns its keep.

Winter: snow and ice without damage

• Use a plastic shovel, never metal, and shovel with the boards, not across them.
• De-icers: Trex allows calcium chloride or rock salt on its boards; Duradek accepts rock salt or kitty litter on vinyl. In both cases rinse the residue off before the spring sun bakes it on. On wood we prefer sand or kitty litter for grip — salts are hard on finishes and on fasteners.
• Keep snow from piling against the house wall at the ledger; that's the one place you never want standing water.

Once a year: the ten-minute structural check

Cleaning keeps the surface; inspection keeps the structure. Once a year, probe the ledger, post bases and stair stringers with a screwdriver, grab the railing, and look at the fasteners. Our guide to the signs a deck needs repair or replacement walks through exactly what to look for, and when to call someone.

Quick answers

Can I pressure wash my deck? Composite: usually yes, within the manufacturer's psi and distance limits (Trex: ≤3,100 psi, fan tip, 8 inches). Vinyl: ≤1,500 psi, 12 inches, away from seams. Cedar and treated wood: not recommended; use oxygen bleach and a brush instead.

How do I get moss and algae off a deck? Remove the debris it feeds on, scrub with the right cleaner for the material (oxygen bleach for wood, soap for composite, vinyl cleaner for membranes), rinse thoroughly, and fix the shade or drainage that's growing it back.

How often should I re-stain a cedar deck in Vancouver? When water stops beading — for most decks here that's every two to three years, and more often on the sunny, south-facing sections.