Is the black staining on my Vancouver cedar siding rot or algae?

The black staining on your cedar siding is most likely algae, mold, or mildew — not rot — but you'll need to do a simple physical test to know for sure. Vancouver's wet climate makes biological growth on cedar extremely common, and the good news is that surface staining alone is usually a cosmetic issue that's straightforward to address.

Here's the quickest way to tell the difference: take a firm object like a screwdriver or your thumb and press firmly into the darkened area. If the wood feels solid and doesn't compress or crumble, you're almost certainly dealing with surface biological growth. If the wood feels spongy, soft, or your tool sinks in with little resistance, that's rot — and it needs immediate attention before it spreads to adjacent boards, sheathing, or framing.

Understanding what you're seeing is important because Vancouver cedar siding faces a relentless combination of factors: 1,200mm+ of annual rainfall, persistent humidity from October through March, and limited drying time during overcast winters. Algae and mold thrive in exactly these conditions, especially on north-facing walls, under overhangs where moisture lingers, and anywhere vegetation grows close to the house. Cedar has natural tannins that resist rot, but those tannins also react with moisture and iron (from fasteners or airborne particles) to produce dark staining that looks alarming but is often only skin-deep.

Vancouver-specific context worth knowing: Many older Vancouver homes — Vancouver Specials, Craftsman bungalows in Kitsilano, character homes in Mount Pleasant — still have original or older cedar siding that hasn't been maintained with regular staining or sealing. Without a protective finish, cedar becomes porous and absorbs moisture, accelerating biological growth. If your siding hasn't been refinished in 5+ years, that's likely a contributing factor.

For surface algae and mold, a homeowner can clean the siding themselves using a diluted oxygen bleach solution (avoid chlorine bleach — it damages cedar fibres and kills surrounding plants). Apply with a soft brush, let it dwell for 10-15 minutes, and rinse thoroughly. After the siding fully dries — wait at least 2-3 dry days, which in Vancouver means planning this for May through September — apply a quality penetrating cedar stain or sealant to protect it going forward.

When to call a professional: If your screwdriver test reveals soft spots, if the staining covers large sections, or if you notice paint bubbling, boards cupping, or gaps opening up between boards, those are signs of moisture intrusion behind the siding. That could mean a failed rain screen assembly, inadequate flashing, or compromised building envelope — all of which require a professional assessment. A qualified siding contractor or building envelope specialist can probe the siding, check moisture readings with a meter, and assess whether you're dealing with isolated rot or a systemic water management problem.

Rot repairs on cedar siding in Metro Vancouver typically run $150–$400 per board for individual replacement (including labour and materials), or $8,000–$25,000+ for larger sections requiring re-siding. A full exterior repaint or restain on a typical Vancouver home runs $4,000–$12,000 depending on size, prep work, and access.

Your next steps: Do the screwdriver test on every dark area you can see, especially at board ends, around windows, and near the roofline where water tends to concentrate. If everything feels solid, plan a cleaning and restaining project for this spring or summer. If you find soft spots, don't delay — rot spreads, and catching it early keeps repair costs manageable. Browse our directory at the Vancouver Construction Network to find licensed siding contractors and painters in your area.