What causes those black streaks on cedar fences in Vancouver and how do I prevent them?

Those black streaks on cedar fences in Metro Vancouver are caused by tannin bleed — a natural reaction between cedar's water-soluble tannins and iron-based fasteners, combined with the region's heavy rainfall washing the reaction down the fence face.

Cedar contains high concentrations of natural tannins, which are part of what makes it so rot-resistant and ideal for Vancouver's wet climate. When those tannins come into contact with steel nails or screws — especially standard ungalvanized or low-quality galvanized fasteners — a chemical reaction produces iron tannate, a dark compound that looks almost identical to ink. Metro Vancouver's 1,200mm+ of annual rainfall then does the rest, washing the dark compound down the boards in streaks that radiate outward from each fastener point. On a new cedar fence, you can start seeing this within the first wet season.

The fastener choice is the root cause, and it's entirely preventable.

The fix starts before the first board goes up. Use stainless steel screws or hot-dipped galvanized ring-shank nails exclusively — no exceptions in Metro Vancouver's climate. Standard electro-galvanized fasteners (the shiny silver ones sold in bulk at hardware stores) have a coating that breaks down within a year or two of wet-weather exposure, exposing the steel underneath. Hot-dipped galvanized has a much thicker, more durable zinc coating. Stainless steel is the premium option and will never react with cedar tannins at all — worth the extra cost on a fence you expect to last 20+ years. Avoid anything labelled simply "galvanized" without specifying the process.

Existing black streaks can be treated, but prevention is far easier than removal.

If your fence already has tannin staining, oxalic acid-based wood brighteners (sold at most Metro Vancouver hardware stores and paint suppliers) will neutralize the iron tannate and lighten the streaks significantly. Apply it with a brush or pump sprayer on a dry day, let it dwell for 10-15 minutes, then rinse thoroughly. It won't always remove stains completely — especially on older, weathered cedar — but it makes a noticeable difference. After brightening, let the fence dry completely (at least 48-72 dry hours, which means planning this for May through September in Vancouver) before applying a penetrating cedar stain or sealer.

Moss, mildew, and algae are a separate but related problem you'll likely see alongside tannin streaks in Metro Vancouver. North-facing fence sections and boards under tree canopy are especially prone to green and black biological growth, which traps moisture and accelerates wood degradation. A diluted bleach solution (1 part bleach to 10 parts water) or an oxygen bleach wood cleaner handles this well. Pressure washing at low-to-medium pressure (under 1,500 PSI) removes surface buildup without damaging the cedar grain — high pressure will raise and fuzz the wood fibres.

The maintenance schedule that keeps cedar looking good in Vancouver's climate:

Clean the fence with a wood cleaner or brightener every 2-3 years, let it dry fully, then apply a quality penetrating cedar stain or oil-based sealer. Semi-transparent stains in amber or cedar tones are popular because they enhance the wood's natural colour while providing UV and moisture protection. Clear sealers are lower maintenance but provide less UV protection, meaning the cedar will grey faster. Pay special attention to sealing all cut ends — any board that's been cross-cut loses its natural oil protection at that surface and is the first place rot takes hold in Vancouver's wet conditions.

Staining and sealing is a reasonable DIY project for most homeowners — the key is doing it during a dry stretch of weather (minimum 48 hours dry before and after application) and doing proper prep work first. If your fence is large or you want professional-grade results, fence staining runs $2-$5 per linear foot for professional application in Metro Vancouver.

Need help finding a fence contractor for repairs or a new cedar installation? Vancouver Fence Builders can match you with local professionals for free through the Vancouver Construction Network.