What causes black spots on my cedar deck in New Brunswick?

Black spots on cedar decking are caused by a chemical reaction between the wood's natural tannins and iron, typically introduced by metal fasteners, railings, or even iron-rich water splashing onto the deck surface. This is one of the most common aesthetic complaints among cedar deck owners in New Brunswick, and while it looks alarming, it is a surface stain rather than a sign of rot or structural damage.

Cedar is naturally rich in tannins — the same organic compounds that give the wood its beautiful reddish-brown colour and its resistance to insects and decay. When these tannins come into contact with iron, they undergo a chemical reaction that produces iron tannate, a dark blue-black compound. The most frequent source of iron is standard galvanized or bare steel nails and screws. Even galvanized fasteners can expose iron as their zinc coating wears away over time, and in New Brunswick's wet Maritime climate, where annual precipitation exceeds 1,100 millimetres in most regions, that wear happens faster than in drier parts of Canada.

You will often notice the black spots concentrated in rings around each nail or screw head, which is the telltale sign of fastener-related iron staining. However, the discolouration can also spread more broadly if iron-laden water runs across the deck surface. Homes in rural New Brunswick that use well water with high iron content can see widespread black staining after watering plants on or near the deck, or from irrigation overspray.

Fixing Existing Black Spots

The most effective treatment for iron tannin staining on cedar is an oxalic acid-based wood brightener. Oxalic acid breaks the bond between the iron and tannin compounds, lifting the dark stain from the wood surface. These products are widely available at building supply stores across New Brunswick. Apply the brightener according to the manufacturer's directions, allow it to dwell for the recommended time, then rinse thoroughly with a garden hose. For severe staining, you may need two applications. Avoid using chlorine bleach, as it can damage cedar fibres and strip the wood of its natural colour in an uneven, blotchy pattern.

Once you have removed the existing stains, the key to preventing recurrence is eliminating the iron source. Replace any standard steel or galvanized fasteners with stainless steel screws. Stainless steel does not react with cedar tannins and will not produce black staining regardless of moisture exposure. Yes, stainless steel fasteners cost more — typically two to three times the price of galvanized — but on a cedar deck that already represents a significant investment, the additional cost is modest and eliminates the problem permanently.

If your black spots are not concentrated around fasteners and appear in random splotches, you may be dealing with mould or mildew rather than iron staining. New Brunswick's humid summers create ideal conditions for surface mould on cedar, particularly on north-facing decks or sections shaded by trees. Mould spots tend to be slightly raised and fuzzy when examined closely, while iron stains are flat and embedded in the wood grain. A deck cleaner formulated for mould removal will address this issue, and applying a quality exterior wood stain or sealer after cleaning will help prevent regrowth by reducing moisture absorption into the cedar.

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