How do I tell the difference between surface mildew and structural rot on my deck in Vancouver?

Surface mildew appears as dark staining that can be cleaned off, while structural rot creates soft, spongy wood that has lost its structural integrity. The difference is critical in Vancouver's wet climate because mildew is a maintenance issue, but rot requires board replacement or structural repair.

Surface mildew and algae are extremely common on Vancouver decks due to our 60-80% humidity levels and limited winter sunlight. Mildew typically appears as black, grey, or greenish staining on deck boards, especially in areas that stay damp longer — north-facing decks, areas under roof overhangs, or spots where leaves accumulate. The key test is simple: scrub the stained area with a stiff brush and oxygen bleach solution (or commercial deck cleaner). If the staining comes off and reveals sound wood underneath, you're dealing with surface contamination, not structural damage. This is purely cosmetic and part of normal deck maintenance in Metro Vancouver's marine climate.

Structural rot, however, fundamentally changes the wood's integrity. Rotted wood feels soft or spongy when you press on it with your finger or a screwdriver. You might be able to push the screwdriver tip into the wood with minimal pressure, or the wood might feel "punky" — like it's lost its density. Rotted areas often have a musty smell and may show white fungal growth (the actual rot organism). The wood fibers have been consumed by fungi, so even if you clean off surface staining, the wood remains structurally compromised. This is especially common on deck boards that stay in contact with standing water, around planter boxes, or where leaves and debris accumulate and hold moisture against the wood.

Check these high-risk areas first on your Vancouver deck: the ends of deck boards where they meet the house (water runs off the roof and pools), any boards in contact with planters or where drainage is poor, the underside of deck boards near the ground (especially if there's inadequate ventilation), and around fastener holes where water can penetrate. In Metro Vancouver's climate, rot typically starts where water sits rather than drains away. Cedar decking is naturally rot-resistant, but even cedar will rot if it stays consistently wet.

The "screwdriver test" is your best diagnostic tool. Take a standard screwdriver and gently probe suspect areas. Sound wood will resist penetration and feel solid. Rotted wood will allow the screwdriver to sink in easily, sometimes several inches. If you can push the screwdriver into the wood with hand pressure alone, that board needs replacement. Don't just test the surface — rot often starts from underneath where moisture wicks up from poor drainage or inadequate ventilation.

For structural elements like joists and beams, rot is a serious safety concern that requires immediate professional attention. If you can access the underside of your deck, check the tops of joists where they contact the deck boards (water drips through board gaps and sits on the joist tops) and anywhere joists are close to grade or in contact with concrete without proper moisture barriers. Structural rot in load-bearing members can compromise the entire deck's safety.

When you find actual rot, the affected wood must be replaced — there's no way to restore structural integrity to rotted lumber. Surface cleaning and staining won't fix compromised wood fibers. For individual deck boards, this is often a straightforward replacement job. For structural members (joists, beams, posts), hire a professional to ensure proper load transfer and code compliance.

Prevention is key in Vancouver's climate. Annual cleaning removes the organic matter (leaves, pollen, dirt) that feeds mildew and creates conditions for rot. Ensure proper drainage both on and under your deck — water should never pool or sit against wood surfaces. Maintain adequate ventilation under the deck (minimum 12 inches of clearance) and consider gravel ground cover to reduce moisture splash-back.

Need help finding a deck professional to assess structural concerns? Vancouver Deck Contractors can match you with experienced contractors who understand Metro Vancouver's unique moisture challenges and can distinguish between cosmetic maintenance needs and structural repairs requiring immediate attention.