Should I use a membrane under my cedar deck boards to protect the substructure from Vancouver rain?

No, do not install a membrane directly under cedar deck boards. This creates a moisture trap that will accelerate rot rather than prevent it, which is exactly the opposite of what you want in Metro Vancouver's wet climate.

Why membranes under deck boards backfire: Cedar decking naturally absorbs moisture from Vancouver's frequent rain and high humidity (60-80% year-round). When moisture penetrates the deck boards — and it always will through gaps between boards, screw holes, and the wood grain itself — it needs somewhere to go. A membrane underneath traps this moisture against the bottom of your cedar boards and the top of your joists, creating perfect conditions for mould growth and wood rot. The trapped moisture can't dry out properly, leading to premature failure of both your decking and structural framing.

What actually protects your substructure in Vancouver's climate: Proper ventilation and drainage are your best defenses. Ensure at least 12 inches of clearance between the ground and your deck joists, with cross-ventilation from multiple sides. Install a gravel bed under the deck rather than leaving bare soil — this reduces moisture splash-back onto the underside of your joists. Space your cedar boards with 1/4-inch gaps to allow water to drain through quickly rather than pooling on the surface. Use joist tape (a thin, adhesive-backed membrane) on top of each joist where the deck boards attach — this prevents moisture from wicking into the joist through the screw holes, which is the most common failure point.

When waterproof membranes ARE essential: If you're building a second-storey deck over habitable space (a room, suite, or carport below), then you absolutely need a continuous waterproof membrane system like Duradek or Tufdek applied over a plywood substrate. But this replaces the cedar decking entirely — you don't have both cedar boards and a membrane. These vinyl membrane systems are heat-welded at the seams to create a completely waterproof surface that protects the space below.

Better protection strategies for cedar decks: Focus on keeping water moving rather than trying to block it. Use stainless steel or hot-dipped galvanized fasteners that won't corrode in Vancouver's moisture. Apply joist tape to prevent moisture intrusion at fastener points. Ensure your deck slopes 1/8 inch per foot away from the house. Most importantly, maintain your cedar annually — clean, brighten, and re-stain or oil every 1-2 years to keep the wood sealed and resistant to moisture penetration.

The bottom line: Let your deck breathe. Vancouver's marine climate requires drainage and ventilation, not moisture barriers that trap water where you can't see it causing damage.