How do I waterproof under my Vancouver deck stair landing?

Waterproofing under a deck stair landing in Vancouver requires a continuous waterproof membrane system that directs water away from the landing structure and prevents moisture from reaching the framing below. This is especially critical in Metro Vancouver's climate where persistent rainfall and humidity can cause rapid wood decay in trapped moisture conditions.

The most effective approach is installing a sloped waterproof membrane (Duradek, Tufdek, or similar vinyl system) over a properly prepared plywood substrate. The membrane must extend beyond the landing perimeter and connect to a drainage system that channels water away from the structure. The substrate should be sloped at minimum 1/4 inch per foot away from the house to ensure positive drainage.

Preparation is critical for long-term success. The landing framing must be structurally sound and properly supported — any flex or movement will crack the membrane over time. Install 5/8-inch exterior-grade plywood over the joists, ensuring all seams are supported by framing. The plywood surface must be smooth, clean, and completely dry before membrane installation. Any protruding fasteners, splinters, or debris will puncture the membrane.

Professional installation is strongly recommended for waterproof membranes, especially in Vancouver's climate where failure means water damage to the structure below. Certified installers heat-weld all seams to create a continuous waterproof surface and properly flash the perimeter connections. The membrane typically extends up any adjacent walls and under door thresholds to prevent water intrusion.

Drainage below the landing is equally important. Install a gravel bed or drainage system beneath the landing to handle water runoff from the membrane. Without proper drainage, water pools under the structure and creates the humid, stagnant conditions that accelerate wood decay. Ensure the ground slopes away from the landing and consider installing a French drain if the area doesn't drain naturally.

Alternative approaches include aluminum landing systems with integrated drainage channels, which eliminate wood entirely and provide permanent waterproofing. These systems cost more upfront ($60-$120 per square foot installed) but require zero maintenance and never rot or decay.

This is definitely professional territory — improper membrane installation leads to expensive water damage repairs that often exceed the cost of the original deck. The membrane must integrate with your existing deck's waterproofing system and meet BC Building Code requirements for elevated structures over habitable space.