The best thing to put between an urban planter box and a wooden deck is a raised, free-draining base—not a single flat sheet of plastic or rubber. Use rot-proof plastic or composite planter risers, pot feet, or a slatted plastic base to create a 1–2 inch air gap. Add rubber or neoprene pads under the feet to spread the weight and protect the deck surface. If the planter has drainage holes, place a shallow waterproof drip tray underneath, but make sure it can be emptied or drains away from the deck. Water should never sit in the tray or under the planter.
The goal is to stop constant moisture from reaching the wood while still letting air circulate. A solid rubber mat, plastic film, landscape fabric, or cardboard laid flat can trap condensation and soil runoff, which increases the risk of rot. Untreated wood spacers can also rot and hold dampness. For heavy planter boxes, use a plastic or composite grid, or several sturdy runners with open ends so water can escape. Keep the base slightly sloped, and direct overflow away from the deck and house.
Before placing the planter, clean the deck, apply a suitable deck sealer if needed, and check that the planter does not block drainage or trap leaves. Use a liner inside the planter to keep soil from washing out, and avoid overwatering. If the planter sits against a wall, leave a small gap for airflow. In short: put down rot-proof risers and a drainable drip tray, maintain an air gap, and keep water moving away from the wood. That combination protects the deck far better than any single barrier material.