I'm in the process of replacing the deck boards and have found rot. The side joist of this second level, 30 year old deck is doubled with what looks like 1/2" plywood sandwiched in between. The plywood is seriously rotten. I have not yet found rot in the joists. What should I do? If I continue to lay the new decking, the rot will spread to the undersides of the new boards. But worse, won't this rotting plywood rot the joists? Is there a solution? Is joist rotting, when you have a rotten plywood 'starter' a quick or slow (10+ years) process? AUGH!