Well the tenants have to leave the property in the same condition they found it, fair wear and tear excepted. That said, you need to make it easy for them to do so. Don’t have experience of decking, but I was a letting agent for many years. Get it in good condition to start and then leave them instructions for maintenance. Repairs though will be your responsibility, as with a fence. If necessary, Aim to share the annual responsibility would be my advice , but cleaning responsibility etc would be theirs.