Deck posts should rest on and be anchored to concrete piers. Footing size are dependent on factors such as live and dead loads and ground conditions. Concrete footings should be at least 12″ below the undisturbed ground surface and in my neck of the woods, below the frost line to prevent heaving. Follow your areas recommended footing depth to meet local code.
Guard Rails and Railings:
Railings are a critical safety connections and should get a thorough inspection for safety.
Railing posts must be attached to the decks rim joist and tied into the deck joist framing. Use two through bolts on your post and use a Simpson’s tension tie # DTT2z or a HD2A hold down for the top bolt connection.
Guard railing posts should be a minimum 4×4 size and be attached to the outside joist and rim joist of the deck. They can be retrofitted with a tension tie, through bolts and or blocking as needed. Follow IRC code requirements when installing fasteners into guard posts.
If guard posts anchoring does not have a minimum allowable tension load of 1,800 lbs for a 36” maximum railing height they should be replaced. The same tension ties used too tie the deck joist to the house can be used on railing posts and attached to the top bolt installed in a post.
If your deck is more than 30″ off the ground it requires railings and guardrails. Depending on the application guard rails can be either 36″ or 42″ – refer to your local code.