My mother-in-law has a deck 8 feet off the ground that is is built with 4 X 4's for support, with only one notched 2 X 6 rim joist on the outerside of the deck which also supports the 2 X 6 joists that are spaced at 24 inch centres. The deck is at least 24 feet long by 8 feet wide.
There are no deflections. It's just old and had some joist wood rot along with top decking rot. I've replaced the three joists that had wood rot. The posts were set on patio blocks about 6 inches max into the ground and surrounded by crushed limestone and interlocking brick. Three of the six posts have wood rot below ground level.
The deck was built 25 years ago and does not meet current Ontario building code which calls for all decks to be built with 6 X 6 posts with 2 X 8 joists, and at least two, 2 X 8 rim joists joined together. Since I'm not building a new deck, you're not required to meet the newer Ontario building code, but I want to at least make sure that it is secure. It seems the 4 X 4's stood up okay for the past 25 years without requiring 6 X 6's.
I could remove one post at a time with temporary side supports, remove the patio stone and dig down four feet as required by code, insert a sono tube, fill with cement and then attach a saddle with a new 4 X 4 or 6 X 6 post. The current floor of the deck is covered with 2 X 6's. Quite a few need replacing. I was going to remove these.
I wanted to enhance the structural integrity of the deck by turning the 4 X 4's into 6 X 6's by adding a 2 X 6, vertically screwed onto one side of the 4 X 4, and then a 2 X 4 vertically screwed into the side that overlaps the 4 X 4. This would then make the post a 6 X 6. Then, for added rim joist support, I was going to lag bolt a second rim joist to the one that is already there. This rim joist would sit on top of the 2 X 6 that I would add to the 4 X 4. I was also going to add an additional 2 X 6 joist between all of the current joists thereby moving from 24 inch centres to 12 inch centres.
Please let me know whether this is a futile effort to enhance the structural integrity of the deck, or would I be better to spend her money and replace the current 4 X 4's with 6 X 6's, still adding a second rim joist attached and notched into the 6 X 6 post? I've already added joist hangers and lag bolted the deck to the 2 X 6 header that is attached to the house. Originally, this deck was built only with 4 inch nails.