How deep does a deck post footing need to be if it’s directly behind a retaining wall?
How close can a deck post footing be to a retaining wall
deckfootings
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They make a metal brack to span the old 2X4 and you most likely can get it to work for the new roof. If not I can most likely get some made at work. You do have some good bracing and support on the one footing you have pictured here and if the other one is the same way it would be just as strong.
I would just pull out the old footings and replace them with the new style footing and be done with it.
Reasons: 1) you would always have questions as to how strong the modified brackets (ie the roof/posts are) and 2) By replaceing the old footing /posts you now have done the job and will pass and inspector.
Burying the wood post will cause it to rot prematurely. Even ground-contact rated Pressure Treated lumber won't last forever.
The only benefit of burying the post (besides it being easier to do) is lateral stability. i.e. you can sometimes neglect lateral bracing this way.
Best practice for wood-post decks is to have the concrete extend above grade and use a stand-off post anchor to hold the wood post to concrete.
Here is an example of this with some knee-braces for lateral bracing.
The braces don't match so I find this somewhat ugly, but the image is clear and easy to understand.
Footings here don't need to be deep so I often form my own square ones but sometimes I use bigfoot forms and I really like those. Although Shirlock's "mushroom" method is more than adequate if you are only required 8" wide.
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Best Answer
Comment by @BillDOe is correct: it should go the required depth for the post, measured from the base of the retaining wall as if there was no wall and no retained earth there. As long as it has sufficient depth, it can be arbitrarily close to the face of the retaining wall.
You don't mention if you are planning/required to pour a footing for the post. That may change this answer as the presence of the wall may have an impact of the options you have for the design of the footing and the resulting distance from the existing wall.
If you have some flexibility and can set the post further back from the wall, your local code might have a minimum distance defined at which you can set the post at the standard depth, without accounting for the wall. Somewhere between 1' or 2' is pretty common. Some require specific types of retention systems to prevent the wall from toppling from the top during high winds.
Another factor to keep in mind is frost heave: if the frost line is 4' in your area, that means 4' in a flat, uninterrupted section of land. If you are directly behind a 3' wall and only go 4' from the top of the wall, it's very likely that the post will experience freeze/thaw cycles during winter. That would be bad for the deck, and could also be a problem for the wall itself.
One last thing to watch out for is any kind of engineering fabric or mesh extending back from the face of the wall, and a few inches deep in the soil. If it's there, you would need to carefully cut through it and avoid disturbing it as much as possible. you do NOT want to try and auger through it...
As always, your best bet is to check with your building inspector, but this is the typical recommendation. Different municipalities have different requirements.