Jack up that portion of the house and place new footers.
How to lift or jack up sunken floor joists.
Anything involving 20 ton house jacks will take time.
Continue raising the jack until the deck is no longer.
It has to be jacked up slowly over days or even weeks to avoid cracking drywall plaster windows and even structural elements.
Once the new joists were up we found locations where the concrete blocks could sit flat on the ground so the pressure wouldn t cause them to crack.
If a pier that the post is standing on has sunk into the ground you will need to lift the beam off the post and replace the post with a taller one.
After they were in place we hoisted up the 4 x6 x8 beam so that it crossed all four of the new joists sliding the screw jacks underneath.
Identify the area of the deck that is sagging and locate the support post that needs to be shimmed.
Generally there are posts and beams under the floor joists.
Get the jacks under the floor.
Jacking up the deck and shimming the post can resolve deck sag problems.