I have an older home 1 1/2 storey home with a field stone/lime-based mortar foundation. The roof and floors have both sagged. I'm wondering if I should take some steps to reverse the problem. The problem could be caused by the deteriorating foundation. The top 2 feet of the foundation are above grade and a lot of the original mortar has turned to sand. 20 years ago a major renovation of the house was completed. which included resurfacing the foundation with what appears to be a Portland based product. Even still the foundation has continued to deteriorate particularly the middle of the south facing wall. The beams supporting the house run east to west. I believe the deterioration of the foundation in this middle section which is causing the floor and roof sag.

I would like to correct the sag over time using temporary adjustable steel columns, raising the house off the middle section of the foundation and build it back up. I don't expect to achieve a perfectly straight roof or floor, but how much can I try to correct the sag without starting to worry about creating other problems?

Any additional advice/suggestions would be appreciated