How can I fix the sag in my floor? Can I jack up the subfloor and sister a joist to the original joist to hold it in place? If not, is there a simple fix? I'd like to avoid jacking up the original joist if possible?

I don't know another way of doing it. You do have an option besides the sister though. You could install a pad and a post support under the joist.

If you want to sister the joist remember to raise it a bit over level and install the sister with the crown up to prevent future sagging.

DO NOT, I REPEAT, DO NOT jack the subfloor without jacking the joists!!!!
That would be a disaster!

The first thing is that if you have screws into the joists (unlikely, but possible), you would tear them right out.

The second, and significantly more important item is that your floor system acts as a deep horizontal beam to distribute lateral loads (wind, seismic, unbalanced lateral earth pressure) to the lateral force resisting elements (shearwalls). This relies on a specific plywood thickness, joist width, nailing pattern, etc. If you start messing with that, by providing a gap between the joists and the plywood then you are undermining part of your LFRS.

There would be a way to do this, but you should really hire an engineer to spec it for you and not have a go at it yourself. If you want to do it yourself, jack the joists and not just the subfloor.

Thanks. I will contact a professional and have it done right depending on the cost. When I removed the carpet their were shims under the plywood. I still have the plywood so I may just shin the floor again, install the plywood and then install new wood flooring over the plywood. I will not jack up the sub-floor unless I jack up the joist. Thanks for saving from myself.

James

Thanks for the advice. The floor has been properly repaired. They raised two joists and installed permanent posts. The 1" sag is now gone. The sag behind the radiator is just 1/4" which I will cover with quarter round molding. The floor has been sanded and finished and looks great. Now I can enjoy the beauty of the home's original wood floors.



Thanks again.

James