I would add blocking between the joists. At 48" OC, no wonder the floor is not stiff, even with 1 1/8" plywood.

If it were mine, I would consider just adding more posts where I had issues, then running 2x6 blocking between the joists on 2' centers.

The problem with sistering 4x6, is that you're not going to get a whole lot more support. Plywood will likely not do much if I understand your design.

So here's an idea (I'm also an ME) - do a prototype. Take an area that's flexing / sagging, and put a post mid-span and some blocking. See what you get. Or just sister one joist and see what you get. Low cost and effort to get get the data you need to invest in all of it.

When you sister, you have to use glue to get the shear strength up. Nails / screws are not enough.

To jack it up level, get a 2 or 4 ton bottle jack (2 is nice and small and adequate) and a number of 2x4s of different lengths. You can configure them to fit - some on the flat and one vertical - at each location you need to jack up. Make a mark 1" up on the side of the joist above each post, and in the center. Use screws / nails to secure the string and the ends, and jack the joist up until the center is aligned.

Best of luck with it.

Consider the flooring when jacking. When you do jack make sure you are leveling the sunken part and not raising the entire joist. Put something on top of the jack to spread the load.

Thanks for the comments. I may try to reinforce a section first to see if it gives the stiffness I am looking for. If it doesn't, then I'll talk with a structural engineer and see what the additional foundation work will cost.

Of course, more ideas/comments always welcome.

I can't imagine adding sub floor between joists would substantially stiffen a floor. Adding plywood to the bottom of the joists might.

How did it go? Did adding plywood to the underside of the subfloor (between the joists) stiffen your subfloor?

How could it stiffen the floor? It needs to be over the top of the joist to do anything. Between the joists you just adding plywood and really not doing a thing except adding weight.

My post wasn’t intended to start another round of speculation, it was to find out the results.

It worked really well

Thanks Troy. Did awong22828 leave the site?

I meant to revise this post to update how things actually went...

The reinforcements worked really well. I did 3 things that really ensured support of the entire floor and that the new materials transferred loads into the existing foundation supports:

- Laminated 1-1/8" plywood to the underside of my subfloor (glued and nailed in place),

- Sistered 2x6s to the existing floor joists to support the edges of the new plywood (glued and nailed in place). To ensure full contact with the plywood, i jacked these 2x6 to ensure full contact (vertically), then used clamps to ensure full contact against the existing floor joist (laterally).

- Added 2x6 blocking perpendicular to the sistered floor joists (16 inches on center). I also used jacks here to make sure the blocking was pushed tightly against the new plywood/subfloor.

Effectively, I double-sheeted the subfloor from the crawlspace and ensured the new plywood was transferring load to the existing floor joists by sistering and blocking with 2x6s. The 2x6s essentially formed a ladder structure under the plywood.

The floor is very solid now. Very little deflection, even with lots of children stomping or myself running on a treadmill. The travertine tile floor in the kitchen has stopped popping and cracking.

I will add that the work was gruelling. It took over a year while working on the weekends, and my crawlspace gets very damp during the winter. I found the easiest way to gain acCess to the crawlspace was to cut a 4ft x 4ft hatch in my living room floor.

Good luck and let me know if you have any other questions.

Perfect! That’s exactly what I needed to know. I am thinking of doing something very similar to what you did, but for a middle floor condo. I can’t raise the floor height, and I can’t get access to the underside of the joists from the unit below, so I was thinking to do something very similar to the underside of the subfloor. This is very encouraging, thanks for sharing!

Really glad to hear this worked! We are in a remodeled home originally built around 1920 as a weekend house on the water. The pine flooring is attached directly to the joists and after all these years there is some sagging. Going to try a section like you did! Thanks for sharing your experience.

The key is to make sure that any added material is able to transfer load into the existing floor joists or foundation supports. I jacked every piece into place to ensure a tight fit and counteract some of the sagging.