I recent bought an older (1890) house. On the original floors, the wood flooring is sagging between the joists–you can feel the height difference as you walk, up and down, up and down. It's firm, it's just not flat. The joists below are in great condition, huge old timbers, they're just far apart and I'm betting the subfloor is not up to the task of staying flat.
Is this something to be concerned about? What complications might this cause (e.g., could furniture legs break through the floor if between joists)? Or should I ignore it until it comes time to refloor the house? Is it possible to shore this up from underneath?
Best Answer
Since you can get to the floor from underneath, you can shore up the flooring by adding new joists between the existing ones. This will not be a FUN process, but the process is simple enough, just labor intensive.
First, add cross braces between each joist at each end, of the same dimensions as your existing joists. You'll want them to be snug fitting between each joist, and well supported with joist hangers.
Now, between each brace, halfway between each existing joist, hang new joists to fit between the cross braces you added. Before you permanently hang the joists, prop them up from beneath with 2x4 temporary posts to force the tops up to be level with the tops of the existing joists (pushing the flooring above back into shape), then secure them permanently in place with joist hangers.
Add blocking between the old and new joists as well, to ensure the new joists don't warp/twist.
See the pic attached to help clarify what I mean