What should I do about a hardwood floor that's sagging between the joists?

Question

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?

Answer 1

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

• Black = OLD work already there
• Red = new cross bridging or bracing
• Blue = new joists
• Green = blocking to prevent warping
• Purple circles = places where you use joist hangers

Answer 2

To make the floor stronger from underneath you have basically two choices.

1) Add new joists between the existing joists, so if they are currently 18" oc they would now be 9"oc (alternating an old joist with a new one you add). This is basically what @The Evil Greebo is referring to.

2) The second option is to sister the existing joists. You glue and screw a new piece of lumber onto the side of each of your existing joists, effectively making them twice as thick. You need to glue and screw along the entire length of the joist because you want it to move with the original joist.

I am not a structural engineer but I think option 1 will actually be a stronger floor.

To answer the other question about furniture legs busting through the floor, yes it can happen if there is no subfloor and the tongue or groove edge of the flooring breaks. This has happened a few times at my house (without a subfloor). Not to imply that anyone has ended up in the basement but the problem areas are if you get the leg of something heavy on just the right spot (a weak board that does not span across two joists) it can crack.

It happened once when someone sat on our sofa and it has also happened with the fridge when moving it around it got on just the wrong spot and broke through. The fix usually involves going into the basement and installing a 2x4 between the joists directly under the cracked board. I just leave the original board in there but now that it is supported underneath it does not move anymore.