Floor is Sinking and Sagging: What’s the Cause?

LESLIE: Carol in Mississippi, you’ve got The Money Pit. What can we do for you today?

CAROL: I have a floor that’s sinking in the hallway and the kitchen and I was just trying to find out what’s the best way to repair that and what type of wood should it be repaired with.

TOM: So do you have any idea, Carol, as to what’s causing the floor to sink?

CAROL: I don’t know if it’s the foundation. I don’t know if it’s the foundation, because it’s in Mississippi and there’s lots of moisture there. And for some reason, the homes there really look rundown and stuff. And I always – and I did research just because it’s so moist there. But the house is really, really old, so I have no idea what’s causing it.

TOM: Is the floor a wood floor?

CAROL: Yes, it is.

TOM: And is it over a crawlspace?

CAROL: No, it’s not over a crawlspace.

TOM: Is it over a basement?

CAROL: No, it’s not over a basement. It’s on the foundation part of it.

TOM: Yeah. OK. Look, there’s only really three types of floor structures. You’re either going to have – your house is going to be built on a slab, which means it’s on concrete, or it’s going to be on a wood-framed floor, which is either going to be over a crawlspace or over a basement. I’m trying to figure out what kind of floor structure you have.

CAROL: OK. It’s up off the ground a little bit.

TOM: That’s a crawlspace. So what has to happen, Carol, a structural engineer has to go into the crawlspace and get under that area and look up and see what’s going on. If you have a lot of moisture, you could have some rotted beams there. And if that’s the case, the rotted beams have to be repaired. That can’t – well, it’s very difficult to do that from the top side; you would do that from the bottom side. And then once you know what the cause of the sagging is, then you’ll know how to approach it.

But let’s say that you’ve found there was a beam that was decayed. Well, what would happen in that case is you would put a new beam next to it. So if one bad floor joist, you’d put another one next to it, maybe even one next to that so you’d kind of sandwich the bad beam between the two good beams. And that would straighten that out.

Once the structure is repaired, then you can go in from the top side and repair any remaining decay, like if it was the wood – plywood – subfloor or something in that nature. But you’ve got to start with the structure, which is what’s underneath there, to figure out why this sag has occurred and why this area has sunken in. And once that’s re-supported, then you can move to the top side.

Does that make sense?

CAROL: Makes a lot of sense. Thank you.