How can I fix a 14-inch dip in my hardwood floors caused by sagging subfloor between joists, with access to joists from the basement?
Asked by Dolores Pedroza 
3 months ago
Last Updated: September 28, 2024
Hey, so I noticed a little dip in my hardwood floors in the kitchen, about 1/4 inch deep. It seems like the subfloor is sagging between the floor joists, with the hardwood running parallel to them. I can reach the joists from the basement. Do you think putting blocking between the joists could help apply pressure to the subfloor? Or maybe using a jack to push up the subfloor via the blocking could level things out a bit? Any advice on how to fix this would be appreciated!
Is the sag consistent between those two joists, or is it only at one end?
(On a side note, hardwood flooring should ideally be installed perpendicular to the joists, unless there’s a minimum 3/4 inch subfloor in place. But I’ll stop there…)
Remember, perpendicular is crucial, especially for thin layer subfloors. Also, don’t forget the joist hangers are too small.
The sag is only on one end of the floor, not the entire span.
Is this a case of the floor dipping due to a weak subfloor? If it is, get a 2×6 block to fill the gap, jack it up until it’s level, then attach 2x4s on both sides for support. Once that’s done, you’re good to go.
Make sure to follow ‘s simple directions closely.
This dip is isolated and caused by a weakened subfloor. Thank you for the guidance.
No worries, adding some subfloor adhesive wouldn’t hurt. Just make sure to address the weakened areas. I’ve done it both ways with consistent results.
The textbook recommends installing hardwood flooring perpendicular to the joists to prevent any corrugation effect.
Use a 4 lb hammer to gently tap the sub floor from below in order to create space for shims to be inserted between the joist and sub floor.
You should totally hire a contractor, haha
Absolutely, without a doubt
Make sure to add blocking/struts to fix the issue. When you use the jack, go slowly. If it becomes too tight, leave the jack under pressure overnight and continue jacking the next day.
It’s not ideal that the new flooring was installed that way. Definitely not recommended, but it can be done with enough layers and thickness of subflooring.
Are the beams aligned properly with the rest? I’m concerned that the beams might be lower in this spot, and raising them here may not help much.
Yep, the joists are nice and level
If you put a 24″ level on the floor and it’s not level, the issue isn’t with the plywood. There’s one joist that’s lower than the others. You’ll need to raise that joist or add shims between the joist and the plywood.