How can I flatten this floor? - DIY Chatroom

JamesEric · 10-18-2009, 11:29 AM

As you can see I have access to the floor from below. Is it possible to jack up the sub-floor and then sister the joist to hold it in place without jacking up the joist? When I put the level on the joists they seem flat enough but the floor above is sagging. The house is 100 years old and I'd prefer not to touch the joist. Any suggestions for good fix that lets me keep the original floor?

Thanks again for the continued help.

James

**Scuba_Dave** · 10-18-2009, 11:38 AM

You want to jack up the floor without jacking up the joist?

The sub-floor is not designed to hold the weight from jacking
If you want to destroy the floor trying to jack up the sub-floor would be a good way to do it

You need to jack up the joists little by little over time to bring this back into place
Do you see any crack or failure in the joist?

JamesEric · 10-18-2009, 11:43 AM

No cracks or failures in the joist. Why couldn't the subfloor sit on the sister joist instead on the original joist?

Daniel Holzman · 10-18-2009, 12:25 PM

If the existing joists are level, there is no reason to jack up the joists at all. You need to determine why, if the joists are level as you indicate, the floor is sagging. Usually a sagging floor indicates sagging joists, but from the first picture it looks to me like the floor sags much more than the joist ever would. Also, the basement photo seems to show the joists in very good condition.

I suspect that the floor is not firmly attached to the joists. This could indicate a problem with the subfloor installation, I could not tell much from the photos. If the subfloor is not properly attached to the joists, you could remove the flooring, and reattach the subfloor using screws or nails as appropriate. You may need to replace part of the subfloor, using identical thickness plywood to whatever was originally used.

If the subfloor is flat, and the floor is not, you have a different problem, which we could discuss once you determine where the problem lies.

**Scuba_Dave** · 10-18-2009, 12:34 PM

One problem will be that the flooring is now warped
Trying to jack up just the one area will push up the floor
That looks like a pretty good dip
It's also possible that the joist is level, but it was installed lower then the joists on either side

JamesEric · 10-18-2009, 01:07 PM

I gave it a second look and that joist is bowed in the center. the joist are 1-3/4" x 11-3/4" and the span is 13 feet. I think the joist are oak. The joist are set in 4" of masonry block and brick. If I try to jack up the joist it will damage the walls above. I'd still prefer to jack up the floor and attach a sister to the original joist. Will that work?

Handyservices · 10-18-2009, 01:54 PM

I would locate the joist and cut the subflooring out above it for access. Then cut out the bowed joist and replace it with a new one. Replace the removed subflooring section and finally the hardwood section.

The joist could also be bowed because of improper installation of the cross braces in that area. Really hard to tell without cutting a section out.