Floor Joist Reinforcement?

[mccroanjj]

I have a 5-year old 2-story house. The kitchen is on the first floor, over 2X10 joists, 16" o.c., but the span is almost 16'. As a result, the center of the floor, where the island is, sags about 1/2-3/4" and flexes significantly when someone stands next to the island. The crawl space is reasonably accessible, so my thought is to sister the floor joists with 2X8s (so as to not have to remove metal cross-bracing). Could I expect enough stiffening and correction of the floor to make it worthwhile? Or, would it be better to remove the cross-bracing, sister matching 2X10s and the reinstall cross-bracing? Other suggestions?

[CurtisS]

Sounds like the floor needs a support beam perpendicular to the joists and a post down to a footing BUT it is likely that this is not possible. Sistering in additional beams may be your only choice. I think your last option (remove cross-bracing sister in 2X10s) would be the most solid solution.

[Rich]

I agree with CurtisS for the most part on this one. The support beam could work without a solid footing. You could go to a precast supplier and buy some 24"x24" pads that are 6" deep to use to support a post for the beam to sit on that could be sufficient. I've done whole interior pony walls in this fashion where below grade foundations were not allowed due to water table constraints that worked out great. Just another option to the problem.

[OHPG]

Hi,

Adding additional joist will not correct the 'sag' you already have. At best it will slow down future sagging.

I'm not an engineer or architect but 16' feet is a huge span to cover for your 2x10 floor joist. I've been building and helping build homes for 12 plus years and I've never seen a 2x10 span more than 10 feet.

They maybe able to span more than that. But I've never seen it done.

Honestly you can only hope to stop future sagging by adding a center support beam and posts under the floor. Place the beam and post as close to the center line of the span as possible. Running perpendicular of course.

You may not want to here this but now isn't the time to be blinded by the up front cost to do it. Consider the fact that.

With time it will get worse!

I recommend adding a support beam and posts on or near the center line under the existing joists.

Then getting someone in there to level the floor from the top. Thus getting rid of the 'sag'

Leveling the floor from the top can be done by removing the finish flooring and adding sheets of plywood that are the same thickness as the depth of your 'sag'

Do not cover the entire sagging area with the plywood. leave an area about 3-4 inch wide. From the edge of the plywood to the edge of the fool that is at not sagging.

When you place a level on the floor you should see a 3-4 long thin triangular shaped open space under the level and above the floor.

This small area is then filled with floor leveling compound. Once it set up you can sand it smooth and replace the flooring.

I don't like using a floor leveling compound over a large area. It's hard to get a good level coverage with the stuff. And plywood that is the same thickness as the sag is easy to install.

I've done this for about 12 customers and my mom and dad over the last 12 years so I know it will work. But consult a local contractor in your area for assistance.

It really hard to explain ever step but talking to a contractor in your area about what I've just told you will help him/here understand what you need done.

Good Luck

[Rich]

Realizing that you've used that method before - I just wanted to add something. I would rather use the center beam, as you describe, but perform the leveling from the beam below by either jacking the beam up little by little or shimming each joist. I've done both with decent results. Once in a small area where we shimmed the joists and once on a larger area where we placed the beam and jacked it up then cut posts to length.
The jacking of an existing floor takes a couple days to get done. If you do it all at once you will probably have a few broken joists - depending on age. You may also have to go back and screw the subfloor down - which I would recommend anyway.
Again.. just another option to a problem.

[grumpydasmurf]

I saw once on "this old house" they had a floor that had sagged in the center and to correct this they used 1/8" thick steel strips.

First they jacked up the floor to be in a position they wanted it to stay. They then "sistered" one of these steel beams in the floor joist. The beams were bolted into each floor joist. When they released the jack the floor stayed where it was.