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Michael Kusz 03-24-2012 12:12 PM

Sandwiching Floor Joists
I have a 40 year old home, the second story floor is sagging in one area, about 1" in the center of a 10' square area as determined using a laser. The overall room is 14' x 28'. I suspect the sag is do to the floor joist age and spacing, the joists are 4" x 8" x 14' on 41" centers.

Can I sandwich several of the joists with 2" x 8" x 14' to strengthen them? There are 8 of these joists supporting the floor, which is a bedroom.

Daniel Holzman 03-24-2012 01:23 PM

The "sandwich" you refer to is called sistering. Yes you can sister a new joist to the old joist, the process has been discussed many times on this forum, do a search for old posts. Your spacing is very unusual, most of the time joists are 16 inches on center, so even if you stiffen up the joists, you are likely to have stiffness issues between the joists. One other detail is that if you want to get the floor back to level, you are going to have to jack the old joist to level before you install the sistered joists, or you could attempt to install the sistered joist level, but higher in the middle than the existing joist. That approach can be used provided you remove the existing floor and subfloor. The jacking approach can be used without the need to remove subfloor and floor, but is tricky to get right.

Michael Kusz 03-24-2012 01:30 PM

Fastener Pattern
Thanks again. I have read the previous posts, however as you pointed out, 41" centers is not the norm. As a follow-up, I have read comments about fastening the "sister" with "lots of nails", screws on 12"-14" centers, 1/2" bolts, etc. I plan on using 1/2" bolts and washers in a w pattern, moving alternatly 2" in from the top of the joist, then 14" and placing the next bolt 2" in from the bottom of the joist.

I may try to level the floor using a hydraulic jack and a 6" x 6" across two of the joists, lifting no more than a 1/4" every 2 days or so.

Daniel Holzman 03-24-2012 03:51 PM

You can sister two joists using just about any type of fastener, as long as it is structurally rated for shear (that means no drywall screws). Personally I like to keep it simple, 16d nails every 6 inches offset top and bottom. I don't bother gluing the two boards together, if you have adequate horizontal shear capacity from the nails, the glue does nothing.

Gary in WA 03-24-2012 11:23 PM

"I may try to level the floor using a hydraulic jack and a 6" x 6" across two of the joists, lifting no more than a 1/4" every 2 days or so."------ Are you saying jack two existing beams (joists) with the decking on them, then sister? Are you installing posts?


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