Sistering Floor Joists - same size required?

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  #1  
Old 01-05-16, 06:30 PM
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Sistering Floor Joists - same size required?

I'm converting a small office into a large bathroom in my house. The room is gutted. The floor is not level, and not flat. Nothing major, but enough to cause a headache. The house is old, so we've done the work in the basement to shore the house up.

Question is, I'd like to sister some new boards to the existing joists, so that I can have a nice floor to work with as I install the tub, lay the tile, etc. My existing joists are 2x10. Do I need to use 2x10s for the sister?

I'd prefer something smaller, due to the wires, rad and plumbing pipes that I need to work around.

I'd use 2x10s when I had a clear run, but downsize when possible to something like a 2x6.

Anybody see a problem with that?
 
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Old 01-05-16, 08:01 PM
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All you are trying to do is transfer the load from the sister to the existing joist, so I think your plan is OK. You will want to make sure you don't skimp on fasteners as they will be taking the entire load if the ends don't bear on supporting structure. Don't use drywall screws for this. And it wouldn't hurt to use construction adhesive between as well. That will also help prevent squeaks down the road. Any bridging or blocking there to resist twisting should be recut and replaced. Adding the sisters will add some twisting force because the load will now be off center to the joist, so the bridging is important.

Have you thought about cutting tapered shims screwed and glued to the tops of the existing joists instead of sistering? You'd clamp a piece of stock in place as if you were going to sister it, but then scribe it along the top of the existing joist, cut the piece off, and glue it and screw it in place. Could be easier (especially if you have a lot of bridging or blocking to redo) and less expensive; final results would be just as good.
 
  #3  
Old 01-06-16, 03:36 AM
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If you plan to raise a section of floor above an area of sagging beam or floor joists that are unsupported, then any under sizing of that section could fail.
There is a big difference in strength between under sizing the sistered section of added dimensional lumber and drilling holes to pass wires through.

A participated in a similar job by removing some duct work so a contractor could repair a sagging beam.
He also has a plumber remove a couple of water lines and an electrician remove wiring that passed through nearby floor joists when he built those sections up up.
 
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Old 01-06-16, 03:51 AM
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What is the unsupported span of the 2x10's that you are trying to shore up? Are they 16 in OC or greater? It may be easier to build a partition wall underneath the bathroom in the basement to stiffen up the floor. Sounds like an over span situation that sistering will not correct. The floor needs to meet certain specs for installation of tile. A stiff floor is needed as well as a flat floor.
 
  #5  
Old 01-09-16, 11:04 AM
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The width of the room is about 9'.

The span of the joists from foundation wall to the center support beam is 14.

The joists are 16" OC.

I've considered shims, but feel like I might lose my mind making the super specific cuts.

If I sister the joists, I'll take the time to back out wires where neccesary.

If this is an overspan problem, I would be open to building additional support in the basement.
 

Last edited by LogiVap; 01-09-16 at 11:50 AM.
  #6  
Old 01-09-16, 01:59 PM
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Unless you used #3 grade lumber, it should be in specs for 2x10's so it must have been something do to the house settling. Can you take a mason line and string it tight as you can get from one side of the room to the other to determine the exact dip involved. Mason line should be a tight as a banjo string (they do stretch quite a bit). Then report back. If you don't object to a support wall underneath, that may be the easiest route to go. I just want to make sure it is a sag and not a wave and it is parallel to the joists and not perpendicular.
 
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