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Background: I have a kitchen supported with 2x8s 16" o/c over a 13' 3" span. I want to install a 12x24 tile floor, so that much deflection is just not going to work.

Question: I have 2 options I am considering. The first is to sister the 2x8s with another 2x8 so that both would be supported at both ends. The second is to sister the 2x8s with 2x12s but the 2x12 would not be supported on either end as they can't fit between the beam and the floor.

Which one will add more rigidity to the floor?

Side question: Which is more rigid one 2x12 or two 2x8s sistered?

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  • Is there a full basement under this kitchen? What about an engineered beam perpendicular at mid span? Maybe several 2 x 12s made into a beam. How much intrusion into the space below is acceptable? Feb 14 '17 at 13:19
  • I have a 13'3 x 40' room under that side of the house. it would lol rather weird and ugly to put a beam and a column right there is the middle of the room. Feb 14 '17 at 17:28
  • Does the room under the kitchen have a finished ceiling that you are going to take down for access to sister in the extra 2 x joists? What is the height of this room? What is the current flooring in the kitchen? Feb 14 '17 at 22:23
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Side answer - beam stiffness goes as depth cubed, width directly.

12 cubed is 1726, 8 cubed is 512

So 2x12 is 1726 .vs. 2x(512) for 2 2x8. roughly 1.7 times stiffer, that is.

You can "haunch" the ends of the 2x12 to 8 inches - it should be done as a taper, not as a notch - 45 degrees, not 90 degrees - it's actually stronger that way, which is a bit counter-intuitive).

Joist hangers may offer other options to retain full depth to the ends.

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  • Could an additional 3.5" be added to the bottoms of the 2 x 8 joists? Perhaps 2 x 4s with slow setting adhesive braced in the center or in 2 or 3 places then screwed at 6" intervals to the existing 2 x 8 joists. Another way to supply extra tension on the bottom, could a steel strap be nailed or screwed to the bottoms of the 2 x 8 joists which would be temporarily braced while the strap is nailed at 8" intervals. (This is how the wooden treads of ladders used to be braced from below with metal tension rods.) Feb 14 '17 at 12:51
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    All possibilities, but all more complicated than the original plan.
    – isherwood
    Feb 14 '17 at 14:11
  • How is the steel strapping more complicated than sistering on more joists? You may know that this would not work, but, if it would, it seems it would be simpler. Do the original joists have any plumbing or wiring in them? What about bolting angle or square cross section steel on one or both sides of the bottom edges of the original joists. The original joists could be temporarily braced from below at mid-span all the way across while the steel was screwed in or bolted through. This would have the advantage of not changing the ceiling height in the room under the kitchen. Feb 14 '17 at 22:41
  • Strapping will not work without vertical tensioners of some sort below the center of each joist, creating a sort of truss. This makes it complicated and time-consuming.
    – isherwood
    Feb 26 '17 at 21:28
  • Ecnerwal: So would this be appropriate? (I modeled it in SolidWorks) a full 45 instead of a notch or should I use a notch and try my best to beat the rest of the 2x12 in between the 2x8 plate and the floor upstairs? ibb.co/gH4BOv Mar 14 '17 at 0:45
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Your second option would result in a stiffer floor. It's not necessary that the ends of the sistered lumber rest on the foundation. The movement that you feel is the result of deflection (bending, and not compression of the joist at the supported ends), as you've correctly described.

The critical concern is that you have solid connections between the old and new lumber. Three 16d nails or 2-1/2" construction screws every 16" and distributed vertically should do.

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    Construction adhesive is helpful as well.
    – Ecnerwal
    Feb 14 '17 at 3:16
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    Glue, then screw before it dries, using screws with 1-1/2" of non-thread, that way all the screw is in the other piece and it clamps them together instead of holding them at gap. Glue is worthless if a gap remains. And epoxy (eg West System with adhesive filler) is the best. Feb 14 '17 at 4:48
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    I considered recommending construction adhesive, but it doesn't seem necessary here. The fastener pattern I suggested will lock things down solid, and noise is unlikely. Couldn't hurt, I suppose.
    – isherwood
    Feb 14 '17 at 16:10

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