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Does adding a sister that is smaller than the original joist actually increase strength at all? If so, does it need to be sistered at the bottom (so both beams touch the sill) or does it still add strength if sistered to be flush on the top?

To extend the question, does a sistered joist that doesn't go the full span add strength?

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Does adding a sister that is smaller than the original joist actually increase strength at all?

Yes. They will both bear the load.

If so, does it need to be sistered at the bottom (so both beams touch the sill) or does it still add strength if sistered to be flush on the top?

As long as the joists are attached together with sufficient strength, the force will be carried by both joists through the entire length of the span, but the larger joist will transfer all of the load to the sill. That will work.

Now if the joists are the same size, the attachment from joist to joist is not so critical - they can share the load from the floor without transferring part of that load from one to the other.

To extend the question, does a sistered joist that doesn't go the full span add strength?

Yes, it will add some strength, although calculating just how much strength it will add may be difficult.

  • This is true, but only to a point. If you were to add 2x4s to 20 foot 2x12s, for example, you'd be adding weight without any significant increase in rigidity. You'd have a net loss in terms of the robustness of the floor. – isherwood Feb 26 at 13:23
  • @isherwood If the 2x4’s were added like an “I-beam” they would significantly increase its strength... – Lee Sam Feb 26 at 17:00
  • @isherwood - I think a 2x4 would significantly stiffen a 2x12. Picture them sitting side by side spanning sill to sill. Let's say a load of 500 pounds on the center of the 2x12 will cause 1/4" deflection. The 2x4 isn't carrying anything. Now remove the load, sister the 2x4 to the 2x12, replace the load. It will deflect less than the original 1/4" because the weight has to bend both the 2x12 and the 2x4. – batsplatsterson Feb 26 at 19:30
  • Have you ever fiddled with a 20' 2x4? It's a noodle. It offers virtually no rigidity within the range of motion we're discussing (which I hope is much less than 1/4"). It's just not tall enough. Now reconfigure to a 1x8 and we have something useful. – isherwood Feb 26 at 19:35
  • To put it simply, you could deflect it an inch with one finger. It makes no difference if you attach it to something else. – isherwood Feb 26 at 19:41
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Joists and beams are sized based on 3 conditions: 1) bending, 2) shear, and 3) deflection. However, only one of these elements will “govern”. That is to say, one will fail before the others.

Long spans usually fail in bending or deflection, while short spans with heavy loads fail in shear.

Adding smaller boards to existing joists is called a “composite” joist. Each member has certain characteristics and determining how (and where) to fasten them together is critical.

If additional boards are not added uniformly then the joist can be eccentrically loaded, which will cause it to twist and possibly fail.

Because of the complexity of calculating these loads (and connections), generally we design joists so loading is evenly distributed to the top of joists (and sistered joists) so the load is applied uniformly to all members. Therefore, we like all members to align on top. If one member is smaller (or has many knots and is not as good of grade) it doesn’t matter, as the load will transfer to adjacent members.

Also, your question about sistered joists not “going the full length” is important, because when joists bend they have tension on the bottom edge and compression on the top edge. So, if the sistered joist is not continuous on the bottom, then it can’t carry the tension throughout the joist. But if it’s discontinuous on top, the ends of each piece just presses against one another and is effective...if ends are pressed against each other tightly.

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