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Improving an attic for live loads. Contractor sistered our 2x6s but due to the plaster ceiling, they weren't able to sister evenly. I realized this means the new sistered 2x6 beams aren't actually touching the walls they span between.

Do I need to go back and wedge something under so they're touching? Or are the nails sistering them together enough?

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It’s complicated and it depends. You’re using 2x6’s so there can’t be extra large loads, so you’re probably ok.

The reason the joists need the correct amount of bearing area is so that the joists don’t crush where they bear. Extremely large loads tend to crush the fibers of the joists at bearing points, unless they have an extra amount of bearing area. You are using 2x6’s which don’t support much so I’d guess you have plenty of bearing with one joist.

A bigger problem is deflection (bending). If you over load the attic, the joists will bend and crack your plaster. I’d keep the heaviest loads against the walls and the lightest loads in the center of each span.

  • Seems like having the new joists actually touching the wall would help on both the front you mentioned. – brentonstrine Feb 25 at 22:42
  • If the joists are adequately nailed together, bearing and deflection should not be a problem. What is the span and how many nails sistering each joist together? What is the spacing of the existing joists? – Lee Sam Feb 25 at 23:10
  • 12 foot span on 16 inches. Not sure the number of nails. Seems to be one about every foot or so. – brentonstrine Feb 26 at 3:29
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    Joists nailed together at about 12” - 16” o.c. Should support about 40 lbs. per square foot live load plus 10 lbs. per square foot dead load spanning 12’ at 16” oc, depending on the grade and species. A bearing size of about 1” long x width of one joist is sufficient. (No waterbeds or refrigerators.) So, 1 joist is sufficient for bearing and the nailing is adequate if it’s staggered top and bottom (about 1 1/2” from the edge of joists) or if the floor sheathing rests on both joists...EQUALLY. – Lee Sam Feb 26 at 4:55

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