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Planning to build a 10'x16' "cabin" with a loft. My idea is to use 2x4-10 for the walls then in the loft area use 2x4-8 for the walls and run 2x6-10 the width of the cabin to create the loft floor joists. Do I need additional posts or beams for support or am I just totally going about this the wrong way?

The loft should be able to support two sleeping adults and a mattress. I'm new to all of this so trying to keep it stupid simple.

  • the cabin will sit on concrete pillar and beam foundation
  • ground snow load in the area is ~100 psf
  • the soil is loam

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  • Will you be getting a permit and hooking up to a power company?
    – JACK
    Commented Apr 3, 2023 at 0:30
  • plib.org/aitc/resources/publications/beam-capacity-tables
    – DIY75
    Commented Apr 3, 2023 at 0:51
  • I will not be hooking up to a power company (not available). Permit is TBD but I probably should.
    – Foopysfarm
    Commented Apr 3, 2023 at 0:59
  • @Ruskes every floor joist of the loft has to be 3.5" width? I am having a hard time interpreting the table you linked.
    – Foopysfarm
    Commented Apr 3, 2023 at 2:11
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    @Ruskes, all your comments should be in your answer, not here.
    – isherwood
    Commented Apr 3, 2023 at 12:45

2 Answers 2

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Here's the approach I would take.

  1. Buy 2x6x10 SPF heat treated/kiln dried lumber

    • standard stuff stocked at big box stores should be fine; #2 or better
    • try to find the better ones in the stack with little warpage; no splits and avoid large knots on the edges

  2. All joists should be crowned up and securely attached at the ends to studs

  3. install the joist at the front as a double (beam); when people go up and down all their weight will initially be on that one beam with little spread to the next.

  4. The joist at the back should additionally be screwed to the wall studs.

  5. I would space the joists at 12"; 16" would be fine as well, but would definitely flex more and the 12" provides more even support for the plywood. I used an online sag calc as a reference.

  6. I would glue (construction adhesive) and screw 3/4" plywood/OSB (2" long construction screws) down to the deck of the loft; the glue will prevent squeaks and help stiffen the whole thing.

    • If you can't buy a single sheet to fit this space, use 3/4" tongue and groove OSB and take advantage of the T&G to lock the pieces together.
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  • It should be noted that joist hangers are typically a requirement. Otherwise you'll have balloon framing, which is generally prohibited in the 21st century.
    – isherwood
    Commented Apr 3, 2023 at 12:47
  • I do like the 12" interval suggestion. I generally don't consider 2x6 adequate for joists spanning more than about 6 feet (regardless of load tables, due to bounce), so that would help.
    – isherwood
    Commented Apr 3, 2023 at 12:50
  • @isherwood Joist hangers are typically required where the are not resting on bearing material; since the OP has a double plate below it, I see no reason to not run the joists onto it and let the double plate bear the weight of the loft. And as far as I know, balloon framing is still legal as long as there are fire stops between floors, which really isn't a concern here as it is a single level and the double plate already provides a fire block.
    – UnhandledExcepSean
    Commented Apr 3, 2023 at 13:06
  • You're right. Apparently I misinterpreted the plan.
    – isherwood
    Commented Apr 3, 2023 at 13:33
  • Since the outer edge of the wall is flush with the rim joist the overall width is 10' 3". This means the loft floor joists can't span the full width and would overlap the walls double top plate by 2" and not the full 3.5". Is this an issue?
    – Foopysfarm
    Commented Apr 3, 2023 at 19:52
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If you put 1/2 inch or 3/4 inch plywood, that floor can carry up to 300 pounds per square foot with the proposed joists spaced at 16 or no more than 24 inch.

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