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I am building two loft beds with desks under them for my 6 and 8 year olds. My question is will the mattress hold safely? It is 3/4" plywood (75" x 55") with 3x 1" x 4" slats under supported by 2" x 6" railings and 2" x 6" under the railings. I am not a professional woodworker so I am sorry if some terms are not correct lol. enter image description here

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  • Tried websearching and comparing your design against the many plans that have been posted? Looks fine to me but I never built one, just had friends who did so.
    – keshlam
    Nov 11, 2023 at 18:18
  • Generally I wouldn't be concerned however with it being beds for my young children I just want to make sure its sturdy because I know they will be bouncing around on it and everything, mainly just looking for a second opinion from someone who knows more then me lol. Thanks !
    – Zatellia
    Nov 11, 2023 at 18:23

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Plywood:

You want plywood that's sheathing. For 3/4" (or 23/32") it needs a span rating of 48/24. Plywood is what's called "anisotropic," where how you orient it determines its strength and stiffness. When constructing the base for a bed it's tempting to cut a little off the sheet's side and to cut a little off the sheet's end, but that typically will orient the sheet's strength axis incorrectly. This mistake would result in a strength and stiffness of 29% and 15%, respectively (computed from Table M9.2-1 of the 48/24 spec). See item 11 at https://osb.westfraser.com/blog/whats-on-your-apa-grade-stamp/ for an example grade stamp that includes strength axis orientation.

Orienting the sheathing's strength axis to span across the bed's side supports, the sheathing is good for 29.5 psf:

(29.5psf)(1ft)(55in)²(1ft/12in)/8 = 930.#-in,

where 930.#-in matches a conservative bending strength for the sheathing from Table M9.2-1 of the 48/24 spec. Sleeping rooms under the International Residential Code get designed for 30 psf live load, so the 29.5 psf is great. This all assumes that the strength axis has been oriented correctly.

Checking for deflection, I get a deflection under 30 psf live load of 0.68" which is very high for a floor. Such a floor would feel tremendously bouncy. For a 200# person the actual deflection is less than 3/16", where that seems fine.

I would get rid of those 1×4 slats. They're counter productive. Instead install a ledger board along the side of the 2×6 to continuously support the sheathing, and fasten the sheathing down to prevent the ledger from bowing out from under the sheathing.

2×6s:

Each of your 2×6s is a lot like a floor joist spaced at 24" centers. If your 55" was 48" instead, then each 2×6 would be carrying the exact same width of floor. This makes the joist tables from the International Residential Code useful. For a 2×6 supporting 24 inches width of 30 psf live load, the worst maximum length is 6'-5" for #3 Southern Pine. Your 6'-3" length satisfies this worst-case maximum.

In case some kid hangs on the 2×6, I compute a maximum midspan load for a 6'-3" #3 Southern Pine 2×6 as 262#. If you use those beefy 2x6 rails, then you can double that 262# by structurally linking the framing and the rail.

Stability:

An elevated bed needs lateral support to prevent it from toppling. For a free standing bed this could be diagonal bracing that would interfere with the space below the bed. The easiest solution is to fasten the bed to wall studs. If it's in a corner, then attach to both walls.

If it only has one wall, then the fasteners joining the sheathing to the frame become important. Without the sheathing, the two posts interior from the wall are somewhat free to wobble parallel to the supporting wall. The fasteners joining the sheathing to the frame will restrain this wobbling. A fastener every 6" to 8" would be wise.

Addendum:

I see that there's a lot of plywood waste from cutting 55" widths from 96" sheets of plywood. The bending strength of 1×4s is going to be totally inadequate to support the weight without the sheathing's bending strength. A roof's live load of 20 psf seems like a decent lower bound to fix for the bed to support. Orienting 2×4s flat, the 20 psf load plus 3 psf weight for materials results in a 17.9" maximum spacing of 2×4s below the plywood if the plywood doesn't span the full 55" width.

If you're building the stairs from the drawing, however, then I suppose that would be the spot to use the plywood waste.

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  • Wow, that is very in depth and somewhat confusing but between me and Google ill figure out the terms. Thank you for the explanation. I actually had to cut the plywood because of its size so it's 2 pieces 55x37.5. I knew I would need something under the plywood to support it from bowing, so 2x4 would be a better bet then the 1x4 (the slats are actually 2x6 I am at work so I had to check the receipt lol. The beds will be in the corner of the room so fastening them to the wall is fine. Yes most of the scrap wood will be used for stairs / drawers. Thanks again!
    – Zatellia
    Nov 11, 2023 at 20:04
  • @Zatellia, if your plywood is 48/24 rated, then it alone is more than adequate. It's 29.5 psf capacity is very high for a bed, where I believe that 20 psf is a good design target. I wrote the "strength axis" stuff more for others that find this question with a 38" twin mattress in mind. Your 55" width forced you to cut the plywood so that it's probably oriented correctly for strength and stiffness. There exists plywood with its strength axis aligned to the short side, but that's very unusual.
    – popham
    Nov 11, 2023 at 20:18

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