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The boards, 2x12's will be used as floor joists in a loft, they will be 16" on center. the entire area is 35' x 6'.

I need to support a lot of weight, about 5000 pounds which will be distributed evenly on this platform/floor.

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Most floors are usually made to ~40 pounds per square foot.

Floor joists for this are usually size so an inch of depth to a foot of free span, 2x6 for 6 foot span.

You have a bit over 24 pounds per square foot, if spread out evenly.

This is just a rule of thumb/napkin math, for a building should have engineered plans.

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    I think your last statement is key here. This is a very unusual load for a (assuming) residential floor. The OP should hire a licensed engineer or architect to do the design and analysis so that it's safe.
    – jwh20
    Mar 23, 2023 at 15:22
  • @jwh20 A lot more information will be needed about the walls/floor covering/foundation supporting the joists to know if it will hold up or fall down. The answer was just for the joists themselves.
    – crip659
    Mar 23, 2023 at 16:20
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Ignoring, since your question does, what holds up the joists?

2x8 16" o.c. will do 100 PSF live load and 20 PSF dead load over more than 6 feet (more than 8 for everything but #2 SYP) per this calculator (no affiliation): https://www.anthonyforest.com/ewp/powerful-solutions-span-calculator.shtml

2x6 will actually do the 6 feet under those conditions in all species listed.

That's 4200 lbs of "dead load" (floor structure, etc) and 21,000 lbs of "live load" (things that are not permanent.)

So 2x12 is a nice waste of money, or an indication that perhaps you should spend some of your budget holding the joists up.

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  • 6/7 foot 2x12s taking up space, this is a decent use. Having to cut long 2x12s or buy them not so much.
    – crip659
    Mar 23, 2023 at 16:27

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