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Thread: How to calculate load capacity on construction...

  1. #1
    Join Date
    Oct 2007
    St Marys, West Virginia

    How to calculate load capacity on construction...

    Can someone share their knowledge with me on how to calculate load capacity on joist etc?

    My building has a loft. I have 2X6's for the joist, 2 feet on center, with 3/4" tongue in groove sheeting.

    I have wondered just how much load I can safely put up there without overdoing it.

    Since I am far from an engineer, I don't know where to look to try and figure that out.

    Any help would be greatly appreciated.
    One good turn deserves another

  2. #2
    Join Date
    Feb 2003
    Conway, Arkansas
    What do you wanna do on it? 2x6 joists are a minimum in standard home construction for stick-built trusses. Storage space? Should be fine. Set up shop? Then you'll need more support.
    Thanks & Happy Wood Chips,
    Dennis -
    Get the Benefits of Being an SMC Contributor..!
    ....DEBT is nothing more than yesterday's spending taken from tomorrow's income.

  3. #3
    Join Date
    Jun 2003
    Portsmouth, VA
    Rick, I use an online calculator for span limits:

    When I use this particular one I select Douglas Fir-South for framing lumber.

    To give you a general idea though, floor joists are sized to support 40 pounds per square foot of "live" load (moving weight) or 10 psf "dead" (stationary) load.

    When I framed in my loft over my shop I used 2x8's, 16 inch on center, with a max span of 11 feet. It's covered on top with 5/8" OSB glued and screwed in place and on bottom (shop ceiling) with 5/8" OSB just screwed in placed. It's rock solid and I have A LOT of weight up there.

    I used 16" OC because that's what the walls and roof are framed in and I just matched them up.

    Here's an odd bit of information though: typically, a 2x10 framed in at 24" OC is stronger than a 2x8 at 16" OC.

    Hope this helps.

    Be well,


  4. #4
    try southern yellow pine council.

    they have span and load tables. some of it gets pretty technical in nature, and mostly relates to pounds / square foot of loading.

    2x6s are generally used for ceiling joists, esp since you said 24" oc and reallllly short spans only (5 or 6 feet ). Most floor loads would be a minimum of 2x8 @ 16" oc and most preferbaly 2x10. There are lots of options, if your load bearin walls are sufficient, you could remove the deck, 'sister' some 2x8s at the proper spacing, and then redeck it. Check with a local (reputuable) contractor to let them think of some good options.
    Last edited by Lee Koepke; 01-31-2008 at 2:26 PM.

  5. #5

    New Floor ? ? ?

    I thought I'd add to the discussion here, since my day job allows me to add "PE" behind my name.

    The previous two replies are correct- your 2x6's are too light to support anything but a short span if that span has any kind of load on it. For either of the joist calculators referenced in the previous postings, some of the critical input are your selections of dead and live load, and you need to have both for a realistic estimate of your requirements.

    For a 2 x 8 joist and a double layer of 3/4" something (plywood/osb; a single layer will probably be too thin to support heavy equipment) you're looking at something on the order of 15 pounds per square foot (psf) minimum for joists at 16" centers. For your case, with 24" spacing, you're probably going to be at least 20 psf (possibly 30-35 psf), depending upon the type of wood in the joists and the type/thickness of sheeting that you use to cover the floor, if you sheetrock the ceiling below, etc.

    THEN, you need to consider live load. That's you, your workbench, tools, and wood stock/etc. that you're going to have stacked or otherwise situated on the shop floor. You don't want to skimp here, and it will definitely impact your selection of joists, spacing, and whether or not you really want to put your shop there. For example, live loads for residential living space are taken as 40 psf (per the current International Building Code), while "light manufacturing" is taken as 125 psf. Big range of values there. In any case, if you want to get an idea of where to go from here, don't go less than 40 psf live load and 20 psf dead load.

    Just a thought, and not to be construed as a completely professional opinion, as I have no idea of the size/spacing of the wall framing and what you'll be putting on the new floor.

    Frankly, I like the idea of contacting a reputable local contractor to see what he thinks.


  6. #6
    Join Date
    Oct 2007
    St Marys, West Virginia
    Great information, I appreciate it.

    I should clarify the spaces use. This area is storage only. My shop is a 11X20 building with gambrel roof. The roof rafters are 24" on center. I ran 2X6" rested on top of the walls and tied to the rafters. The free span is about 10 1/2 feet (11 ft minus two 3 1/2" walls equals 10' 5").

    I only put it up there to utilize the head space. I will store some lumber, but very little as it can get heavy. Mostly some various working peices. I will use the area for jig storage, extra hoses, and pretty much anything I want out of my way. No heavy tools.

    Placing wood up there is where I dont want to overdo it. So I thought if I could figure out the safe load I will always stay under that.

    I'll look at the links posted and see what I can calculate.

    For what its worth the shop flooring is 2X8" and 16" centers and the walls are all 16" center. I dont usually do 24" center, but felt with the roof design there wasnt a need to beef it any more than that. Now that I am in the shop I think I would have liked to went ahead and built the loft heavy duty, but its still better than no storage.
    One good turn deserves another

  7. #7
    Join Date
    Oct 2007
    Pacific Northwest
    Something else not yet mentioned - those values for dead load INCLUDE the weight of the actual framing, which can add up quickly. I'm on the wrong computer (other one is temporarily dead) or I would post a partial listing of different framing choices and their PSF weights, as well as weights of different sheathings, etc - or you could go here

    forgot I'd posted it to my studio build forum... Steve

  8. #8
    Join Date
    Apr 2006
    Near saw dust
    If you are worried, just add another in each bay.

    Knowing the load will not really help as estimating the weight of your stuff is going to be hard to do. I would put a string (this is nuts but I think I would do it if I were in your shoes annd I was really concerned- and I have an engineering degree) across the top plates and stretch it tight between 2 nails along side the middle joist. This will allow you to monitor deflection as you load it and over time. I think anything more than about 3/8" in a shed/shop would be bad and should be unloaded/
    Strive for perfection...Settle for completion

  9. #9
    Join Date
    Dec 2006
    Auburn, ME
    I concur with Brian on this one. If you are that concerned I would call in a contractor or an engineer to help you out. There are too many variables to just give a size with a load rating. Another resource might be your local building inspector. It could be hit or miss but could be helpful. One very overlooked piece is all the connections. (BTW that is why the bridge in the twin cities collapsed..the connections failed)

    Another question for you, can you really look at 'stuff' and determine how much it weighs? Are you going to have a scale to weight everything? Will it all be a uniform loading over the whole joists or maybe just point loads in the middle?

    If it were me I might use it for very light storage, boxes, small jigs etc. and call it a day. Get someone to go over to your place and take a look at all the framing to determine what you could use it for.

    good luck,

  10. #10
    Join Date
    Oct 2007
    Pacific Northwest
    Rick, with those spacings your loft won't even support 30 PSF live/10 dead load - I'd get uncomfortable putting more than empty cardboard boxes up there, no joke. Even doubling those joists won't quite get you 40 PSF, which is NOTHING when trying to store lumber.

    Seriously, I would forget using that space unless you have a large quantity of ping-pong balls you wanna save... Steve

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