Weight limit of 2X6 attic floor?


I am nearing completion of my attic project. I recently put in an attic stairs and putting in 3/4" OSB plywood for the floor, replacing the old T&G planks I had up there. I live in a split level 50 year old house. The attic is 2X6's spaced 16"OC. It spans approx 20' X20'. Below are some walls forming a bedroom closet .Although I am not sure if this is a load bearing wall for the attic, I would think it does add some support. But realistically how much weight can be put up there? Is there some sort of guidline?
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Mikepier wrote:

Probably a lot more than you care to haul up those stairs.
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Ditto...As long as your just puting Christmas decorations , boxes of seasonal clothes ect. you should be fine...Just don't put a pallet of bricks up there...LOL...As usual use common sense...
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I saw pics of some attics that there were some 2X4's nailed from the roof rafters down to the ceiling joists I guess for extra support. Can I put one or two in for the long span above my bedroom?
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Mikepier wrote:

Sure, and that may help stop the roof from blowing off in a major hurricane.
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Not very much according to my reference manual. A 2x6 16OC is only rated for 30 live floor load to about 11'2". A 20 foot span 16" OC need at least a 2x10 for a 30 pound live load.
For a 20 lb live load, limited storage with SYP joists 16" OC is good to about 14.5 feet of span. This calculation includes the weight of a drywall ceiling under the joists.
The good news is your old wood most likely is SYP and better than anything you can buy today so I provided the ratings for Select Structural grade instead of the lesser grades. I suspect you can exceed the numbers a little with no real fears.
I would not fill it up and I would keep the heavy stuff to the sides. Sorry to be the bearer of bad news.
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Colbyt wrote:

Those load specs are likely based on acceptable deflection under load (L/360?) for comfort in a living space i.e. not too much bounce, not the limit that will cause structural failure. Cracks in the ceiling sheetrock are probably a greater risk than actual failure.
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I want to clarify, the entire attic is about 20X20, but the largest span is 15 1/2' in my master bedroom then it rests on a wall that creates the bathroom. all the other spans are 12' until hit rests on a wall below, like the closet, bedroom wall, etc I also heard it was good to screw down all the OSB to actually strengthen the floor. So far it seems solid. I am not looking on putting anything massive up there, just basic stuff.
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Very simple: put stuff up there until it crashes down. That's the limit. 3/4" plywood sounds a little heavy to me by itself.
Steve
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On Sun, 7 Feb 2010 13:38:48 -0800, "Steve B"

Then you put up less than that.

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On Sun, 7 Feb 2010 06:06:21 -0800 (PST), Mikepier

I would think that what makes a load-bearing wall is not what is in the attic. Because anything near the wall and in the attic will rest on the wall to some extent.
And I didn't think what determined a load-bearing wall was the construction of the wall, assuming it is made like a standard wall is made and not of some flimsy construction.
I thought a load-bearing wall was determined by what is beneath it. Is there another wall directly beneath the closet walls that hold them up. All the way down either to the foundation or to steel girder that spans the basement.
Am I right?

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>snip<

OSB and plywood aren't the same thing. Maximum strength will be achieved if you use 3/4" real plywood glued and screwed to the joists. Your call as to whether that might be necessary.
Joe
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I would be a bit surprised if the original 2x6 are 20' long. If they met at some mid point and were side lapped, this would have occurred over a wall, that wall was then a load bearing wall. That load bearing wall would have another wall or girder below it, also load bearing. I assume you do not have trusses if you were able to deck out large expanses. I don't know that you gained much trading out 3/4 T&G for 3/4 plywood, they would both be rated about the same.
The length of the joists is measured from bearing to bearing. If they truly span 20' they have no load bearing ability. #2 SYP joists carrying a ceiling are maxed out at just over 15'.
here is an easy calculator that will let you play with the numbers: http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species=Southern+Pine&size=2x6&grade=No.+2&member iling+Joists&deflectionlimit=L%2F360&spacing&wet=No&incised=No&liveload&snowload=-1&deadload&submitlculate+Maximum+Horizontal+Span
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numbers:http://www.awc.org/calculators/span/calc/timbercalcstyle.asp?species =...
Thanks for the link. How do I know what kind of wood I have? I assume Douglas Fir, but what is SYP you mentioned?
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Add to what Dan said:
SYP Southern yellow pine sometimes labeled SP. Still sold even at the BORG when the length of a 2x6 is over 12 feet. Almost surely used in a 50 year old house. Only moderately better rated. 50 years ago the grading standards were much higher than today.
Colbyt
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Southern Yellow Pine, the other structural lumber. They both have a similar modulus of elasticity. Lumber normally has a grading stamp on it. If you've already decked the thing, you won't be able to find one. You may see one on one of the rafters, the lumber all came out at once from the lumber yard and is probably the same grade and type.
I doubt that they would have used much better than #2 SYP. That calculator calls it southern pine.
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