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Old 10-03-2012, 11:01 PM   #16
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so, i used 2x6's ....


The combination of a 2x6 (actual dimensions 1.5x5.5 inches) and a 2x4 (actual dimensions 1.5x3.5 inches) is approximately equivalent to a board 1.5 inches wide by 6 inches deep. Of course this is not a standard size board, but that is the approximate equivalent. This assumes that the 2x4 is adequately fastened to the 2x6 so the two boards act like a composite board.

Using an allowable maximum bending stress of 1200 psi, and a modulus of elasticity of 1.2 million psi, your combination can span not more than 11.5 feet assuming 30 psf live load and 10 psf dead load, based on the joists being spaced at 16 inches OC. Your actual load sounds like it is less, however those are the code required loading for your space based on your description, assuming you fall under the IRC.

Of course your lumber may be superior grade, and could possibly have greater allowable bending stress than 1200 psi, but that is a pretty typical number. Based on your dimensions, I would be very careful about loading the floor with anything heavy, however as you seem to be limiting the load to insulation, you should be OK. Of course, the building inspector might have a very different opinion, since your project does not seem to meet International Building Code, however it is possible that your town operates under a different code, or perhaps there is no building code where you live.

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Old 10-03-2012, 11:20 PM   #17
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so, i used 2x6's ....


Quote:
Originally Posted by GBR in WA View Post

What R-value on the cellulose?

Gary

i have to get to bed. but i will answer this one for now.

well, idk yet. 40-50 i guess. it depends on how deep it will be. and the cost, (though i don't think this will be an issue). and how much i should have. i will be covering an approx 25x25' area.
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Old 10-04-2012, 09:48 AM   #18
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so, i used 2x6's ....


Using Daniel's figures for 'I' and 'E', and assuming live load 20psf, dead load 10 psf, maximum deflection at centre is just over 1" (probably slightly less due to the outward push of the rafters, assuming they are well-fixed to the ceiling ties).
This = span/168. Maybe his code for ceilings would be span/180 or thereabouts, but not significant. When all's said and done, it's only a garage ceiling; don't know what all the fuss is about.
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Old 10-04-2012, 01:16 PM   #19
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so, i used 2x6's ....


Waiting on the ink-stamp picture.

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Old 10-04-2012, 09:02 PM   #20
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so, i used 2x6's ....


pic isn't working. here are the markings :
16' 8rd Trk = 2291062 A 9
220 2810 08/07/12 11:49:58
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Old 10-04-2012, 09:07 PM   #21
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so, i used 2x6's ....


oh. i nailed the 2x4 to the 2x6 the whole length about every 16".
and the ends are nailed as well as i could get in there to do so. they didn't move when i push/pulled on em.
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Old 10-04-2012, 10:10 PM   #22
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so, i used 2x6's ....


those notched 2x6 added strength to the 2x4 ceiling joists that were up there alone. If you are making an access or something fancy up there then something else will need to be built, What was on the ceiling before, plaster? consider a light weight drywall...

You might even add stiffness to the structure by blocking between the joists along the top of the wall.

Last edited by hand drive; 10-04-2012 at 10:25 PM.
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Old 10-04-2012, 10:26 PM   #23
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so, i used 2x6's ....


there was P&L up there before, with only the 2x4's holding it. P&L is heavier than 1/2" light weight drywall, that is on there now. so removing the heavy P&L and old FG insulation, adding 2x6's and lightweight drywall = sounds pretty good to me. and i have been up there, on it. it is pretty solid. perfect ? no. but way better than it was.

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