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harrymanimus
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# Posted: 10 Jan 2019 18:37
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I have a loft that is 12x12 and held up by 2x6 beam boards spaced 2 feet apart. I purposely compromised with a bit less head room under the loft to get more head room in the loft area. I'd like to cut the bottom 2 inches off of the 2x6 boards to give more headroom below. So I was thinking if I do that I should reinforce the modified boards. I'd sister some 2x4 boards in. Should I sister every board or every other board? Should I glue and bolt? I suppose I should use 12 foot 2x4 boards.
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fiftyfifty
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# Posted: 10 Jan 2019 19:52
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What is your height under the loft?
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rockies
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# Posted: 10 Jan 2019 20:04 - Edited by: rockies
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The strength comes from the depth of the beam. It's not just the live load from you walking around in the loft that those beams are supporting but also the dead load of the building's roof structure.
Don't do it.
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ICC
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# Posted: 10 Jan 2019 20:09 - Edited by: ICC
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What is the loft used for?
FYI, 20PSF is the figure used for an uninhabited loft according to code. A sleeping area loft uses a 30 PSF figure to follow code.
Plus do you know the species and grade of lumber that has been used for the existing joists?
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ICC
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# Posted: 10 Jan 2019 20:22 - Edited by: ICC
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Using SpanCalc those 2x6 are undersized as it is,using 30 PSF, #2 grade S-P-F lumber. Cutting the joists down would not be a wise move.
Even using a lower deflection allowance (L/240 instead of L/360, or L/180 which would be very springy) ) than what is normally used for a floor AND having 2x4 on 12" centers the 2x4 would be inadequate.
Quoting: rockies It's not just the live load from you walking around in the loft that those beams are supporting but also the dead load of the building's roof structure.
The loft joists are not supporting the dead load of the roof, only the load placed on the loft floor. Those joists do also restrain the walls from pushing outwards from roof forces on the rafters above...
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harrymanimus
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# Posted: 10 Jan 2019 23:58
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The loft would be used for sleeping but only if not enough room down stairs. The loft seems super sturdy to me, so I'm scratching my head about it already not being adequate. The joists for sure do not hold the roof up.
The osb type stuff nailed to the top is pretty thick with the water impermeable resin.
I don't think I saw anyone specifically mention the pros/cons of sistering boards. What if I cut it down to a 2x5 and sistered a 2x4 in? What if I used a different kind of wood for sistering, or some other material? What if I added 2 sister boards?
I don't have a great pic of the loft but here is something.
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KinAlberta
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# Posted: 11 Jan 2019 01:23 - Edited by: KinAlberta
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Add flitch plates maybe???
On depth, subject to someone with expertise or depth of knowledge correcting me, I think every 2†of depth adds almost 50% to beam strength. So take 2†away and you’ll loose a lot of strength.
It’s a great question. If one can’t add depth, how much width does one need to add.
https://www.quora.com/I-saw-a-beam-which-is-more-in-width-than-depth-It-is-safe-in-de sign-also-Is-it-allowable
Also, the 2’ centres for a loft floor seems rather under built. Build it and they will come - and test its limits - by jumping up and down and piling heavy loads in it, etc.
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Nate R
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# Posted: 11 Jan 2019 08:00
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My 10' shed with a loft for storage (Loft is 6'4 under joists, I think) uses 2x4s on 2' spacing....It's VERY springy. I would NOT go 12' with that. Even doubling up as others said may not be enough.
Do you NEED more headroom? Why? Not sure an inch or 2 will make it FEEL much better.
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ICC
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# Posted: 11 Jan 2019 10:16
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Quoting: KinAlberta Add flitch plates maybe???
that is probably the only good way but is the expense and bother worth it for only a couple inches.
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Tonerboy
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# Posted: 11 Jan 2019 10:19
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Not to be sarcastic, but why would you go to all the work to modify the current 2x6, which is a little underrated in the first place to gain just 2". If the loft is not used that much I'd put my effort into raising the 2x6 since they don't appear to be resting on the headers.
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KinAlberta
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# Posted: 11 Jan 2019 10:31 - Edited by: KinAlberta
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Quoting: Tonerboy they don't appear to be resting on the headers.
I see that now. If it were my problem, I’d unscrew the decking and pull the nails on the 2x6s and then start fresh. I suspect that it would go very fast. (Years ago I bought our house and it came with an large older poorly placed deck of 2x4 cedar and 2x6 fir framing all put together with drywall screws. Taking it all apart went amazingly fast. And I was able to save and reuse thousands of dollars worth of good wood.)
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harrymanimus
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# Posted: 11 Jan 2019 11:15
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Maybe it's not worth the trouble.
There are 2 lofts at different heights. I did the tall side at 6'11" from floor to loft beam bottom, and 6'4" on the other side. The 6'11" side is for hanging out, living area. The 6'4" side is for sleeping/bathroom. Between the beams is another 6 inches up, so doesn't feel very cramped, there is room to put lighting out of the way. I did the short side to keep the loft more usable as a sleeping area without having to bend over so much. We don't have anyone super tall so 6'4" has not been a problem. But when I decided on that height I thought I could recover at least an inch if I wanted going to 2x5 without much work. Later, that got me to thinking about going to 2x4 size.
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Fanman
Member
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# Posted: 11 Jan 2019 12:48
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Beam strength is proportional to depth cubed. It would take four 2x4s sistered together to have the same strength of one 2x6.
(3.5/5.5)^3 = 0.26
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Just
Member
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# Posted: 11 Jan 2019 13:41
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lf you could shorten the span some how with a beam the 2x4s may work i build them with a 6 ft. span on 16 centers No 2 spruce.
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harrymanimus
Member
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# Posted: 11 Jan 2019 18:55
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I'm wondering if anyone is taking into account how a load spreads out with more surface area support.
If I stand on a 2x4 all the force is against that 2x4 at a smallish point. If I lay down on a 2x4 my weight is spread out. If I stand 1/4 plywood on top of 2x4s above a 2x4, that doesn't do much to spread the load to other 2x4 boards because 1/4 inch plywood is pretty thin. 1" resin reinforced osb should spread the load to the other supports quite a bit better. Going wider by sistering seems like it would also help. To me seems like there are more variables here that need to be considered.
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Fanman
Member
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# Posted: 11 Jan 2019 19:14
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Quoting: harrymanimus I'm wondering if anyone is taking into account how a load spreads out with more surface area support. If I stand on a 2x4 all the force is against that 2x4 at a smallish point. If I lay down on a 2x4 my weight is spread out. If I stand 1/4 plywood on top of 2x4s above a 2x4, that doesn't do much to spread the load to other 2x4 boards because 1/4 inch plywood is pretty thin. 1" resin reinforced osb should spread the load to the other supports...
The standard span tables already take that into account, by using live and dead loads expressed in pounds per square foot.
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rockies
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# Posted: 11 Jan 2019 19:16 - Edited by: rockies
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Why don't you take a 12' long 2x4 (which is in fact only 3 1'2" wide) and lay it on edge with the ends resting on two boxes and stand on it in the middle? Then you could try two 2x4's on edge spaced 24" apart and put a sheet of plywood on top and test it again, standing and lying down.
Nothing like a test run to see if it bends too much.
ICC: when I said that the joists were also helping to support the dead load from the roof I meant to say that they were helping to prevent the side walls from bowing out, as you stated more clearly.
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ICC
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# Posted: 11 Jan 2019 19:38 - Edited by: ICC
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As fanman has pointed out already the span tables that we have used for decades as well as the SpanCalc I linked to above already use a fully distributed load. The load amount is spread 100% evenly across the entire area, something that does not happen in the real world once people start placing furniture, etc.and using the space. The tables and the AWC calc also take into account that when joists are placed is groups of three or more as in the usual floor or ceiling, there is a gain in strength of about 15%. That is to say a single joist with a load of "x" placed on it will deflect, or sag, 15% more than the group of joists we find in a normal floor. Engineers are a careful bunch and think of things most mortals never do.
Thicker floor sheathing does increase the stiffness. a 1" thickness will stiffen more than 7/16". But at the same time, the thicker flooring weighs more and will increase the total load which will add to the deflection.
Of course, if you persist you might find someone who will agree with you. That won't make what you propose any more of a good idea then as it is now.
Quoting: rockies ICC: when I said that the joists were also helping to support the dead load from the roof I meant to say that they were helping to prevent the side walls from bowing out, as you stated more clearly.
Just trying to keep things clear for any who read this.
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