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Old 03-08-2009, 09:57 AM   #1
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Joist question - sistering


I have a home built in the 50's, originally as a one-story, flat-roofed convenience store. It's roof was built with 2x8's at 12" OC, spanning 15'. In the 80's, somebody added on a gabled attic (and I could strangle the idiot for not giving me more headroom for a decent build-out!) The original roof is now the floor of my attic, which I would like to finish out for more bedrooms. We have noticed the floor bounces a little, but not dangerously,and sometimes not even noticeably at all. Questions are 1) Is the original sizing, spacing, and span of the 2x8's acceptable for a small bedroom load 2)If not, will sistering 2x4's or 2x6's correct the inadequacy 3) The house is slab on grade, so if I install a beam on the first floor tocut down the span, and add a post column, do I need to rip out the slab at the column location and have a cyndrical footer poured below frost line, or can I just use a 12"x12" base plate to spread the load over the 4" thick slab?

We are cutting in a new interior stair to access this attic floor, so the post question pertains to that situation as well. Is it expensive to have somebody come in and do the footer?

Thanks in advance
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Old 03-08-2009, 10:47 AM   #2
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Joist question - sistering


yes you need footers under the posts. Cost should not be that much. No you cannot use sistered 2 X 4 or 2 X 6... you need 2x10 minimum for this span.
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Old 03-08-2009, 10:53 AM   #3
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Joist question - sistering


Sistering the joists with 2x8's will significantly stiffen the floor.

I'd also recommend adding 2x8 blocking between the joists, about every 6 or 8 feet. They'll help take the bounce out of the floor.

If you decide to break up the span with a beam (upset or underneath), you do need a footing. The 12" bearing plate idea won't work because a 4" slab isn't adequate for use as a footing. You'll need to saw cut out a square of your slab and dig it down for a pad footing. If the footing is contained within the house a frost-depth footing isn't necessary. The size of the pad footing will be determined by the imposed loads. Around here, most interior pad footings are at least 8" thick under the slab, and are typically 3'x3' with several pieces of rebar in them.
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Old 03-08-2009, 01:20 PM   #4
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Joist question - sistering


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Originally Posted by thekctermite View Post
Sistering the joists with 2x8's will significantly stiffen the floor.

I'd also recommend adding 2x8 blocking between the joists, about every 6 or 8 feet. They'll help take the bounce out of the floor.
Assuming a 2x8 sister joist, if I can't get to the original bearing points on the ends, can I cut it a few inches short, to allow for installation? The overiding important priority is providing more mass in the middle of the span, right, basically the lower edge of the joist. I would try to install these from above, and not demo the ceiling of the remodeled bedroom below. I assume one 2x8 sister per joist, nailed and glued like crazy? 2 1/2" ring-shank nails?

Thanks in advance
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Old 03-08-2009, 01:28 PM   #5
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Joist question - sistering


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Originally Posted by thekctermite View Post
You'll need to saw cut out a square of your slab and dig it down for a pad footing. If the footing is contained within the house a frost-depth footing isn't necessary.
How do you guard against moisture working its way up along the joints between the old slab and the new pour, or is that not a problem/concern?

Given the existing 2x8 and 12" OC, what span is acceptable, or at what point do I place the beam? I appreciate the feedback, and know this is stuff I can find in a table somewhere. If you have a link, I will look, though I would probably come back here and get some verification that I am reading things right. ;-)
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Old 03-09-2009, 08:11 AM   #6
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Joist question - sistering


I can't help you with the beam question, but I do have a couple of other tips. I recently sistered 2x8's in my bonus room area. To get the full length of board in the space, I removed a few sections of siding on a gable end, then cut a slot in the plywood so I could slide the boards in each bay long ways. I used Spax, self drilling 1/4 x 3 inch hex head wood
screws to connect the timbers. C clamped the boards together while screwing. It came together very nicely. Good luck!
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