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Old 05-03-2007, 02:58 PM   #1
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sistering joists


i have a full loft above my 20x24' garage. the 1/2" plywood floor is laid on 2x7x20' joists. the span is 16" oc. to what degree would sisterig with 2x12x20's help eliminate the deflection i now have?

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Old 05-03-2007, 03:34 PM   #2
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I'd like to know where you get 20' lengths from, no way that's what you have and even 20' 2x12s won't give you enough strength in the flex area to do anything with, you need to shorten the unsupported span, or you probably already have strips of wood in the middle of that span up to the roof rafters holding it all up, that's what they did in my garage, I have a 22' unsupported span, but they aren't one piece, they double up in the middle.

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Old 05-03-2007, 03:34 PM   #3
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It would make a tremendous difference. Adding 2x12 Southern Pine in #1 or select structural grade would be adaquete by themselves. Sistering to the 2x8 you now have (1.5x7.25) would be great. But the ends would need to be supported too. If it were me I would sister with the same size you now have making it a much easier job.

Double check the thickness of the plywood subfloor, hopefully its 5/8 and not 1/2". All Plywood measures 1/32" under the stated thickness. What do you plan to use the loft for? Oh, the span is 20 ft. not 16".

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Old 05-03-2007, 06:13 PM   #4
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Remember to use construction adhesive when you sister and some bolts and nuts wouldn't hurt either
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Old 05-05-2007, 06:57 AM   #5
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[quote=R&D Tile;43410]I'd like to know where you get 20' lengths from, quote]


why? I assume he means 2x8's but I can even get 2x6's in 20' lengths?
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