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Old 02-16-2008, 07:20 PM   #1
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Sistering a floor joist?


Here is the deal; I have to finish my basement to get my wife off my back! As part of this project I want to get the bounce out of the above 2x8x12 floor joist by sistering 2x8x10s to them.

What do you think about a 10 footer sistered to a 12 foot joist. Will this work or make it worse since it is not the same lenth and not sitting on the plate and beam. It is not possible to use a 12 foot joist because pipes are in the way. Thank you for your advice.

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Old 02-17-2008, 09:18 AM   #2
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Sistering a floor joist?


As you seem aware of, the correct way to get maximum benefit from sistering the joist is to go from plate to plate. Sistering them as you suggest will most likely help to remove some of the bounce in mid span as a real world situation, but may be a pretty hard sell to the building inspector. Even if not inspected, and this is seen by a home inspector at some time in the future if you sell the home, it will be made into an issue.

I am assumming that you have pipes that are bored through the existing joists, near the ends, if you can get a 10 footer in place. What I have done a few times similar to this is get the heaviest STRUCTURAL gauge metal studs available ((look in phone book for large drywall suppliers who deal with commercial work). Measure the clearance between the top of the offending pipe and the bottom of the sub floor above to see what is the deepest stud that will fit between them, and get enough 12 footers to put one on each side of the existing floor joist. Slip them in place above the pipe, inserting the end above the pipe first, apply a liberal bead of sub floor adhesive to the top flange and side of the metal jamb it tightly to the sub floor and hold it in place with a few self drilling metal screws available from the supplier (short enough so that they do not go completely through the wood floor joist). Do the same on the opposite side of the joist, then drill through the assembly, and install some through bolts every 2 feet or so, at random heights so that they are not all in a straight row. Now cut a squash block to fit between the bottom of the new metal and the plate or beam at the ends and fasten it in place. This will give you plenty extra stiffness at a reasonable cost, as the metal is not that expensive.

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Last edited by troubleseeker; 02-17-2008 at 09:24 AM.
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Old 02-17-2008, 10:02 AM   #3
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Sistering a floor joist?


There are pipes that are attached to the bottom of the floor joist that prevent attaching an equal 2x8x12 to the joist.

What about sistering and scarfing a joist to make it a 2x8x12. The last thing I whant though is a home inspector having a problem with it when I sell the house when house values go up again.
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Old 02-17-2008, 10:38 AM   #4
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Sistering a floor joist?


Boy, a picture would be worth 2000 words. I'm trying to guess the locations of the pipes; so they must run parallel to two walls in that they won't allow you to slip one end of a 12' piece in first?

A properly tapered scarf joint( 6 to 1 length to depth ration) ,glued with a structural glue like resorcinol or epoxy is structurally sound. Couple this with a generous application of adhesive between the laminated joists, and fasten with screws or nails, alternating the ends with the splices on existing joists, and you should not have any problems.
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Old 02-17-2008, 08:16 PM   #5
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Sistering a floor joist?


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Originally Posted by troubleseeker View Post
A properly tapered scarf joint( 6 to 1 length to depth ration) ,glued with a structural glue like resorcinol or epoxy is structurally sound. Couple this with a generous application of adhesive between the laminated joists, and fasten with screws or nails, alternating the ends with the splices on existing joists, and you should not have any problems.
I would also add some 3/4"+ plywood (3/4"x8x~32") gussets over the joint. and glue and screw them off.

Last edited by LakeTahoeDan; 02-17-2008 at 08:18 PM.
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Old 02-18-2008, 07:38 PM   #6
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Sistering a floor joist?


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I would also add some 3/4"+ plywood (3/4"x8x~32") gussets over the joint. and glue and screw them off.
Certainly woul not hurt anything, but I must have missed the part where he said he would be training elephants above
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Old 02-18-2008, 07:41 PM   #7
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Certainly woul not hurt anything, but I must have missed the part where he said he would be training elephants above
bathtub above ?= elephant

even if it is just a shower you do not want any flex or grout lines etc will be compromised.

comprimised grout lines= water damage=mold
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Old 02-19-2008, 10:34 PM   #8
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Sistering a floor joist?


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bathtub above ?= elephant

even if it is just a shower you do not want any flex or grout lines etc will be compromised.

comprimised grout lines= water damage=mold
Not discouraging your method. Like I said "Can't hurt". Just trying to throw a little humor in.
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Old 02-19-2008, 10:35 PM   #9
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Not discouraging your method. Like I said "Can't hurt". Just trying to throw a little humor in.

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