Sister joint and size

bigsoldierJuly 27, 2006

We are thinking about sister two joints under first floor from basement. First of all, is this a do it yourself project? We believe we have basic capenter skill.

Also, our house is about 30 years old. The measures I got are 1.5x7. I search on the web and found all the joints are in size 2x8 or 2x10. Did I measure wrong or this is the size the builder should use?


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They are called "joists" not "joints." 2x8 lumber actually measures 1 1/2x 7 1/4 (2x8 is called it's nominal size) so it sounds like you have 2x8's. You don't say why you are sistering the joists. Is the floor sagging? It's not hard to sister joists, however if the floor is sagging you will have to jack up the existing joist before adding the sistered joist. This can get tricky and IMHO is not a DIY job unless you have the right tools and some experience doing it.

    Bookmark   July 27, 2006 at 4:41PM
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Thanks for the correction and comments.

Our new front load washer vibrates badly. The floor feels bouncy when it spins. We are thinking to enforce the floor by adding cement board and tiles on top of the sub floor (it's vinyl now). But these will add a lot more weight besides the weight of the washer and dryer (about 300 lbs). We are worrying about these are too heavy to the floor. Sistering joists underneath should add better support? Thanks!

    Bookmark   July 27, 2006 at 5:26PM
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Probably a good idea, expecially with the ceramic tiles. They crack easily if there is any bounce at all in the floor. I know that alot of homes have been build with 2x8 floor joists, and I think they all suffer from the bounciness you describe.

I would check for levelness of the floor to see how far off you are. If it's pretty close you could probably sister them as is and maybe build up the low spots a bit with mud when you install the cement board. They also have a commercial product available for levelling floors.

I'm not an expert or anything, but you might want to consider sandwiching some heavy duty construction adhesive between your old joists and the sistered ones. This should add to the stiffness of the assembly.

    Bookmark   July 28, 2006 at 12:56PM
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You should also add some blocking. Blocking is short pieces of 2x8 running between the joists (perpendicular to them). Believe it or not, blocking can add a lot of rigidity to the joists by tying them together so that a heavy load in one spot (your washer for instance) is spread over the other joists. The blocking should fit snugly and is atached by hanger brackets or nailing.

    Bookmark   July 28, 2006 at 5:55PM
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Check the appliance forum 1st, there are lots of threads there about front loader washers that vibrate. You may be better to try to level and balance the machine to doing all the work you propose! You might want to isolate it, too. Try a foam yoga matt over a sheet of plywood. Only need to be big enough to support the machine's feet, so you would not see it otherwise.

    Bookmark   August 2, 2006 at 2:30PM
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