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Old 07-24-2009, 09:54 AM   #1
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Connecting Sister Joists


I am new to this site and it is awesome. I am hoping someone can help with a few quick questions.

I am about 90% finished with a bathroom remodel (along with just about everything else in the house). I recently discovered that a few of the original floor boards in front of the tub were rotted due to water damage.
Unfortunately, the boards that were damaged run underneath the tub. The joists are not damaged or rotted.

Since the tub is new and all of the tile work has been completed, I really didn't want to remove the tub and tile work. The tub itself sits across two joists so it is supported well. The skirt of the tub extends just past the joist.

Here is what I have done so far to replace them.

On the one end of the damaged area, I removed the tiles, cement board (1/2 inch) and sub floor. I cut out the damaged boards so that approximately 1/2 inch of the existing joist is exposed.

The other end of the damaged area sits underneath of the tub. Since I didn't want to remove the tub, I cut the damaged boards from underneath so that what is left of them is now flush with the existing joist. Fortunately, there is no further damage to this side.

The boards that were damaged were 1x6 t&g. I replaced them with 1x6 no 2 pine. The approximate size of the damaged area is 15" x 21". I cut the new boards to fit the damaged area.

In order to have something to nail the boards to on the side that is under the tub, I sistered a second joist next to the existing joist. The existing joist is not damaged. The new joist is 2x8. Its span is 13'8". This is the same size and length as the existing joist. At one end, both of the joists rest on the existing beam that spans the length of the basement. On the other end the existing joist rests on the concrete block wall and continues into a pocket between the blocks. At this end the new joist also rests on the concrete block wall. I have everything in place, but have not nailed it together yet.

My questions are:

1. What is the best way to secure the new joist to the existing joist? I was planning on bolting and nailing or screwing them together.

2. What is the best type of bolts to use and how many should I use? I was planning on putting about every 4' with one in the top and bottom?

3. I may not be able to nail the one end of the new boards to the new joist since it is just inside the skirt for the tub. There is access from an adjoining closet, but I may to be able to have enough room to hammer. If this turns out to be the case, can I nail them from underneath or do I need some sort of "L" brace so that I can secure the boards from underneath.

4. In order to put in the new joist, I had to remove one of the X braces that was between the original joists, do I need to replace it with another X brace or can just use 2x4 or 2x6 blocking? Also, how much blocking should I use.

Thank you in advance and sorry for the long post.



Last edited by tdiantonio; 07-24-2009 at 09:57 AM.
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Old 07-24-2009, 01:41 PM   #2
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Nail with 16d nails 1" from the top and bottom edge 24" o.c. staggered. Three 16d nails at the ends. You can replace the bridging (X) with blocking.


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Old 07-24-2009, 02:27 PM   #3
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I nail them 16d at 16" on center, two rows- one top, one bottom. (Same as a built up header, per code). There is no code on sistering but I want them to be as strong as a header. Also, I keep the nails away from the edges 2" so not to be in the tension or compression area of the boards.(Top 1'--1-1/4" of edges) No floor joist can be notched in the middle 1/3 of it's span, for that reason. It may not be necessary with just nails, but twice when bolting rim/ledger joists to glue-lam beams, the Structural Engineer required 2" clear area - top and bottom. And one other time, lag bolting a rim and floor joists to a new carrying truss, same requirement. Maybe nit-picking, but why risk splitting. My two cents. Be safe, G
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Old 07-24-2009, 06:34 PM   #4
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Thank you both for the responses. I will use the 16d nails.

One correction on my original post regarding the following:

At this end the new joist also rests on the concrete block wall.

This is not 100% correct. I went back and doublechecked today and the new joist is resting on a framed 2x4 wall that is just in front of the concrete block. The new joist is approximately 2" from the concrete block.

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