Sistering joist questions

orourkeJuly 24, 2009

I am planning to sister a joist that has sagged over time and have the following questions:

(A) As shown in the diagram, the original (old) joist (and floor) are bowed by about ¾" in the center. So how can I fit the sister underneath, assuming the new sister is a straight 2x8 ?

One way, I thought, would be to notch the ends of the sister (where the sister will rest on the sill) to compensate for the bow in the original joist. If so, the sister will end up not resting completely on the sill (there will be a small gap between sill and sister as it will be impossible to get the notch depth just right).

Another alternative, I thought, would be to use a floor jack to put pressure on the center of the old joist until it crowns and then slip the sister underneath. But IÂm skeptical I can completely undo a ¾" 50 year old joist bow that way.

Is there an alternative?

(B) Given the fact that joists are 16" OC. seems to me, that it will be inevitable to cut the sister joist a little shorter than the old joist in order to get the sister into the joist bay. Now again the sister does not fully sit on the sill. Is there another way?

(C) I will attach the sister to the original with bolts or lag screws (one better that the other?). Should I also glue them together (thatÂs what I saw on a Bob Villa YouTube video)? If so, what should I use? Wood glue or some sort of construction adhesive such as "Liquid Nails" or "PL 400"?

Finally, am I making a big deal out of (A) and (B), fretting about the fact that the sister will not fully sit on sill and that it will be a little shorter than the old joist when, perhaps, the objective here is just to reinforce mainly the center of the original sagged joist?

NOTE: Joists are 2x8s on 12Â span. Sill is 6", foundation wall is 16".

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Trimming the sister shorter does not make much difference since sag is not a bearing area issue.

The problem with any type of jacking is that the old joist may have a set that will not go away very quickly.

If you want to try and straighten the old joist before sistering watch what is happening very carefully as you start to lift the middle.

Some joist will give enough, but many will not and the end(s) can actually be lifted off their bearing location.

Raising very gradually (1/8 inch per week type numbers) can give the old wood some time to move back into a straight line.

Glue is not normally needed, and in most cases is assigned a zero strength since application is so variable.
All you need is a slightly thicker glue line to render it useless, and correct glue line thickness is very thin (thousandths of an inch over the entire glue area).
Clamping is required to even get close to a correct glue line.

16d nails are more than adequate.
While bolts and lag screws may appear better if you use fewer of them tan you would nails you can easily exceed the compression limits of the wood.

There are nailing rules for new work that can serve as a starting point, but older lumber may have been ungraded or just over spanned, making a more frequent nailing schedule needed.

    Bookmark   July 24, 2009 at 9:47AM
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Thanks for the info on gluing. IÂll add some nails to the sister to be safe, nailing goes quickly.

I have vaguely read the "1/8 in lift per day" rule when jacking up houses to avoid cracking of wallboard/plasterÂ

Ummm I applied about 1200 lb lift to the middle of that joist and it only lifted ¼". Seems unlikely that IÂll be able to straighten this old joist. But IÂll wait a day, see if it relaxes some. Have to do other prep work in the meantime anyway. Would wetting the joist help? I think that was one method that was used to bend wood boards in shipbuilding.

    Bookmark   July 24, 2009 at 3:05PM
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Sistering is allowable wherein only one end is sitting on bearing. Even if neither end is on bearing. So you have nothing to worry about, and you can bear one end of the new joist on the ledge, and jack the other end into position.
It is only permissible when the existing joist is of sound constitution. If completely degraded by insect damage, for example, both ends must bear, because then it's replacement without removal, and not sistering.

    Bookmark   July 24, 2009 at 8:01PM
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