i have seen questions about sistering joists here before, but would like to ask more specific questions to clarify the best methodology.
the sistering that i have done has been in locations where i have needed as much strength as possible in a small depth, for example adding to existing 2X6 ceiling joists or 2X8 floor joists. it is extra work, but since i am not doing a lot of this, i have ripped LVL material to the correct depth, belted one face to remove the slick outer surface, and glued and nailed (or screwed if there wasn’t room to swing) these to the faces of the existing lumber using subfloor adhesive, thinking that this created a strong and solid union. nailing has been done in a staggered pattern, top and bottom edges except at the ends, where it is nailed both top and bottom.
recently i have heard comments suggesting not to do this, only to nail the pieces together. what is the appropriate thing to do, and is gluing them together wrong, or just not advised, and why?
Also, if the existing joist is not plumb, what is the correct thing to do. if the sistered joists are kept in intimate contact with the existing joist, then they will all be off plumb, and not sit on the plate or in a hanger correctly and may need to be shimmed to maintain correct ceiling or floor height. if they are shimmed plumb to the existing joist, then they will not be in as intimate of contact with the existing joist.
any comments would be greatly appreciated.
nick
Replies
I don't see any advantage or disadvantage in gluing sistered joists together. Your fastening pattern should suffice to hold the two together as one. Just a thought here, but is there any advantage whatsoever in sistering joist together? The added joist material will function as well independently on its own, as it will if fastened to another joist, I think. I may be wrong about that, though, and someone with a better understanding of timber design may state that the two together function better than independently. Perhaps the area where you want to add glue is to the top edge of the joist, to make a more positive connection to the sub-floor above. There ought to be some advantage in that, as the two components work together.
If the existing joist are not plumb, it is worth your time to make a real effort to plumb them. Obviously, a joist will offer the most stiffness when loaded perpundicular to its grain. Your joist that are not plumb are actually robbing you of much stiffness as they are deflecting the load into lateral movement, rather than absorbing it in internal shear forces. Another reason to plumb them is that their bearing points need to take advantage of the full width of the board to spread the loads evenly through the member. What I'm saying is, you ought to go in there and put a real effort into straightening them, through the use of blocking, and working the joist at the bearing ends. Do the best you can. Use some strong language, if needed. It's sad to think someone would install a bunch of joist that aren't plumb...stupid stupid stupid.
intimate contact....I like the sound of that....
later
first off, thanks for the comments. in this most recent application, the joist that i am sistering to, is carrying the load for a hearth extension. so my need to join the two joists together, is to pick up some of the load that will be on this framing when i pour the hearth. but you raise a point that i can't really answer. i just assumed that two joists nailed together would carry the load better than if independant. perhaps someone else will comment on this point.
as far as using blocking goes, what i have no experience with, is how much movement can be coerced out of old lumber? the existing joists are over 60 years old, so they are definitely dry, and i am concerned about torsioning any one of them too much. for example, one of these 2X8 joists is out of plumb 1/2" in the 71/4" depth. that is more than i would like, but how much could i expect to change that by blocking in a span of 9'? by half? that would certainly be better than adding to it the way it is now.
and by the way, that comment about the "intimate contact" made me laugh....i guess it sounds a lot less like a framing question on the re-read than i intended.
nick
FWIW, the way I was taught was the new "sister" could be essentially independent if it spanes the width of the original span; i.e., is supported on both ends.
If it is to repair decay - rot, WDI, etc, it needs to extend past the damaged area at least 3 feet and needs the nails, screws & glue.
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"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
Nailing (or better yet screwing) together would keep the new and old joist from moving vertically at different rates which might create floor squeeks from subfloor nails in the old joist.
Personally, I wouldn't worry much about plumbing the old joists but do try to install the new sisters plumb, even if that means no "intimate contact". If the sisters are able to acheive decent bearing surfaces on the ends they will gain very little from being fastened to the existing joists. Normally I would nail them as you have done but the gluing as you have done it, is more extra work than it is worth. Blocking that is wedged in at an angle and then hammered toward a ninety degree orientation can remove the worst of the twists and cants of the old joists, but put in the new sisters first so that your blocking doesn't get in their way. The strengths here are mostly calculated in an additive way and are affected only slightly by attachment to the existing joists, UNLESS; you cannot seat the new sisters on the existing bearing surfaces. In which case it is essential to attach to the old joists as you are really just stiffening their centers rather than adding new additional bearing joists.