My first post, so here I go –
I’ve searched through a lot of the old posts, but couldn’t quite find what I needed. I have a 1920’s colonial revival with a very bouncy living room floor. The joists are accessable from the basement and are 2″x 8″, with a span of 14′ I would like to sister joists to them and have heard/read different ideas.
1) Can I use 2″x6″ and keep the bottoms in line? This would help me get bearing on both ends, as I think it will be difficult to get full 8″ joists into the sill space.
2) Should I use 2″x8″ and squeeze them in? Do I need full bearing on each end? I’ve read that for deflection, bearing is not necessary?
3) Or do I use 2″x10″, again, with or without full bearing on each end? I have water lines about 18″ in from the sill end attached to the bottom of the existing 2’X8″ joists. Can I cut out the bottom in that 18″ section? In this way, at least I have 10″ through most of the joist.
The idea of a midspan girder is out, I’m planning on finishing my basement once this is done. Attachement is another question, I have a palm nailer and plan on nailing, but I’ve seen a lot of posts about lag bolts or carrage bolts. So again – how many, how often, what size?
Thanks for the help
David
Replies
Well, first off, anything you do will help. Even if all you do is sister a 2x4 alongside. The question is, how much help do these joists need? Certainly 2x10s would be stiffer than 2x8, which would be stiffer than 2x6, etc.
Do you know about flitch plate beams? Even a thin pice of metal, sandwiched between two 2x's so that it can't "roll" mid span will add tremendous strength.
You can also gain strength by adding joists BETWEEN existing joists, in effect reducing the "on center" spacing
But a few things jumped out at me when I read your questions, so I'll address those -
"...I think it will be difficult to get full 8" joists into the sill space."
Rip a slight bevel along the top edge of the joists you'll be adding, that will allow you to roll them in much easier without resorting to downsizing to 2x6.
"Do I need full bearing on each end? I've read that for deflection, bearing is not necessary?"
No. You don't "NEED" it. But it's better - that is to say, you'd gain more strength that way.
"I have water lines about 18" in from the sill end attached to the bottom of the existing 2'X8" joists. Can I cut out the bottom in that 18" section?"
Sure, you could do that. But is there some reason you can't cut the water lines, insert full sized joists, then renovate your water lines as needed?
Just remember, there's more than one way to skin a cat. Keep the end goal in sight. Don't get caught up in what's "easier" or "faster". Those are important considerations, yes, but not the most important. Think first about what's best going to provide the results you're after.
But just by reading your questions I can see you have a pretty good handle on this job. Whatever you do will help.
Romania wasn't built in a day.
jim i don't disagree with you, i too think that even adding 2 x 4's will help, and you may well know more about this than i, i am just concerned that even though it "helps" it is still a bouncy floor with spans of 14'. it is a very subjective thing as to whether its just a little better, or much improved.
i like to try not to span over 10' with 2 x 8
i would opt for 2 x 10, and i would put at least one run of solid blocking down the middle of the span as well, and a lot of how effective this will be has to do with the amount of detail and close tolerances in construction.
i would use construction adhesive and nails to sister in the joists, and i would use a framing nailer not a palm nailer, and i would tighten them all up by hand, the swinging of the hammer works the best to get everything nice and tight.
i would also try not to notch for the water line, and try to move it out of the way temporarily, and then back up to bottom after sistering.
i would also try to set up a system of jacks or braces to hold the sister up tight while you attach, and then attach a new ledger underneath the bottoms of the 2x 10's mounted into the sidewalls so that the full width of the new 2 x 10's have bearing.
i wouldn't completely eliminate the idea of a midspan girder either, i understand you want to finish basement and don't want to lose headroom but, if you brace up the joists either side of center with temp walls, and cut out carefully the 3 - 1/2 inches needed for a glulam beam, and then slip a beam in and reattach joists to the beam with joist hangers, you would keep your headroom by installing the girder flush with the existing joists except for a couple of bearing posts. just another thing to think about. it may probably need to be engineered, maybe steel.
even with the 2 x 10's you may be doing a lot of work for not much improvement. however i am not an engineer, just a carpenter/ construction type. i think you would be wise to get an engineer to look at it, but i also think its smart to come up with some suggestions for the engineer. i would also take some photos and the dimension perpendicular to you 14' run.
Welcome to BT!
The 2x10 would give a stiffer floor, but there is no reason additional 2x8's wouldn't work.
When we stiffen up an old floor such as yours we typically add a new joist to each side of the old. Often by the time a new joist is jacked into place it's having to fight both the weight of the floor and the bend in the old joist so two new joists are definitely not overkill.
Getting them in will be the hard part, but it's quite doable. Keeping at least 1-1/2" of bearing on each end is ideal and simplifies life.
Cut any nails that stick through the floor in the area where the new joist is going in and at least a few inches to the side. A dremel tool with cutoff wheel will work although a small grinder with cutoff wheel is probably the fastest option.
Measure the length and height for the new joist and try to anticipate the maximum length you'll be able to use. Old floor joists may be any depth or even a combination of sizes, shimmed to various degrees.
I like to run a string line along the bottom edges of the existing joists to get a feel for how hard it's going to be to get the new joist in. It helps a great deal to support the old joists with some verticle 2x4s wedged in to help bring the floor up where it should be. Walls built over floors may make it impossible to straighten everything out.
Sometimes it is necessary to taper the top of the joist on one end to be able to get it tipped up into place. I like to wax the bearing surfaces on top and bottom of the new joist so it slides easier.
Ok, now the joist is stuffed alongside the old joist on one end and the other end is just short of the opposite sill plate. To hammer or pry the joist in to place is quite challenging if possible at all, depending on how tight the fit and how far the floor has sagged.
A much more effective way to move the joist is to use a come-along. Rock climbing stores will have bolt rings (below) for a couple bucks each that greatly simplify using the come-along. Use a hardened structural screw, such as a 1/4" simpson screw found at HD or Lowes, to attach one ring on the new joist and the other end to the old. It doesn't matter where along the joist the come-along is placed. The rings will accept up to a 3/8" bolt so it's also acceptable to through bolt the rings.
Be careful when winching the joist into it's final spot since the comealong will be putting a great deal of force on the new joist. Keep your fingers out of harms way.
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It's best to use PL10 construction adhesive to bond the new to old, but it's so messy if you aren't able to get the joist in that it's probably best to do a few without the construction adhesive and simply nail or screw securely.
If the new joist isn't tight up against the old use big C-clamps, wedges, or whatever it takes to get it to move over. In the worst case it might be necessary to use a small hydrolic jack to force it over.
Good luck
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