joist sistering question

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To beef up some 2x8 floor joists, I'd like to at 3/4" plywood to both sides.
Glue and screw or Glue and nail?
It'd be easiest to use the pneumatic nailer but would screwing be stronger?
Also, what spacing for either screws or nails?
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Franz Fripplfrappl
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Why dont' you just put in another 2x8 instead of some ply? less screwing / nailing.
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On Tue, 22 Jul 2008 05:46:03 -0700, Zephyr wrote:

good 2x8's are difficult to find
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franz fripplfrappl wrote:

???
They are available at the box stores. They don't have to be GOOD, just strong.
Having said that, I lather on a shit-load of glue, then oodles of lag bolts to hold the boards, whether 2x8s or plywood, together. Maybe strapping.
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Go to a real lumber yard and get some douglas fir if you want strong, you're right to stay away from the "prime" grade "whitewood" at HD, that stuff is soft, does not have a long grain, sure it looks perfect and straight but it's better for walls than joists. Some fir, even if slightly twisted makes a better joist than "whitewood", main thing is that it be straight along the small sides because the twist will be flattened out after you bolt it, nail it, glue it, etc anyway. Lay the small edge on concrete if it rocks or gaps too much, then pick a different board, but a slight crown is not necessarily bad either.
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RickH wrote:

I don't know what "whitewood" is but my local Home Depot has structural grade Douglas fir, same as the local "real lumber yard".

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On Tue, 22 Jul 2008 08:37:22 -0700, RickH wrote:

Good tips. Thanks.
I've seen some very bad boards at the big box stores: warped, checked, split, open knotholes, etc.
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Pull-out is not an issue for this application. Plus, the glue will do a great job of keeping the ply against the joist. Spare yourself the wasted time. Glue, Clamp, Nail. That's what I've done to 'cap' trusses in 100yr old house and garage. Plan to do the same with the roof joists to remove some sagging.
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Bart wrote:

FWIW with an impact driver Spax screws go in just as fast as nails.
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On Tue, 22 Jul 2008 15:02:39 -0400, J. Clarke wrote:

And, Spax screws are the finest I've used.
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Why either? I put bolts through, so as to enable tightening equally from either side.
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jack the centers up a little to crown the old joist before you sister anything, and the job will come out better and stronger. I'd just go with another 2x8 that way you are not limited to the 8 foot span of plywood.
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I'm about to do this myself for the master bedroom on the second floor. I'm using glue and screw. I too have a pneumatic nailer but disappointed with the poor pullout force even with 3.5" nails. I think 12" on center with two staggered rows would be plenty with glue.
Nails have better shear but screws better pullout. How about glue and staples? (My house use 2" staples on the shear walls for earthquake resistance. Pullout better than the shinny nails. Its about 2" on center, no glue.)
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I'm going to be doing the same thing to some 16' spans of 2x8 rough cut oak joists. I'm using 1/4 X 5 flat bar and 3/8 nuts and bolts. Right now, i'm in the process of jacking them up slowly from underneath to take the belly out of the floor. After I go about 1/2" past level, (about another month) we'll bolt the steel plates to the side of every other joist and we'll have a solid and level floor once again.
s

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On Jul 22, 10:19am, "Steve Barker DLT"

Steve-
You state: >>>>>I'm going to be doing the same thing to some 16' spans of 2x8 rough cut oak joists. I'm using 1/4 X 5 flat bar and 3/8 nuts and bolts. <<<<
Do I understand correctly that the 1/4" x 5" flat plate will be added to the side of the oak 2x8? If so, depending on how it is placed, a plate added to one side will only increase the 2x8 by about 50%. Two plates nearly double. If you really want to boost the joist stiffness the, plate needs to be deeper (closer to the 8" dimension of the oak). Through bolting, for all the extra work compared to short lags from each side (Simpson drive screws), doesn't really buy you all that much.
cheers Bob
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Thanks Bob, for the input. My dad ( a lifelong boilermaker ) and I kind of farmer engineered the idea to stiffen up and straighten a bellied upper floor. I already have the cribbing and jacks in place slowly 'bending' the joists back the other way. I'll consider putting a wider plate on. As it is, i'm spending about $500 to do every other joist. Perhaps i should go to a 6 or 7" plate eh? It'll just be my wife and I upstairs, one side is her sewing room and the other side is our office. We already have it about as loaded as it will be. I hate the bounce though. This house is 1877 and was built to the specs of the day for balloon construction. I see why they don't allow 2x8 to go over 12' nowadays. <G>. I already re-did the entire downstairs floor and put a support in the middle as well as 2x10's in the middle. (the heavy traffic and stairs area).
steve

<You state: >>>>>I'm going to be doing the same thing to some 16' spans of 2x8

cheers Bob
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On Jul 22, 6:23pm, "Steve Barker DLT"

The depth on the plate gives you the most bang for your buck. I'd go thinner & deeper rather than shallow & thicker,
The stiffness of the side plates (& beams for that matter) go as the depth cubed but only linear with thickness thus a 7.75 deep piece is about 3.7 x stiffer than the 5". Will a full 8" piece fit or would it have to be trimmed? :(
Are we talking about stock "flats" or are they going to shear it out of plate? If its going to be sheared oyu can get any depth you want. Sheared plate is fine, don't need any fancy CR flats here. Yeah, the CR flats will be staight, flat & true but sheared out of HR plate should be a lot cheaper. Plus the oak joist in the weak direction is WAY stiffer than the plate, so the screws will pull it fla, if necessary. Just start screwing mid span & work symmetrically to the ends to avoid a bulge.
I'd have the shop punch holes for the Simpson drive screws (the short ones), for oak you'd have to pre-drill. I'm thinking two rows 12" o/c staggered is probably enough. Six inches o/c is a little crazy. Maybe 8"? The screws are there to keep the plate for buckling out of plane. With the joists jacked, you'll be sure to get the plate to take load.
I'm not a huge fan of asymmetric installation but I would consider doing every joist but only a single side plate to keep the material cost close to the same .............but now all the joists are nearly 2.5x stiffer than the oak alone. You'll be getting performance more like 2x10. :)
maybe someone else in the ng can take a look at my concept & make sure I didn't make a gross error.
cheers Bob
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More good info here, thanks. So you think i could go how thin? 3/16" even 1/8" and still do the job? Oddly enough, the old rough cut joists are almost a full 2" thick, but they are only about 7-3/8" wide like a modern one. I do have a friend in the steel fab business that could shear the pieces of plate and punch them.
steve
The depth on the plate gives you the most bang for your buck. I'd go thinner & deeper rather than shallow & thicker,
The stiffness of the side plates (& beams for that matter) go as the depth cubed but only linear with thickness thus a 7.75 deep piece is about 3.7 x stiffer than the 5". Will a full 8" piece fit or would it have to be trimmed? :(
Are we talking about stock "flats" or are they going to shear it out of plate? If its going to be sheared oyu can get any depth you want. Sheared plate is fine, don't need any fancy CR flats here. Yeah, the CR flats will be staight, flat & true but sheared out of HR plate should be a lot cheaper. Plus the oak joist in the weak direction is WAY stiffer than the plate, so the screws will pull it fla, if necessary. Just start screwing mid span & work symmetrically to the ends to avoid a bulge.
I'd have the shop punch holes for the Simpson drive screws (the short ones), for oak you'd have to pre-drill. I'm thinking two rows 12" o/c staggered is probably enough. Six inches o/c is a little crazy. Maybe 8"? The screws are there to keep the plate for buckling out of plane. With the joists jacked, you'll be sure to get the plate to take load.
I'm not a huge fan of asymmetric installation but I would consider doing every joist but only a single side plate to keep the material cost close to the same .............but now all the joists are nearly 2.5x stiffer than the oak alone. You'll be getting performance more like 2x10. :)
maybe someone else in the ng can take a look at my concept & make sure I didn't make a gross error.
cheers Bob
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I've done this sort of thing in various forms of the years.
And the question always comes up "how thin can we go?"
Well, thinner is cheaper but at a certain point it would not be much better than paint.
I have a machine shop buddy as well & he keeps the "aerospace engineer" in check. He may not be able to "run the numbers" but I've never seen anything he's built fail & nothing he build is wasteful of material. Working together we've come up with some really cool, cheap & workable designs. I guess in 30+ years working metal he learned some stuff. :) And his limit for this sort of thing is 1/4"
I could give a long winded (typical, I guess) answer about thickness but the short answer is:
I'm ok with 1/4" but thinner (3/16 or less) makes me nervous.
Long answer:
The 1/4" added to a single side more than doubles the joist stifness, so thicker really isn't needed. Thinner runs the risk of the plate not acting as a unit with the joist>>>>>>that means more screws.
The out of plane (sideways) buckling of the system is driven by the oak joist. The joist in the weak sideways direction is ~30x stiffer than the steel plate. That's why we need to screw the plate to the joist. If we went thinner the sheet would be even more likely buckle & curve away for the joist. it would have very little sideways stiffness of its own (~40% of the 1/4" plate) so you'd need more screws
How many more, I'm not sure (I'd swag it at ~2x) & I'm really not sure how to calc it.......time for a lunch with some of my former students to get their input. :)
Going from 1/4 to 3/16 saves ~25% material cost but IMO drives the risk of the retrofit not working, way up.
BTW the span on these joists is ~16', what is the spacing?
The reason for this question is......are you sure the bounce is coming from the joists? If the joists are more than 16"oc the subfloor could be the problem.
IMO 1/4" is still real "plate" material......3/16 drops into the category of sheet.
Steve, Wayne is correct about oversizing the hole for the SDS. I'd go only about .010 but check the proposed hole size with some screws.
cheers Bob
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Ya, they're 16" OC , give or take an inch. <G>. You know in 1877 i rekon they only could measure with their forearms or something. It's funny, the vertical studs run from 13" to 19" spacaing. you'd think they could have cut a stick or something and at least made them all the same.
thanks for all your input.
s
BTW the span on these joists is ~16', what is the spacing?
The reason for this question is......are you sure the bounce is coming from the joists? If the joists are more than 16"oc the subfloor could be the problem.
IMO 1/4" is still real "plate" material......3/16 drops into the category of sheet.
Steve, Wayne is correct about oversizing the hole for the SDS. I'd go only about .010 but check the proposed hole size with some screws.
cheers Bob
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