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Old 01-22-2019, 11:54 PM   #46
jadnashua
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The majority of the strength is in the top and bottom of the board...the middle just holds them in place. AN I-joist is an extreme example of that effect, which is why you don't want holes near the edges. I would use smaller diameter fasteners and keep them away from the edges so you're cutting the minimum number of strands of the wood in those critical edge areas.
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Old 01-23-2019, 07:12 AM   #47
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Are these the SPAX ones you were thinking, Wayne? SPAX 1/4 in. x 6 in. Torx Powerlag T-Star Drive Washer Head Yellow Zinc Coated Lag Screw https://www.homedepot.com/p/SPAX-1-4...1527/202041029 The GRK are only 1/16 bigger, is that a significant problem? The price isn't a big deal since I only need 3 of them. I don't really want to try to skimp on this, I'll throw more money at the problem if it actually helps.

And more numerous smaller diameter fasteners brings me back to the thruloks. Again, expensive but I'll live and the other studs can get lag screws. Small holes, clamp things together, good shear strength. Alternate top and bottom every 12" driven 2" from the edge of the stud? I'll look up exact specs and recommendations on these when I get home tonight.

Question: there's some space between the sisters, maybe around 1/8". The floor boards are nailed to them from above. If I draw everything together, is it going to conflict with those fasteners in some way?
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Old 01-23-2019, 09:03 AM   #48
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I was suggesting the flat head version of that SPAX 6" x 1/4" fastener, so you could drive it flush, so it wouldn't interfere with the new sister. You could use those and just counterbore them, the head is close to flat so it wouldn't take much counterboring. Same for the GRK.

I'm thinking of 1/4" lag equivalents as in the category of "small fasteners" versus 1/2" or 3/4" through bolts.

If the two joists currently have a noticeable gap between them, and the subfloor above is nailed to both joists, there's no upside to trying to close the gap, and there could be a downside. Instead I suggest adding a filler material at the location of each fastener that is going to penetrate both joists. For gaps 1/16" to 3/16" or so, you could use 1-3 drywall shims. [Although I might ignore a 1/16" gap, and I happen to have 1/8" hardboard lying around, so I'd use that instead of 2 drywall shims.]

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Old 01-23-2019, 09:29 AM   #49
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Ok, understood. The only flathead 6" screw I'm seeing that I can get here is this fastenmaster heavy duty screw: https://www.homedepot.com/p/HeadLOK-...INDY/204307979 I can shell out for 6 of these. Acceptable?

I've got drywall shims and 1/4" (3/16" actual) plywood. I can also rip shims out of 2x3s. Would that be better?
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Old 01-23-2019, 09:31 AM   #50
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I wouldn't think that there's be any adverse effects to the nails holding the sub floor boards to the joists by closing the gaps in the joists. Unless the gaps are huge I think closing them will only pull the nails sideways in their holes in the sub floor boards, and you're gonna want to screw those boards down anyway after the joist work is finished.

Seems to me the larger the gap between joists the more you rely on the diameter of the fastener to resist bending. May not matter though, if the fastener cannot draw the pair together at the top, and because of the joist depth finding a clamp with a deep enough throat will be tough, you may just have call it good enough.
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Old 01-23-2019, 01:15 PM   #51
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I'll go back and check how big the gaps actually are. Unfortunately, I don't have access to the floor over these joists because there is a wall there. If I did, I would just pull the nails and replace them with screws afterward. I'm also not sure if the boards are nailed to both joists or just one and I don't think I can find out reliably. Hopefully most of the stress will be between the new sister and the old good sister, not the cracked one, and those will be flush.

Questions:
1). I've been trying to find 2x8x12 douglas fir for the sisters. No one here stocks douglas fir except home depot and I don't trust their stuff. It can be ordered but most places will only do a whole pallet. Select grade SPF also needs to be ordered. I would rather go through the effort and do it better but is this a complete waste? Place nearby has LVL 2x8x12 in stock, can I use that and would I have to work it differently?

2). I'm morbidly tempted to jack up the joists under the bathroom even a 1/4" just to put some of the strain on the new sisters and keep them from sagging more in the near future (plus straighten the floor a bit). The bathroom walls above were meticulously shimmed, shaved, and reinforced into straighteness. All the gypsum and CBU are already up and finished. Is this a bad idea?
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Old 01-23-2019, 03:08 PM   #52
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I would jack it up. Slowly, a little at a time, and let the wood relax between jacking. Listen while you are jacking the joists up.
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Old 01-23-2019, 07:12 PM   #53
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0) Those HeadLOK screws are not flat head, they are washer head.

1) I'd be happy with any dry material. LVLs are fine but overkill and expensive.

2) I'm not 100% clear on the wall locations relative to the joists and to the point on the joists where you would jacking, but if you jack I think you should be prepared to make (hopefully) minor repairs to the walls above.

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Old 01-23-2019, 09:48 PM   #54
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0). FastenMaster calls them flat head but I do see they don't drive flush, so I follow. That's the closest I can find to actual flathead in that length. Can I just get a 7" and drive them through all 3 2x8s?

1). Understood. I wouldn't sister each joist with LVL but I'll pay extra for overkill on this one joist. It's supporting a bunch of heavy stuff. Also its near impossible to get anything but rained-on #2 SPF 2x8s here, this will save me a lot of time. Of course I've seen LVL kept outside too...

2). I was mistaken. The broken joists are just beside the footprint of the wall. The wall runs parallel to them, the stacks run up inside it. Doesn't change much, that side has cabinets, a dishwasher, and flooring over it. I'm not going to jack up the studs. I'll attach it to the whole joist where it is. I really don't want to ruin what I just finished in the bathroom and the floor is not that out of whack.

I checked the gap between the sisters: it's as wide as 1/4" in places but the joists are tighter together at the top. There are only a few spots where I can get a drywall shim all the way up to the floor boards.

I'm making this too complicated, had a long week. Wayne you gave me a good plan on page 3 of this thread: new sister attached tightly to whole joist with 3" lag screws and glue, attach blocking on cracked side to good joists with longer screws with cracked joist acting as spacer (fill gap with shim), secure important parts of cracked joist to good joist with a few additional screws, let bad part of cracked joist hang there and do nothing (maybe trim it). Ends of new sister don't rest on sill/beam, no through-bolts, old joists aren't drawn together, nothing is jacked up (maybe just 1/8" in the center to load the new sister a bit, that seems about as hard on the wall as bouncing on the floor, right?) Sound reasonable?
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Old 01-24-2019, 12:36 AM   #55
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Yes, with one amendment: if the gap between the joists is at the bottom only and not at the top, I would be inclined to draw them together with clamps or the new fasteners. The angled gap you describe means one joist has rotated, and it would better to pull both joists plumb.

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Old 01-24-2019, 07:30 AM   #56
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May not have rotated, may have simply cupped. Lay a straight edge against it and see if it's flat.

If it's cupped, and there's a 1/4" gap, I'd be concerned it may crack if you try to close the gap.
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Old 01-24-2019, 07:49 AM   #57
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First, Dan, I re-read my last post and I didn't mean to imply that your advice is bad.

I'll check the joists more carefully tonight but that cracked joist is in pretty rough shape. It may be twisted or cupped and the cracked portion is jutting out the sides. I'm afraid if I try to do anything big to it it's going to deteriorate a lot more. Over toward the sill, it's got a longitudinal crack and the side facing out from the sister is convex so I had to plane it down a bit to attach blocking. The other joists look pristine in comparison. Either the cracked one is original and all the others in that area were replaced more recently or it was the one at the top of the stack and got the most abuse. The floor planks were nailed to all the joists under a mud bed inset with small pink tile and I see a strap mark in the middle of the cracked joist so it's probably a bum piece of lumber. I'll try to take a panoramic shot to show the whole thing.
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Old 01-24-2019, 09:35 AM   #58
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No worries at all Paul, I'm just a DIY'er so I'd certainly defer to someone who actually KNOWS what they're doing.
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Old 01-24-2019, 10:53 AM   #59
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Paul, let me toos in my couple cents and say that no matter what mechanical fasteners y'all decide is your favorite for the application, if you don't also glue those sisters with a good quality construction adhesive you're not going to get the long-term rigidity you're looking for.

My opinion; worth price charged.
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Old 01-24-2019, 11:41 AM   #60
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Thank you, CX. What do you consider to be a good quality construction adhesive and what would you consider to be a appropriate amount per foot of 2x8 sister?
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Last edited by SpaceCadet; 01-24-2019 at 11:42 AM. Reason: wording
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