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-   -   Best way to sister these joists? 3 choices... (https://www.johnbridge.com/vbulletin/showthread.php?t=91793)

pghsebring 03-04-2011 01:05 PM

Best way to sister these joists? 3 choices...
4 Attachment(s)
Here's what we have now - this is the side view of the floor:

Attachment 93820

Now, what i have is just barely sufficient for stone tile, and i'd like to stiffen it up a little. However, to the right is the breaker box, and all of my wiring for the entire house runs through those joists - so no way i can run a sister all the way to the bearing point on the right. At best i think i can go about 3'. I'm thinking of using 2x8's to sister with, and here are the 3 ideas. Which will stiffen up the floor the most?

First, just about a 4' long piece nailed to the joist that goes to the right:

Attachment 93817

Second, a longer piece that goes to the right - i'm just worried about it putting more pressure on the middle of the joist, because it stops closer to the center of that joist:

Attachment 93818

Third, i was thinking of a cantilever setup - the new joist goes to the left about 6 ft, no nails past the bearing point - about 2':
Attachment 93819

So, which will work best?

jadnashua 03-04-2011 02:18 PM

If this will help, you only need to sister the middle 2/3'rds of the joist to gain most of the benefit - yes, it is stronger if you get the whole length, but not if it's a major pain and you're already close. So, with that in mind, I wouldn't worry about doing anything special at or even over the bearing point. Use it if you can, don't worry about it if you don't. Some construction adhesive and screws to hold the new to the old, and it should improve things, especially if what you have now is close to the requirements for stone.

pghsebring 03-04-2011 02:36 PM

I know sistering the middle 2/3rds would be the second best option after sistering the entire joist, but i can't get to the middle 2/3rds. I can only get to the closest 1/3 of the joist to the beam on the right - and my bathroom is sitting almost centered on the beam - offset just enough for the toilet drain to almost touch the beam. I cannot get to the middle of the joist without opening an absurd can of worms, pulling every single wire for the entire house out of the joists - all because my deflection is 719 instead of 720. Are you saying i'd be better off with doing nothing instead of doing any of the 3 examples i've posted, considering i can't get to the middle of the joist?

Honeydo 03-04-2011 02:53 PM

Thought I understood from your description til I looked at the pics. :confused:

But after seeing your pics I'm really lost at what you are trying to accomplish. or at least the way you are going about it. I see what you say is existing tile but where is this new stone tile going exactly?

If you are worried about the splice over the beam, tell us more about that? overlap? fasteners? joist footprints on beam?

jadnashua 03-04-2011 04:33 PM

To increase the rigidity, you need to get to at LEAST the middle 2/3'rds of the joist...patching something around the end support won't help much. If it really calculates to L/719, considering the calculator is pessimistic, you should be fine if the rest of the install is good (i.e., the subflooring is proper and intact). You could buy yourself a little extra breathing room with using Ditra or Ditra XL rather than cbu and save some height...it would decrease the dead load on the floor as well, which isn't a bad thing, especially with thick natural stone.

pghsebring 03-04-2011 05:36 PM

Honeydo -

I'm replacing the current tile with stone tile. I have it all torn out down to the joists. I'm gonna put down a layer of 3/4" ply, 1/2" ply, then 1/4" hardi, then stone.

The overhang of each joist is about 8" past the beam. The drawings are pretty close to scale - the bathroom is only 3' wide, it is sitting damn near on the beam, offset by about 4" to one side so that the centered toilet drain misses the beam. The left side of the bathroom has two joists essentially - the left ones, and the overhang from the right ones. The right side of the room has just the ones going to the right. I think the footprint is 9" on the beam. The fasteners would be just nails....16d i imagine.

Honeydo 03-04-2011 06:40 PM

Given the position over the beam I wouldn't worry at all about the joists. l719 equates to a deflection of a littke less than 3/16 of an inch at midspan on your 10.5 beam.
The actual deflection at 18inches or 2 feet away from that beam is only a few hundreths of an inch.

I'd be more concerned with a hard spot from uplift. Picture a very heavy load (a party with lots of friends,) fully loads the beam so it deflects to maximum at midpoint, and that results in a tiny opposite uplift at the end depending on the splice and how it's supported by the beam. Shouldn't be a big deal at all but to be safe you can just sand, plane or chisel a 1/16 or so off the top of last 3 inches or so of the joists and that will negate any possible uplift problem.

Just do the joist ends, the untouched wood of the opposite joist next to it carries the subfloor in plane. When fastening the subfloor only screw into the untouched joist areas.

Mountain Tile 03-04-2011 07:10 PM

I like to cut 3/4" ply to the size of the joists, glue and screw it to the joists, it's only 8' long, so I center it in the middle, then butt the pieces tight to the middle one, then overlap another layer another layer of ply, this time offsetting the plywood joints. I glue with a premium adhesive and use deck screws every 4" 6" apart.

cx 03-04-2011 09:32 PM

Stuart, is this the bathroom project you have in another thread?

The only real way to prevent the uplift problem Ed is describing is to reduce the joist deflection on both sides of the support beam. That or provide a different, and better, connection of the joist ends at the beam. But that would require that the beams be longer at the splice and I dunno how a fella'd do that. :)

It's still an issue, though. And if I could, I'd sister one of the joists as far from the overlap as I could, then add another joist piece that would span the splice as far as possible in each direction to eliminate as much of that uplift as I could.

And I see where Ed has mentioned a joist deflection of L/719 on something but I see no indication of where that might be. Judging from your drawings you've got 2x8s spanning nine or ten feet and you ain't gettin' that kinda stiffness from that. Can somebody 'splain me that part? :)


Depends what you're trying to accomplish, Chuck, but if you're trying to reduce deflection of joists by sistering, I don't think you're gaining much with that method.

dhagin 03-04-2011 11:00 PM

Not sure how anyone is getting an accurate measurement of deflection??? I don't see joist spacing anywhere.

... with regards to deflection, doesn't matter anyhow. I'm with Ed here. Over that beam you have essentially -0- deflection or L/lots, especially with walls in those positions. Go ahead, give it all the jump testing you want. No additional joists needed IMO, unless the joist spacing is 2' OC... or more.

The potential uplift at the joist end opposite the unsupported span is a real issue. What experienced framers do here is to cut the joists off as close to the beam as possible with their skilsaw, usually 1-1/2" or so away is fine. Make sure the remaining joist overlap is nailed well with 6 or so good 12d's. You can use steel straps between the joists to make the tension connection if needed

Then, proceed with the plywood as you describe. :)

edit; drawings show 2x10 joists, max span of 10.5', unknown spacing... and 2x8's for proposed sisters. and CX, how did you know his name was Stuart? you clairvoyant? :D

pghsebring 03-05-2011 12:26 AM

The joists are actual 1.5" x 9", 16" OC, 10.5' span. L/719.

I was thinking of using 2x8s for sistering because 2x10s would have to be ripped down.

CX, i don't exactly follow what you think i should do here...

Dhagin's vote seems to be for cutting the overhang off, nailing the joists together well, and calling it a day...

If the deflection to one side of the joist is so miniscule, a couple hundredths of an inch, wouldn't that be the same on the other side of the beam for the same joist? That the uplift would be virtually nil too?

I can't wait to do my other bathroom...its clear what the answer is there...

Honeydo 03-05-2011 07:43 AM

I used the 719 number the op qouted, As his drawing showed 2x10s nominal and his unsupported span, I just ran that back through the defleto to see what variable (16"oc) he had used to get 719.

The Op has since conformed that.

ImO as I said, the deflection movement is insignificant when you convert the deflection number to actual inches within 18" of that beam. I often see people do long span ijoists floors not realizing a normal deflection number can easily result in a 1/2" of defection movement which I have always felt is a weakness of just using a defelction number in judging suitability for stone or tile alone. Here we have somewhat the opposite as he is working ionly within a foot and a half of the beam.

With less than 3/16 of an inch at midspan of calculated fully loaded deflection, uplift is really pretty non consequential also.
I mean we are talking about a relatively stable floor. It's all relative. 719 on a 10 foot span is a whole lot more stable at the splice than 719 on a 20 foot span. The uplift on a 10ft or so span even with 3/16 of max defectionI is tiny. (a couple hundreths of an inch) just don't see the big iissue. IMO either shave the hotspot or cut the tail and good to go

cx 03-05-2011 08:34 AM

OK, I somehow shrunk his 2x10s to 2x8s. Sorry about that. Without knowing what kind of wood he's got I'd hafta use a deflection from the Deflectolator of about a quarter-inch at full loading.

But my point is that I don't think it prudent to disregard the distance of the movement at the end of that joist on the other side of that support beam. That slight vertical movement, given the significant force behind it, is enough to cause a problem in a tile installation.

Cutting off the joist ends, or shaving them down ,as Ed recommended, might be a remedy for the situation, but something needs to be done there.

I don't favor just pretending it's too little to cause a problem.

My opinion; worth price charged.

Mountain Tile 03-05-2011 08:45 AM

Cx, I can't produce numbers on my method, but yes it creates a very strong joice and will decrease deflection. If you really think about it, it's like your creating an engineered beam, Stand a piece of ply on edge and it's extremely rigid, attach it to exsisting lumber and you've increased the strength trememdously. Not trying to be argumental or give undocumented info, just saying this is how I myself handle these situations. Cut an 8" strip of ply and stand it on edge and try to make it bend, not gonna happen, it's almost as strong as steel on edge, lay it flat, different story.

Mountain Tile 03-05-2011 08:53 AM

Also, I might add, to achieve the strongest joice, ply glued and screwed in between sistering with lumber will give you the most strength. This is how we built beams, when removing weight bearing walls.

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