Questions About Sistering Joists - Flooring - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Questions about sistering joists
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03-16-2012, 04:07 PM   #1
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I am redoing a bathroom in a very old house (100+ years). I am adding a corner hottub and was wondering about a few things:

1) Is sistering the joists needed? To be honest I would guess the standard cast iron tub that was there weighs about the same as the bigger corner unit when both filled with water. (The despite it being bigger new tub is very light compared to the old unit.) The old tub was the standard 5', the new tub is 5'x5' pentagon

2) Can sister joists be installed from above? I have pulled up the plywood in the bathroom to expose the beams from above. The ceiling below would need to be destroyed to do it from below.

3) Do sister joists need to run from a support beam to support beam in order to provide support? I can see where the existing joists end and hit the support wall below on one side. As far as I can tell I cant see where the next support beam is. I would guess it is under the floor in the next room. So will sister joists that are only supported by one beam provide any support?

Thanks

03-16-2012, 04:42 PM   #2
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Generally sistered joists run full length from one support to the next. Under certain circumstances this is not possible, and you do not get full capacity from a short sistered joists. The mathematics is complex, and an engineer or architect can tell you how much capacity you get from a less than full length joists. Avoid it if at all possible.

Do you need to sister? Well, the critical question is whether you are increasing the load on the floor. Guessing at the weight of the old tub versus the new tub is not a very good way to evaluate load. Usually most of the load is from the water and the person in the tub, so a good way to figure out the load is to know the total gallons of water in the tub. That should be possible to figure out for the old tub by filling it with five gallon buckets if nothing else, and the new tub certainly must list capacity.

Once you know the total weight of the unit, you need to determine how the weight is distributed with respect to the joists. Does your tub span two, three or four joists? Are you relocating the tub so the load falls near the midpoint, whereas before the load fell closer to the end of the joists? These will affect the bending moment on the joists. The closer the load is to the midpoint, the greater the imposed moment on the joist.

If you need to sister, the crucial question is how are you going to get the new joist in place? That will determine if you can sister from above or whether you need to come in from below. It can be a very awkward job at best. Before undertaking it, definitely make careful calculations as to the bending moment on the joists. If you are not able to make these calculations, you may want to consult an engineer or architect, after all they are trained to do this sort of problem.

 The Following User Says Thank You to Daniel Holzman For This Useful Post: Doorman54 (03-16-2012)
 03-16-2012, 04:45 PM #3 Pro     Join Date: Jan 2012 Location: Lagrangeville NY Posts: 1,020 Rewards Points: 502 When you sister a joist you are doing it for leveling reasons, say your floor isn't level which I don't think it would be if it's that old of a house so you will want your floor and tub level if your floor is t level your tube will be that much off if any. To add extra support you will want to what I call cat the joists that is we're you run your 2x4 between the joists and this is done more under the tub then anywhere else in the bathroom.

 03-16-2012, 05:05 PM #4 Member   Join Date: Feb 2012 Location: SW burbs of Chi-town Posts: 87 Rewards Points: 75 Daniel..... I always enjoy your responses!!
03-16-2012, 05:29 PM   #5
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Quote:
 Originally Posted by JetSwet When you sister a joist you are doing it for leveling reasons, say your floor isn't level which I don't think it would be if it's that old of a house so you will want your floor and tub level if your floor is t level your tube will be that much off if any. To add extra support you will want to what I call cat the joists that is we're you run your 2x4 between the joists and this is done more under the tub then anywhere else in the bathroom.
So if the floor is level, sistering is not beneficial to added weight?

Running 2x4 between the joists seems much easier than sistering, so I would prefer that.

 03-16-2012, 05:40 PM #6 Pro     Join Date: Jan 2012 Location: Lagrangeville NY Posts: 1,020 Rewards Points: 502 Yes, if you need to bring the floor up on one end you sister the joists so your floor will sit level on the new sisters not the joist. so cat them between the joists level with the joist if your floor is already level.
03-16-2012, 06:51 PM   #7
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Jet said;

Quote:
 When you sister a joist you are doing it for leveling reasons,
Definitely disagree with that. What if you sister (to stiffen them), and do not remove and replace the subfloor?

Maybe the laws of physics are different in NY?

Jaz
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 03-16-2012, 07:41 PM #8 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,638 Rewards Points: 4,856 Jetswet, your comments are confusing at best. The main reason for sistering a joist is to increase the moment capacity (strength) of the joist. Adding blocking (I think you mean blocking, you used the word cat, which I have never heard in this context) does nothing to increase the moment capacity of the joist, it does improve stiffness marginally, and it reduces buckling potential, neither of which are directly related to load capacity. If you need additional load capacity, sistering is the most common way to get it. Occasionally you will encounter a situation where the joists have sagged, or were not installed level, or one or more joists is lower than an adjacent joist. Sometimes the framer installs a sistered joist to level the floor rather than planing down the existing joists, installing self leveling compound, or shimming the joists. In my experience, it is rare that sistering a joist to level a floor is easier than self levelling compound. If the floor is out of level AND requires additional strength or stiffness, you can kill two birds with one stone by sistering new joists, and installing them flat with respect to each other. You need to start by determining IF you need to add strength or stiffness to the joists.
03-16-2012, 07:42 PM   #9
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Quote:
 Originally Posted by JazMan Jet said; Definitely disagree with that. What if you sister (to stiffen them), and do not remove and replace the subfloor? Maybe the laws of physics are different in NY? Jaz
? Huh.... Jaz, if I think what your saying is yes you can widen the joists if you put a sistered 2x4 but what's the point? just put cats across.
If you need to raise it for level the floor has to be out.

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