Joist meets Drain - now what?

[Randyj]

Just to jump in on this thread.... I wouldn't worry about the deflection for this area. Just keep the boxed in area as small as possible. The cut joist in a properly boxed in area will be well supported by the two joists it is tied to. It is a bit heady to think about it but seems to me that the cut joist would actually gain strength since its span does not run the entire length...but the joists to which it is connected would gain load by the headers tied to it since they would have to carry the load of the cut joists. I have added 2 x 4's (blocking) around pipes to support the floor which was no longer supported by the joist which was cut..... maybe all this makes sense...
As for the pipe size... you'll need to cut out that tee the trap is connected to and replace it with a tee for the 2" drain.

 

[Geniescience]

Measure the width of the open space between the two joists

linda

The Kerdi drain is less than 6 inches on each side of its center so it is too far from the wall to need being cut.

In Photo #1 we see a joist (cut) and another one at the back of the photo, under the wall. Is the space 14" between the two sides of the two joists? The norm for joists is to place them every 16" and after removing the width of the joist itself you should have about 14" available space.

Is the space 14" between the two?

David

 

[Sponsor]

 

[AdrianMariano]

I got a laugh out of the idea that you can make your floor stronger by cutting the joists.

Unfortunately, it isn't true. I asked a structural engineer friend about it, but it sounds like detailed calculations are necessary to understand the implications of installing one of these boxes in any particular situation. One thing is certain, however: cutting a joist and putting in a box weakens the floor. The deflection will be larger than if the joist were intact. If you make enough of these boxes close together you could be in real trouble. If you were planning to install tile or natural stone I think you'd want to be sure that the box wasn't increasing deflection beyond the tolerance of the flooring. If your joist structure was marginal to begin with, or if you wanted to install natural stone which has tougher requirements, you might well run into problems.

 

[Leejosepho]

I got a laugh out of the idea that you can make your floor stronger by cutting the joists ...

That is not what Randy said. Rather:

The cut joist in a properly boxed in area will be well supported by the two joists it is tied to. It is a bit heady to think about it but seems to me that the cut joist would actually gain strength since its span does not run the entire length...

In other words, and for example: Cutting a 2' piece out of a 16' joist and attaching the ends to a "box" properly supported by neighboring joists essentially leaves you with two 7' joists, and those two resulting 7' joists will have less deflection each in their own respective spans than did the original, overall.

As for the pipe size... you'll need to cut out that tee the trap is connected to and replace it with a tee for the 2" drain.

No, the drain can be bushed down beforehand, and I submit there is no point in running any 2" pipe or fittings between the drain and the existing 1-1/2" tee. Furthere, neither is there any point in installing a 2" tee in place of the existing tee unless the entire line is replaced with 2" all the way down to wherever it eventually dumps into an even larger pipe.

Thinking of a funnel: What would be the point of making its top larger if its ultimate outlet (such as at the sewer line) yet remains the same size as before?

 

[Racer814]

cut it, header it and get on with it... .

all this talk of load deflection is overblown....cut out a section and header it correctly(double the header and use joist hangers) and the deflection across that area will be no more than the rest of the floor system.....

 

[AdrianMariano]

The posted analysis is not right. Cutting a 2 ft section out of a 16 ft joist is only like having two 7 ft joists if you install a support pillar under each end of what's left. If you don't do this then you have weakened the structure. There is no way around it. There's less structural material holding up the floor because you now have one less joist doing the job. It can't get stronger. If you connect it to adjacent joists then you have increased the load on the adjacent joists. They now have to bear the weight of the cut joist and they have to bear the load that the old joist used to bear, so deflection will increase.

 

[Leejosepho]

The posted analysis is not right. Cutting a 2 ft section out of a 16 ft joist is only like having two 7 ft joists if you install a support pillar under each end of what's left. If you don't do this then you have weakened the structure. There is no way around it. There's less structural material holding up the floor because you now have one less joist doing the job. It can't get stronger. If you connect it to adjacent joists then you have increased the load on the adjacent joists. They now have to bear the weight of the cut joist and they have to bear the load that the old joist used to bear, so deflection will increase.

What you are saying about overall deflection in a floor system is definitely correct: The overall strength of any joist system is weakened (however much or little) whenever any joist is cut. However, a 7' joist still has less individual deflection within its own span than does a 16' joist in its respective span.

 

[Hube]

install the HEADER as was suggested by jadnashua and racer814 and others. It's done all the time, no problem.
above all DON'T put a couple of posts under it as (lol) adrianmarino suggested lol.
OR,you could raise the tub a couple of inches to allow for an gentler offset for the drain, frame the base of the tub in, and then you won't have to do any header work at all .

 

[Leejosepho]

install the HEADER as was suggested by jadnashua and racer814 and others. It's done all the time, no problem.

Sure, but why? Unless the entire plumbing from the new shower to the sewer line is updated to 2", the drain can be connected to the existing plumbing without doing that. So, what would be gained by the extra work?

 

[AdrianMariano]

What you are saying about overall deflection in a floor system is definitely correct: The overall strength of any joist system is weakened (however much or little) whenever any joist is cut. However, a 7' joist still has less individual deflection within its own span than does a 16' joist in its respective span.

A 7' joist has less deflection than a 16' joist as long as it is supported at both ends by something which doesn't deflect. But a 7' joist that is supported at one end only by being hung off of the middle of a 16' joist is not the same as a fully supported 7' joist because the support at one end deflects.

Because the 7' joist is hung off of the 16' joist the 7' joist will necessarily deflect at least as much as the 16' joist because the 16' joist is supporting the 7' joist so as the 16' joist deflects, the support for the 7' joist will also be deflecting by the same amount. You can't have the 16' joist deflect down and the 7' joist somehow defiantly remain undeflectected because the 7' joist is actually hanging off of the 16' joist. Then you have additional deflections due to the 7' joist itself bending.