Joist meets Drain - now what?

[lhartl]

Hello,

I am new to this forum.

My husband and I are currently renovating our ensuite. We have removed the bathtub and are planning a large walk-in shower unit in it's place. We have purchased the 32 x 60 Schluter Shower System with an offset drain, hoping to utilize the current bathtub drain location (upgrading to a 2" pipe of course).

Upon examining the existing plumbing, we can see that the bathtub drain was placed directly over a joist (notched to accommodate the drain fittings) - and upon examination of the Schluter drain system, we realize that we need to go almost 4 inches deeper with the new drain fittings.

The 10" joist (on 16 inch centres) is currently notched about 3" and we will need to cut another 4 inches to build a 2" drain/trap to accept the Schluter drain. That would leave less than 3 inches of joist. The shower is on the outside wall, we estimate about 24 inches to the outside wall would be compromised.

The offset drain in the Schluter foam tray is 10" from the edge, and to keep the slant from the edge to the outside of the drain, we have very little to play with.

I am anxious to hear any and all ideas from the experts.

Cheers,
Linda

[jadnashua]

I think you'll need to cut that joist out and put in some headers on either side of it so you've got a reinforced box around the drain..

[Racer814]

street 45 in the drain to angle out of the joist and then another to offset it back


or cut and header the joists

[lhartl]

Thanks for the ideas, what exactly is a 'street 45' ? Can you explain a little more about these fittings suggested?

thanks,
Linda

[leejosepho]

Thanks for the ideas, what exactly is a 'street 45' ? Can you explain a little more about these fittings suggested?

A street fitting is male at one end and female at the other, and the suggestion here is that you first angle away from your joist rather than cutting it more. So, something like a couple of street 45s would let you dog-leg over a bit before going farther on down below the existing notch in your floor joist.

[lhartl]

Hi again, I am attaching a picture of the situation today. The circle that I drew on the board was from the shortest 90 degree elbow that I could find attached to the drain. This circle marks where the curve of that elbow would sit if the joist weren't there. As you can see, there is not much room from there to the main pipe and I need to get a trap in there and also a conversion from 2" to 1.5". I understand that I need to maintain a slope also.

does this give you any other ideas? Should I just get a plumber in to look at this and make it right? I think I am over my head a little.

thanks,
Linda

[Gary Swart]

I would not use 2 45's that just makes the potential for a clog. The best thing to do is as Jim suggested. Cut the offending joist out and box around the area. This will return the support lost when cutting out the joint and provide the space needed.

[markts30]

As you can see, there is not much room from there to the main pipe and I need to get a trap in there and also a conversion from 2" to 1.5". I understand that I need to maintain a slope also.

1st - you cannot place a 2" trap here and reduce it to 1-1/2" to fit into the existing tee....
You have to replace the SanTee with a 2" tee and plumb it from there...
Once this is done, you can figure out the best way to run your drain - in the long run the boxing out of the joists will be the best option...
If you are not sure of what you are doing, take lots of pictures and make lots of drawings and, for the most part, we can talk you through the piping part - for the tiling, go to John Bridge's forum http://www.johnbridge.com/vbulletin/index.php? .

[lhartl]

thanks. Just to confirm, the San Tee is the pipe furthest away in the picture? I hear you say that I need to cut that one out and replace it with a 2" T, but there is no room to accomodate an even bigger pipe and 2 to 1:5 connectors or is there? I haven't seen them at HD but I will go back and look again if you tell me that I should be able to squeeze this in.

I have new respect for plumbers! How do you work in such small spaces and keep your sense of humor? :)

thanks,
Linda

[Gary Swart]

Hello, what is it you are not understanding? Remove a section of the joist that is in way and box around the work area. Also, you can not reduce the size of the drain from 2" to 1-1/2". That would be an open invitation for clogs. You really don't have a serious problem here if you will just remove the joist.

[leejosepho]

I will probably get shot for this, but it looks to me like that circle you have drawn is near the center of the joist, and if that project was my own, I would plate both sides or "make a sandwich out of" that joist to return it to its original strength (or at least very close), then drill the hole you need where you need it. And rather than trying to retrofit to 2", I would keep that bit of pipe (aws a nipple) already coming out of your tee and use it for connecting your new trap. But of course, neither was McGyver a plumber ...

[lhartl]

Lee,

Thanks for the suggestion. The Schluter shower drain is 2" and the old (existing) tub pipes are 1.5", so I must make the conversion somewhere in there. Mark has suggested that I swap out the current 1.5" SanTee and replace with a 2" SanTee - but this will require that I change the pipe connections coming in and going out of the SanTee. I can't possibly replace all the plumbing to 2", somewhere I have to end the 1.5" and I am up against another joist at that point.

Do you think that I need to start at the SanTee with the 2" ?
thanks,
Linda:confused:

[Racer814]

45'ing out of the joist ain't gonna cause a clog...lets be serious, it's not like we have solids going down here....I mean...I might not get out of the shower to go #1(don't tell my wife)....but I ain't going #2 in there.....:rolleyes:

[Racer814]

you may not have enough room for 2 45's anyway...2 streets maybe, thats the easiest way if you have room....headering the joist on the 2nd floor aint no picnic , I would avoid it if possible...........the shower drain should be 2" by code....how hard would it be to change it? .....would it work going into a 1 1/2" drain...yes....is it a really good idea?.....no



you would do yourself a favor by getting a proffesional in there...

[adrianmariano]

What exactly do people mean when they say to build a box around the drain? I can't figure out how this would work without compromising the strength of the joist. I think the idea of bolting a sister joist in next to the existing one and then drilling out the hole in the center for the new drain is good for the structural integrity. The result will be stronger than what was there before.

[lhartl]

The picture may be confusing everyone - the circle that I drew on the board shows where the drain and a short elbow WILL REST on the joist, it was a 'depth' indicator. So you see, the problem with sistering and drilling a hole won't work because I actually need that much depth in the board for this Schluter drain system and an elbow to get off the joist. I will attach a picture of the Schluter drain as fyi.

Otherwise it would have been a great suggestion. I could still bolt a 2x4 (on end) across the length of the joist if I cut down to the 7" mark on this 2"x10" joist, then box it in from there - do you think that would be strong enough?

We never had a problem with the 1 1/2" drain in the tub and we have showered there for 25 years, seems like the code is there for new installs maybe and setting some standards ?? Anyway, it doesn't matter now, we have this Schluter drain and it's 2". We must deal with it.

thanks for the comments, it really helps to talk this thru. I have left messages for two plumbers in Coquitlam, no answers, I think everyone is too busy for such a small job. Any plumbers out there close to Coquitlam?

another day....:)

Linda

[adrianmariano]

What kind of flooring is this joist going to be supporting? If it's tile or stone then you need to pay careful attention to your structure because if it's inadequate the floor can crack.

Deflection of joists is inversely proportional to the cube of the joist height but only one times the width. A 10 inch joist has 7.5 times less deflection than a doubled 4 inch joist (your 2x4 sistered to the existing joist) so it is a LOT "weaker." (So the floor will sink 7.5 times farther when you step on it with your 4 inch wood in there when you step on it than it would with a 10 inch joist intact.) The floor will be weakened for the entire length of the joist back to wherever it is supported. (I got the above number by computing 10^3 / (2*4^3).)

If you need to maintain the full strength then perhaps bolting steel plates to the joist is a solution. I did a quick estimate and this would appear to require approximately 1/2 inch thick quantity of steel if it's four inches high. If you want to go this route you should probably recheck my quick calculations, or maybe check with someone who really understands carpentry or structural engineering. (This is carpentry, not plumbing, by the way.)

Now houses are overbuilt. You can probably cut that joist out entirely or execute some ineffective fix and as long as your flooring is flexible you won't have any serious problems. The floor might feel a soft and springy along the line supported by that joist. I had a joist in my house completely rot all the way through. It was the one at the edge under the wall. And yet nothing obvious had happened to the house as a result.

[geniescience]

Adrian is right. Every time I look at this thread, I wonder what I am missing, and I move on instead of re-reading carefully every word to see if I have missed something.

Maybe we all need to see a large-scale photo. A joist, going from where to where, holding what up over how big a distance. That kind of thing.

Adrianmarino, I'm glad you are here and contributing your knowledge. I also like the fact that you are I both use metal to build or reinforce structures.

David

[lhartl]

Hi again,

I am attaching another picture of the area. The Schluter Shower System is 32 x 60 with a premade shower bed and drain kit. The drain in the premade bed is offset exactly the same place as a tub would be - 10 inches from the plumbed wall and centred at 16". I am planning to install porcelain tile over the walls and floor, so I am laying 1/2" backerboard over the new 5/8" plywood floor, using thinset of course. The Schluter polystyrene floor pan goes down next with more thinset and the entire floor and walls are covered with another layer of thinset and the membrane prior to the tiles. I expect this will be a very solid surface on which to tile. (This system is for the DIY'er but had I known that the drain would be such a problem, I may have opted for a custom mud bed and placed the drain where there is more room. I suspect that the Schluter drain is much larger than standard and this is another source of issues for me.)

I can't imagine much standing weight over the drain and in the top ten inches of the 60" shower, but there will certainly be the weight of the tiles to consider.

I am sorry this is so confusing, articulating this and using the plumber lingo doesn't come naturally.
:rolleyes:
Linda

[adrianmariano]

I don't think she has pictures of the whole joist span. It looks like just that one little hole in the bathroom is opened up and there's a finished ceiling below. It should be possible to figure out what the joist span is, though, and obviously what it's holding up.

Is using metal such a radical idea? My 50+ year old house has a metal I-beam down the middle. I can't say I've actually built with metal myself, but it's the only way I can see to get the strength to match a 10 inch joist in a 3 inch height.

And actually I made a mistake in my calculations of the thickness required (in addition to using 4 inches instead of 3 inches which makes a big difference). Calculating again based on a 9.25" joist height, 1.5" joist thickness, and steel with E=30 vs douglas fir with E=1.6 (E is the modulus of elasticity that determines how the material bends) I'm looking to find the required steel thickness only 3 inches heigh that bends as much as a 9.25" high 1.5 inch thick joist made of douglas fir. I solve this equation:

30*t*(3^3) = 1.6*1.5*(9.25^3)

And get

t = 2.34 inches thick

That's a pretty serious amount of steel and probably impractical to handle. Maybe you can get 1.5" thick I-beams and bolt those to the joist? I don't know. Is there such a thing as a "half-I" beam where you have flanges only on one side? Then you could bolt one to each side of the joist.

If you do or will have tile or stone flooring above this joist (anywhere on its length) you might try asking for advice on the johnbridge forum (previously mentioned) for advice on reinforcing your structure.

[lhartl]

Without ripping up the entire floor, I could not tell you how long the joists are. I will also post on John Bridge site and see if there are more ideas out there for reinforcing the joist.

Linda

[adrianmariano]

Tile must be supported by joists which don't deflect too much. If the joists deflect too much then the tiles, which are very rigid, will crack, or the grout between them will crack. If you plan to install tile you MUST have an adequately rigid floor. Most floors are adequate for tile. (Natural stone requires an even more rigid base and presents more of a problem. My bathroom has joists that are (barely) adequate for tile but definitely not good enough for natural stone.) The allowed deflection for tile is L/360 where L is the length of the joist. (Unfortunately I'm a little hazy on what the load is that you're supposed to assume when doing these deflection calculations. I think it's the live load---people walking around---that matters since that's what will make the floor go up and down and stress the tile installation.)

If you cut a joist then the floor for the entire length of that joist (possibly the entire width of your house) is weakened along that line and it will deflect more along that line. If "more" equals "too much" then you could get cracked tiles or grout. You could get cracked tiles at the other end of the house if this joist happened to hold up a tiled kitchen floor, say. If you do all the work and then something starts to crack later it's going to be VERY hard to fix at that point.

So the issue about cutting this joist raises questions about EVERYTHING which is above the joist, not just about stuff in the bathroom near where you make the cut. A careful analysis of the minimum you need to do if you cut out 7" of the joist depends on knowing how long the joist is, what the joist spacing is, and what it's holding up over it's entire length, not just about the bathroom you're working on. You need a fix which guarantees the L/360 deflection maximum.

Depending on what's underneath the joist you might be able to get an idea of how long the joist is. It has to rest either on the foundation or on a big beam of some sort. In my house the basement is partially unfinished so I can see what's going on. There is a big I-beam which actually rests on steel pillars that goes down the middle of the house. The joists span from the edge of the house to this I-beam. The joists can't just end abruptly.

[geniescience]

linda

yes or no: you need to cut the joist a whole lot because the drain is located right above it. Something like that?

Apart from the reason why you need to cut the joist, it is clear to me that you and others believe you need to cut it.

Thus the big question is whether or not cutting the joist is going to be serious damage to the structural strength of the floor. If you floor becomes springy or springier because of this weakened joist, you may get hairline cracks in the tiles or grout joints. It may rip the orange membrane and you end up with moisture rot in your wood structure.

Joists are better left uncut. Second best is to strengthen to compensate whenever you do cut them. Least best is to do nothing to strengthen and just cross your fingers.

So you need to cut the joist a whole lot because the drain is located right above it. Something like that? Yes or no.

info: Coquitlam BC is a 15 minute drive from the US border, in Canada.

david

[lhartl]

YES, I need to cut the joist a whole lot because the drain is located right above it.

Earlier posts suggested that I should cut the joist but more recent posts have me worried about the porcelain tile that I plan to put in the shower. I have no way of judging what is sensible risk management and what is overkill.

Two inch steel seems lke a big hit for a shower drain problem, so my only other alternative (that I see) is to abandon the Schluter shower kit for a mud deck and drain placed between the joists.

Linda :(

[leejosepho]

The Schluter shower drain is 2" and the old (existing) tub pipes are 1.5", so I must make the conversion somewhere in there ...
Do you think that I need to start at the SanTee with the 2"?

A transition from larger to smaller would have to be done vertically. To make the same transition horizontally would be like expecting a funnel to work sideways.

[leejosepho]

YES, I need to cut the joist a whole lot because the drain is located right above it.

No, the bottom of your drain fitting is a 2" hub, and a 2" x 1-1/2" bushing placed right there would accept a 1-1/2" street 90 to go sideways immediately and on into a trap connected to the nipple already available at your tee ... if that first 90 is at least just a little higher than the nipple at the tee.


What exactly do people mean when they say to build a box around the drain?

Here is a crude graphic: ----I I----

The dashes are the cut joist, and the top and bottom bars of the "I" used in that graphic are the neighboring joists on either side of the cut joist, with the vertical line of the "I" being a short piece of joist added at each cut end to pass the load along to the neighboring joists.


I can't figure out how this would work without compromising the strength of the joist.

The cut joist is no longer capable of bearing any load across the original span. And, therefore, the neighboring joists are now loaded more heavily than before the "box" was added.


I think the idea of bolting a sister joist in next to the existing one and then drilling out the hole in the center for the new drain is good for the structural integrity. The result will be stronger than what was there before.

Yes, it would, but a hole (up to a certain size) in the center of a joist would not actually weaken it in the first place.

[geniescience]

(too much advanced detail and information, leejoseph.) Linda needs less data and more overview right now.

Linda, it is the styrofoam tray. It's the tray that is the problem. You can keep the rest of the Kerdi kit. Linda, I don't want to go over to the JBF forums and see what they have been advising you.

B.T.W., you also have other options besides mud and Kerdi. Later, if needed.

Yes, you can off-center the drain in the shower pan. You can also keep it in the center, cut the joist and work around that in many possible ways that Adrian and Lee and others are discussing with you. The best person to advise you on-site is someone intelligent and experienced in home renovating. Not a repair plumber, and also not most plumbers who build new construction either. This joist thing is a structural thing.

Adrian already mentioned that it is important to know how long the joist is. Go downstairs and measure the distance between walls. All your parallel joists will be resting on the structure inside the two walls. Hope that is simple. Can you do it?

David

[leejosepho]

(too much advanced detail and information, leejoseph.)

Really?!


Adrian already mentioned that it is important to know how long the joist is. Go downstairs and measure ...

Hope that is simple. Can you do it?

Oh, come on!

The length of the joist is irrelevant -- a cut joist is a cut joist, and a joist should not be cut!

[geniescience]

i'm referring to the joist length in order to calculate the deflection. Nobody is going to deny that this is a good thing to do.

i realize that a lot of one joist has been cut already. Let's wait for the facts before advising on what to do. Joist length, of all the joists, or all except the cut one.

david

[leejosepho]

i'm referring to the joist length in order to calculate the deflection. Nobody is going to deny that this is a good thing to do.

I apologize for not thinking of that thought of yours prior to posting.

Overall, this shower (replacing a tub full of water) appears to be going into a corner of a room, and there are likely walls (or foundation) below those walls, thereby making the matter of a weakened joist or even that entire corner of the floor system less critical since a little tile is going to weigh far less than a tub of water. Nevertheless, yes, let us be sure the floor is quite solid!

[lhartl]

There is a major wall running through the main floor of the house at 15 feet. I am sure that this is the joist that we are looking at upstairs. This bathroom is directly over the laundry room on the main floor. The very next joist is above the wall with a door to the garage.



Linda

[geniescience]

.... at 15 feet.... the joist that we are looking at upstairs..... The very next joist is above the wall with a door to the garage. Not yet as clear as it could be. Sorry about that. Linda, all these things called joists are parallel to each other, and the whole set of them run from wall X to wall Y, and that is the "ensemble" we all look at to ensure that the floor is ready for tile.

If I understand what Lee said, and what you said, then your answer implies that the one joist is .. (sorry, I don't know what you have to say about it) but then it mentions another joist, close to a wall, and that is good news.

Your shower is in a corner and the joists are 15' long. Is this right? Please answer this, even though I am going to continue by writing out the next question.

Next question: have you told us somewhere how many inches the drain is from the edge of the shower pan? Your first and third photos show the drain close to the wall. I guess you are thinking of using the new model of Schluter shower pan. Until last year, Schluter pans had the drain the center (centre) of the pan. Now they have another option. But doesn't if give you some wiggle room? That is my third and final question.

david

[lhartl]

Hi David,

Yes the shower is in a corner.
Yes the joists are 15 feet long.
From the wall to the centre of the drain is 10" (exactly the same as a tub).


.... at 15 feet.... the joist that we are looking at upstairs..... The very next joist is above the wall with a door to the garage

I guess this wasn't very clear. Let me try again. The very next joist aligns with the wall downstairs that envelops a door to the garage. I hope that is clearer. I guess I see this as positive because it means we are extremely close to another major structure.:)

Linda

[jadnashua]

This is what we are talking about...cut the joist in the way, add a header on either side to hold up the ends. Joist hangers can be used. Depending, you might want to double up on the headers. Cut out enoughto do what you need to, but keep it as small as practical.

[lhartl]

No, there is no wiggle room because the drain lip goes almost to the edge of the foam tray and built into the slope. (I thought of cutting off some of the foam but there would be a steep slope to the perimeter of the wall.)

[geniescience]

Linda

If you had a couple inches play, would you use it?

If you cut the tray and a little bit of the large flat plate of the drain, the slope of the tray remains the same and you don't need to increase it just to align the tiles at the corners. The height differential would be about 1/16th". The "crease" line separating the flat sloped plates in the tray would not go totally into the corner, it would run up to, and stop at, the wall near the corner. In case you have a perfectionist's eye.

My asking you this question is not an indication that this may be the right way to go. I am asking if you would use the wiggle room.

david

[geniescience]

hi jim

Adrianmarino raised a big concern, and I think it is interesting to you too. Did you see his post from today? Joists are necessary and can only be tampered with after a serious amount of study. I'm sure you know more about this than I do. What did I miss here?

david

[jadnashua]

Boxing things out and putting in headers is done all the time. You need to do it for say a skylight, an opening into the attic, stairwells, etc.

You need to be careful when you do it - you might want to install some temporary bracing until you get the things back together.

Check with your building inspector. But, be careful, it will NEVER be accepted to downsize the drain from 2" to 1.5". To do this right, you really should replace that 1.5" line with 2" until it connects into something either the same size or larger.

Now, if you don't get it inspected, will it give you grief? Maybe. Drains should not be necked down - it is a likely location for a clog, and if you ever wanted to snake it, it might not work. If the inspector ever DOES find out you did work and it wasn't permitted and then inspected, they CAN make you show them it was done per code. This MIGHT mean you have to tear it all out. Then, you will likely still have to pay a fine, and still get a permit.

Sometimes, when you want to sell the house, if a problem is caught during the inspection, you won't be able to complete the sale until it is fixed if it is determined that it was done after the code changed (i.e., not grandfathered). Normally, any time you change something, what replaces it needs to be brought up to current code to the extent you made modifications. If it is simply a remove and replace and the part you changed was not impacted by code changes, you don't need to tear up more to make it right (although it might not be a bad idea).

Another thought, ask this over at www.johnbridge.com (http://www.johnbridge.com). They have a structural engineer that can give you some professional opinions.

[lhartl]

David,

Yes I would consider using the wiggle room if I thought I could do a good tile job and not compromise the slope. I am not sure that Schluter would support cutting the plate. The drain is a pretty integral part of the system. This may be better than starting from scratch and building the mud bed myself.

I have posted on the John Bridge site, maybe their engineer will have a solution for me.

I can see a larger pipe connnected to the SanTee so it is possible to do the entire drain in the 2" pipe. BUT, it appears to me that the 2 to 1 1/2" bushing and elbow may take up less vertical space (i.e. the joist) than a complete 2" elbow, although I haven't measured that out. I definitely want to do this right.

Linda

Linda

[jadnashua]

If you box it out with the headers, you have the entire height of the joist to work with in that boxed area...i.e., there is NO joist right there.

[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]

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

[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...:confused: .

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.