View Full Version : Why 2" drain for shower

04-15-2007, 09:51 PM
I'm working on remodeling a small master bath. I removed the cast iron tub by drilling a series of holes cross-wise across the middle & then breaking the tub in half with a sledge hammer. I'm begining to doubt the feasability of installing another exact replacement cast iron tub because the length of the tube is the width of the room (alcove design). Anyhow, to the question at hand. The tub was plumbed with 1-1/2" drain. Redoing the drain in 2", if I want to convert to a shower only, looks next to impossible. The bathroom is on the 2nd floor & the drain plumbing runs into a false wall cavity. If the 1-1/2" drain was OK using the shower in the tube, why won't it be OK if I use a shower base instead? Comments & suggestions welcome.
Thanks, FJK

04-16-2007, 02:34 AM
I have wondered about the same thing: Why would a smaller-capacity vessel -- damp pan versus tub of water -- need the larger drain?

04-16-2007, 03:51 AM
I have wondered about the same thing: Why would a smaller-capacity vessel -- damp pan versus tub of water -- need the larger drain?

Because the smaller capacity can't serve as a "holding tank" waiting for a slow moving drain.

There also seems to be little updating of codes relating to drainage to reflect flow limiting fixtures. Perhaps because many restrictors are so easily removed.

04-16-2007, 07:46 AM
my view: Code sets standards to account for hazards and danger. Sometimes Code has to set minimums for things that are not all that serious after all. Perhaps in some future decade, century or millenium, there will be two different documents covering the same as (or more than) what Code now covers: the first one covering hazard / danger / safety concerns, and the second one covering permissible calculations allowing one to build options for non-crucial work-arounds.

That being said, let us now look briefly at the diameter of the pipe that takes shower water away from the shower floor and carries it downstream to the stack. Did you know that Code used to set that diameter at 1-1/2" (1.5") ? That minimum was raised to 2" at some point and this has now become the "norm" everywhere in the US and Canada and perhaps in a few other places that I don't know.

To go back to first principles analyzing why 2" now and why 1.5" was once deemed acceptable, can be a short or a long discussion depending on how much depth you want to dig into the issues.

Carrying capacity of pipe is the short answer.

If your 1.5" pipe alreay in place has the right slope (and not too much slope, i.e. steeper) and if the distance is short enough before the first Vent, then its internal capacity to exchange air and water will be as good as the original (1.5" shower drain) Code called for, and it'll work well BUT it is very likely that no-one will want to leave any trace of anything hinting at a clear recommendation that you do anything regarding this well-installed 1.5" drain pipe.

Not a plumber, I looked into this same subject for many years. I renovated my own two bathrooms. In one bathroom a Master Plumber who I hired installed a 1.5" pipe and P trap (instead of a 2" which I had specified and which he had accepted to do); he claimed that for the short distance involved it would work fine, and that he didn't find the necessary space to put in 2" fittings and pipe.

:) Do not ask me now whether "Xx distance is short enough" so that you can safely .... since three variables, 1. slope, 2. venting and 3. distance, all combine to make this situation a bit too complex to talk about unless A. you are truly competent and B. the person responding is truly competent. I think a Master Plumber may be professionally accredited and allowed to make some judgement calls based on serious measurements and knowledge of the interrelated constraints, as well as a licensed Contractor in some places, as well as a homeowner in most places.



04-16-2007, 08:14 AM
This question is mentioned in an article in the Mar/Apr issue of "Official", the magazine of IAPMO. They are actually studying a possible revision or exception to that code. The reason is our "aging" society. They see that more and more, there is a need to remodel a bathroom to make it accessible for disabled and elderly. Allowing a shower to be installed on an existing 1" tub drain would simplify this task. Lots of discussion to take place, but apparently there is some inclination to accept the fact that a 1" line can drain a shower, and if people want to go there and accept the risks and responsibility, then why not? I expect the 2" requirement would remain for new construction, because the logig is basically sound.

04-16-2007, 09:43 AM
Geniescience, reading between the lines of your response, I think I got my answer. I assume your remodel job (bath to shower?) worked fine or else you would have commented differently.

Jimbo, yours was an interesting observation as well, ... showers for the elderly, yeah that be me, real soon.

Thanks everyone who replied. May have more questions later.

Regards, FJK

04-16-2007, 09:44 AM
Saying it in another way...the area of a 1.5" drain opening is 1.77 sq in, a 2" drain's area is 3.14 sq inches - nearly twice as large. That small increase in diameter makes a big difference. A shower curb is often not that high. Standing on the drain, dropping a washcloth, etc. can quickly fill up a shower to the height of the curb if you aren't watching, and spill water out into the room. A tub has an overflow, so if everything is working right, you can't overflow the tub. You could create some big waves maybe, but not cause it to overflow. So, being able to drain the shower even when the drain is partially covered (or just faster when you realize you are creating a pool) as the result of the larger drain line is a big plus.

04-16-2007, 01:29 PM
Because the smaller capacity can't serve as a "holding tank" waiting for a slow moving drain.

Yes, got it! That hit me a few hours later, but I had not also thought about the foot-flaps Jim mentioned.

04-17-2007, 06:52 AM
yeah you got it FJ.

DIY folk ought not to be told how to work around code. Great risk they'll end up doing the wrong thing. Also, leaves a permanent record that can haunt the poster for all time to come. And, a hundred thousand future lurkers will read about work-arounds, short cuts, variances etc all of which may not be good for them but could embolden them to do the wrong thing.

A scupper drain is ideal to get water out of a pan without risking blocking the drain with a big foot or a cloth.

A bathroom floor drain outside the shower is a good way to add insurance (i.e. redundancy) to your bathroom drainage.


04-17-2007, 01:10 PM
DIY folk ought not to be told how to work around code.

Why do you single out DIY folk -- Do you know? -- and why do you even mention that at all?

Unless I have missed something, that is not done here on this board for anyone.

04-17-2007, 07:23 PM
Because DIY'ers are the ones trying to do it the "easy way", and often, if not usually, rankle when they hear that the code does not permit it.

Dan Pick
04-18-2007, 06:51 PM
I wouldn't make the statement that DIY'ers are doing it the "easy way". The easy way would be a call a plumber or contractor and have him do it. Give him or her all your money and take what he provides as quality work.

Most DIY'ers are looking at a cost effective alternative to having a professional. Usually I can upgrade the product and do a great installation by knowing 100% what I'm doing before hand.

Thats why forums like this are so great for an DIY'er to get free advice from a seasoned professional.

04-18-2007, 07:00 PM
Yeah, some DIY's might look for the easy way, but I always aim for the perfect way, minimize the oops, and always try to get as close to perfect as possible. I know one thing about myself, if I have to ask for guidance, I'm not smart enough to "cut corners".

In this case, I'm really puzzled with the logistics of moving & installing a 330 lb tub. The shower deal might not be the alternative I was hoping for. This is a small bathroom. The tub is 30 X 60. Shower platforms (Bases) I was considering were 36X60. The extra 6" probably will put me on the edge of the toliet.

More suggestions welcome for either tub or shower. I just want something that is first class looking & durable.

Thanks everyone,

04-18-2007, 08:27 PM
Go to www.schluter.com (http://www.schluter.com) and look at their Kerdi shower system. They have a pan made to replace a standard tub with the drain at one end, or if you can move it easily to the middle, in the middle. 30x60 if I remember. Note, the curb is fairly wide (this is in addition to the width of the pan), but you could build a narrower one if you didn't want to use theirs.

04-18-2007, 08:35 PM
post a diagram showing the current layout, including the toilet.

04-19-2007, 02:45 AM
DIY folk ought not to be told how to work around code.

Why do you single out DIY folk ... and why do you even mention that at all?

Because DIY'ers are the ones trying to do it the "easy way", and often, if not usually, rankle when they hear that the code does not permit it.

So then, and for as long as he was not merely trying to do something "the easy way", it would be okay for one pro to tell another how to do a work-around?

Codes are meant to keep professionals in line, not homeowners.

Or to say that more clearly: Codes are meant to keep professionals in line for the sake of homeowners and other customers, not to keep homeowners in line for the sake of professionals or to force them into becoming customers.

04-19-2007, 04:27 AM
hi jlo

let's not get sarcastic and cynical on someone else's thread. Let the thread originator have his original topic back.

start a new thread to discuss a new topic.
e.g. opinions vary on what Code is for
e.g. why treat DIY differently when they ask
e.g. make up your own


04-19-2007, 01:00 PM
hi jlo

let's not get sarcastic and cynical ...

There was no sarcasm or cynicism there.

... on someone else's thread ...

start a new thread to discuss a new topic.

You had raised the issue with editorial comment having nothing to do with the question at hand, and since there is no specific place for either that or for my own related comments following, these kinds of friendly exchanges occasionally appear in one happenstance place or another.

04-22-2007, 09:56 AM
I am as my screen name states, a serious DIYer. I get aggrevated when reading things from the professionals who are jealous of homeowners who have the same skills that they have. They think that we are just a bunch of cheap skates who would prefer to half-a** something than over pay one of the professionals. I completed a bath remodel a year ago that took the bathroom down to studs and floor joists, added a tile floor shower with a mortar pan, a corner whirlpool tub with an inline heater and custom tiling throughout. a 5 month project for someone who works 50 hours a week in the manufacturing business. I complete everything to code as it would cause serious problems and more than likely more costly repairs later on if I were to sell my home. You see, I called 2 different professional plumbers in to give me an estimate at plumbing in the new 2" drain and hot/cold water feeds dor the new shower. The 6 inch main waste was 5 feet from the new drain location. Nothing rally difficult other than they would have had to get into a closet in the basement that holds my evacuation pump to make the connection of the new 2 inch waste line. After receiving a $2,400.00 and an $1,800.00 quote, I went to my local home center, purchased everything I needed for about $120.00 and completed the plumbing myself in 4 hours. I don't see where ther was a need to pay $1,800 - $2,400 for 4 hours labor and $120.00 in pvc and copper. So you see, if we, the DIYers can save the extrodinary amount we pay for the labor and the costs to cover the insurance payments for the professionals, why is it that we are considered incompetent, cheap want to be's. Sorry if this insulted anyone, but if the shoe fits!

04-22-2007, 10:12 AM
FJK the remodel I just completed in the above comment about DIYers was completed in a bathroom that was 9' x 6'. I was able to get a corner whirlpoll, a 5' x 3' shower and a large vanity with upper cabinets all in this location. You just need to sit down and determine wether you want the tub, the shower, or if feasable, both. You can, if designed correctly, fit a lot into a little space, making the space both efficient and useful. Granted, I can't dance in the bathroom, but I have used all available space effectively. Can you maybe make your own shower base? You could custom fit it into the space you have. I found a great sight on building a shower pan from scratch, see: http://www.ontariotile.com/preslope.html for complete details.

04-24-2007, 12:49 PM
hi FJK

just for the record, i'll say I probably spoke too casually in my post far far above where i named DIY folk instead of speaking more precisely; (then again, it maight not have helped to name all the other categories like handymen etc. :) )

As you and I just corresponded on another thread, about two traps in series (not doing it) and about (not) reducing drain pipe size in the middle of a run, I'll add here now for the record that I don't intend to represent code or the enforcers, and i do agree with the point of view that says that a few of the articles in the code are more like best practices standardized rather than true safety issues. It is hard to discuss these ones in public. We need an open discussion forum for that. That is why I came here.

No-one will give a blessing to a 1.5" pipe for a shower, but some will discuss the factors that make it work as drains are expected to work. Slopes, venting, distances, etc.


master plumber mark
04-24-2007, 05:13 PM
it will work ok, a 1 1/2 drain for a shower..

it is just not desired ......

if all you got to work with is 1 1/2

without tearing up the whole house just to make it bigger

it will work..... the hair might or might not clog up quicker in 1 1/2

Now if you can make it 2 inch in the shower pan

and then reduce it down to 1 1/2 somewhere down the liner aways it will work....

sometimes you got to do what you got to do...

to win....

in my very own home I had a terraza 36 inch shower drain on a concrete slab since 1964 that was only an 1 1/2 drain and trap and I did no tknow it till I tore the whole thing out of the slab to install a whirlpool tub...

so it will work.

04-30-2007, 09:54 AM
Sorry I haven't got back to the forum for a while.... So, given everything I've read so far, how does this all get pulled together if I replace the tub with just a shower and use a 1-1/2" drain instead of 2"?

The existing tub drain & overflow get removed, the respective t-fitting for the preceding, and finally the slightly downstream trap. Now, considering I have not seen the shower drain hardware, I assume it is designed to plumb to 2". So, coming off the drain for the shower base, what size trap is used? Then, if there is some portion of 2" off the shower base drain, when do you transision to 1-1/2"? Or, is there simply a 1-1/2" base drain you start with?

Thanks, FJK

04-30-2007, 10:43 AM
this is a delicate situation here. No matter what anyone says here, the advice is all geared to doing the opposite of what Code calls for; furthermore you are a true DIY and some of your questions and descriptions show you lack some basic knowledge, not a big thing, but it makes the situation all that more delicate... So it is hard to know how to respond, in a public forum. Many reactions possible.

Some will say you shouldn't reduce a drain line at any time, so best is to go with the existing 1.5" P trap you already have in place. Others will suggest a new 2" P trap, then either a reducer or a street elbow (search on those terms) to connect into the 1.5" pipe. "Who knows?" said rhetorically is the best answer you'll get. This may be the right time to hire a Master plumber; let him make the judgement calls necessary.


04-30-2007, 02:20 PM
... considering I have not seen the shower drain hardware, I assume it is designed to plumb to 2". So, coming off the drain for the shower base, what size trap is used? Then, if there is some portion of 2" off the shower base drain, when do you transision to 1-1/2"? Or, is there simply a 1-1/2" base drain you start with?

If you are going to stay with you existing 1-1/2" drain line, it is not going to make any real difference how large or small anything connected to it might be. In other words, the 1-1/2" line is going to pass the water from your shower no matter how that water is directed there.

Personally, and even though I prefer the larger trap, I would use the 2" shower drain and immediately reduce to 1-1/2" (either vertically or horizontally) before going into an 1-1/2" trap and on into the drain line so you do not end up having a horizonal reduction from 2" to 1-1/2" where the larger trap would otherwise be connected to the smaller line.