What is your plan for a water heater to actually supply that GPM for longer than 5 minutes? Your typical 40gallon tank ain't gonna cut it.

I have a similar shower, I built from scratch so went with a 3 inch drain that can handle 50gpm of intermittent water but a two inch drain is meant to handle 30GPM. I was worried about hair hindering flow on a small drain so went bigger. How is your water pressure? Will you have an open 3/4 pipe discharging the water or will it pass thru a shower valve?

I have on demand hot water and an indirect tank gas boiler operated system and have never run out and we take three,multiple headed, showers simultaneously at times.

Sophie Wheeler, I live in a condo high-rise so the water heater isn't an issue.

roarah, water pressure is pretty good. I plan on doing a Kohler DTV Prompt setup (K-557-K-NA three-port valve, and a K-558-0 interface).

Tundra Finish Works LLC, thanks for help!

Pitch of the shower drain has a lot to do with the drainage and important factor is to have the air above the water as it drains.

Usually 2" drain @1/4" pitch can handle 10 GPM. If the pipe pitch is 1/8" it will handle around 8 GPM.

With that said, you shouldn't have any problems considering you have a good pitch and proper vent system in the building. 2" pipe is used 90% of the time for any residential shower custom or not.

GN Builders L.L.C, Awesome, I appreciate the detailed info!

If you're concerned with building/plumbing code compliance, a 2" drain can receive the output from three shower fixtures. You have five. Code doesn't care if only a few of the heads can be on simultaneously. To be compliant, for those five heads you'd need two 2" drains or a single 3" drain.

Great advice by both MongoCT and GN of the above posts and I Eat my words, I have had this same situation pass with 2" point drain with 2 jets totaling 4fix.

MongoCT is that code for everywhere? I'm in San Diego, CA. My plumber says this shouldn't be an issue? So confused. :(

Tundra Finish Works LLC thanks again!

It shouldn't be an issue, most of the time you cannot install a 3" pipe for shower drain with 2- 2" drains branching off of it, there is not enough room in the floor cavity.

nyc_designer-

You're in San Diego, CA, where water use is a big issue and some restrictions are in place, and the permits department is ok with a shower that can use 11 gpm? I'm surprised and curious. Can you provide more info?

You will be restricted to a single 2.5 gallon fixture used at once. While you can have a dozen different fixtures if you want that many transfer valves and piping, you will only be able to run one at one time.

"MongoCT is that code for everywhere? I'm in San Diego, CA. My plumber says this shouldn't be an issue? So confused. :("

This is not always easy to figure out, especially in California. CA has always had water use restriction proposals and code requirements that are more stringent than national code. They've had proposals stacked upon more proposals, I do;t live out there so I'm not certain what actually made it into the code books.

With that written...

There is the basic IRC, which is the residential code that governs most of the nation...and understand that it is NATIONAL code. Then there is State code, state code can have additional requirements (seismic for example) and it can be more restrictive than national. Then there is local code (country or city/town), local can be even more restrictive than state. But for the most part, local and state code can not be more lax than national code. I hope that's understandable.

As far as IRC (International Residential Code) goes, a shower head is
assigned a value of 2DFUs (drainage fixture units). If the shower has more than one head, each individual head is assigned a value of 2DFUs. You then add up all of the DFUs in the shower and that allows you to calculate the required size of the drain.

Code does not care if only one head can be on at a time, it doesn't care about diverters. It only cares about the total number of water outlets in the shower. In your case, you have a total of five outlets, for a total of 10DFUs.

Now we go on to drainage. Code requires the shower drain to be a MINIMUM of two inches. A 2" drain can is assigned a value of 6DFUs. By trying to feed 10 DFUs of water down a 6DFU capacity drain, you've exceeded the capacity of the drain.

To get back within compliance, you need to up the drainage capacity. You can add a second 2" drain that would give you 6+6 for a total of 12DFUs of drainage capacity, or you can go with a single 3" drain which has a capacity of 20DFUs.

With all that written? If California is restricting shower heads to a lower GPM value per head, then they may have also adjusted the DFU value assigned to each head. example, they may say the first head counts as 2DFUs and each additional head counts as 1DFU. But I have no clue. Your code officer will be most up on your local code. Your plumber? I certainly hope he's aware.

You're in California, you mention using two 2.5GPM shower heads. I believe CA just restricted shower heads to 2GPM effective just this summer. Maybe July? I know it was a proposal, but you might want to ask your code officials.

CA had some other shower restrictions regarding multiple head showers back in 2013. I was able to find this old thread where I posted them a few years ago. Again, this was a proposal. I don't know if it was ever enacted. Or if it is being enforced.

Proposed 2013 Restrictions

I know I wrote a lot, don't let any of it scare you. I live on the east coast. Talk to your code office to get the official scoop.

nyc_designer-

If you haven't already, you may wish to speak directly to the code department that will be doing the inspection before you spend a lot of money on this project.

I called the city a month ago, detailed what I had and wanted to do, and they said I didn't need a permit.

To clarify, I would be using the CALGreen approved Kohler controller that limits one head on at a time. I would like to have the option to use multiple at a time in the future if/when that's OK.

Wow! I 've never lived in a city or town where that level of plumbing work didn't require a permit. But it sounds like you're in the clear, so good luck.

Don't kid yourself. CA code inspectors tour around to catch unpermitted renovations. If you do that project, and your neighbor's turn you in, you will be paying a BIG fine.

You, you need a permit. Any multi family dwelling required permits to touch anything behind the walls.

I'm not "kidding myself" -- this is literally what the city told me. But I will check again...

If it's a direct replacement and you will not be moving vents or drains, many towns don't require a building permit.

With that said, being this a building, some building associations will require HO get a building permit and provide proof of insurance and that a licensed contractor will be doing the work.

If the city told you that you don't need a permit, you don't need a permit.

Good luck

Yes, definitely getting HOA approval. They are VERY nitpicky about even minor aesthetic changes.

Thanks again for the help everyone! It's greatly appreciated.

I continue to be curious on why the city told you you didn't need a permit since you must be opening up the wall and redoing the plumbing inside it. When I've had to get permits in the past, the deciding factor has always been: Are you adding to or modifying the existing plumbing in any way? Swapping out one showerhead for another, or replacing a toilet doesn't really count, but cutting in to piping and doing things that will later be covered does.

When you called the city for permit guidance, did you tell them you were in a multi-unit dwelling? That often makes a difference. Also, have you run this by your condo association?