Can anyone explain to me why a shower requires a 2" waste?

I'm converting an OLD bathroom (cast iron claw foot tub - original) to a shower enclosure and find the waste line is 1 1/2". It's well vented, but, looking at the code, a Shower requires a 2" waste even though it's only rated at 2 DFUs. Can someone please explain this to me?

To me, the shower has FAR less flow than a full bathtub with the plug pulled (especially given low flow rate shower heads). Why on earth does it require a 2" drain???

Tom

In Canada, a 1-1/2 is still ok.

I see a lot of new showers with 6-10 spray heads.

Low flow- HA!

Most codes in the US like to see a 2" shower drain.
Until the 50's, we used 1.5" for showers. I don't think it's a big deal either way. If I'm retrofitting a shower where a tub used to be, I go ahead and use the 1.5" waste line. If I can bump it to 2" easily, I do.

New work, 2" every time.

I think in many cases a 1 1/2" drain line is fine. The size of the drain line is only one factor in the amount of water a shower's drain can handle.

Weither or not the water drops straight down or slides out at an angle can affect flow rates.

Also the shear drop into the drain as well.

For Example:

A 1 1/2" drain line with 7" off free fall will drain faster than a 1 1/2" drain that has 4" of fall and angles off at a 45 degree angle.

I have measured in the field slower flow rates when fittings are slightly out of wack. My thought here is that little eddies are created slowing the flow or choking the air these drains need.

Before getting to committed to your new drain line do a dry run. Test the drain and see if it can handle the flow rates. You do not want to start tiling and find out that the shower's drain is undersized.

I have some of my favourite drains listed here in an Idea Book.

http://www.houzz.com/ideabooks/3650734/list/How-to-Choose-the-Best-Drain-for-Your-Shower

JW

A tub has an overflow, a shower has the curb and it flows out onto the floor. Plug the drain by dropping the washcloth over it, or standing on it, and the level could rise a bit before a big mess...a bigger drain can resolve the issue better. Cross section is 1.5^2 verses 2^2 or 4/2.25 the 2" drain is 1.77 times bigger...not inconsequential.

You can fill the tub with water and let it drain as fast as it can. If it does not drain as fast as the shower fills it, then you shower in a pool of water. With a shower stall, you CANNOT fill it very much, and if it does not drain fast enough it will overflow onto the floor. THAT is the reason for a 2" drain, regardless of what the FU, (which is an arbitrary number with no relationship to the flow of the shower head), rating is.

Great info John . To summarize it " It isn't the size of Your drain, but how You use it " GOT IT !

Boston1 said:

Great info John . To summarize it " It isn't the size of Your drain, but how You use it " GOT IT !

Click to expand...

Boston1 I wish I had more data for you on this. This info was shared to me by one of my drain suppliers as he was skirting the issue for why his drain was air locking. I went home and tried to see if the drain would lock but could not create the same environment.

I did find that the water level was higher in my clients home (where the 1 1/2" line angles off) vs my home's (my wife's laundry room aka mad lab) mock up.

Often the 1 1/2" lines we work on are in condo's - there we have sometimes 7" - 10" of free fall before the P-Trap. Never once a problem with flow rates here.

In the residential homes where we are building curbless showers often the free fall is 4" (2"x10" framing) and here you need to be very careful.

I have started measuring this as we build and then going back and measure water flow with a timer and 5 gallon bucket. Hopefully in time some kind of useful formula shows it's self.

In renovations that have multiple shower heads you really need to see how much water they can produce. When we pressure test the lines I ask my plumber to hook up two hose bibs and we simply fill a bucket. If the flow rate is in excess on 6-7 GPM then we send the water down the drain. Anything more than that should have a 2" line.

All the new fixtures have restrictors build in so most times it is a non issue. But rumour has it clients hate these and call the plumber back to rip them out.

JW