Shower drain size

Old 03-28-07, 07:28 AM
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Shower drain size

I'm helping my neighbor redo his 2nd floor master bath. He's removing a tub/shower surround and replacing it with a separate shower and tub.

The existing tub had a 1 1/2" drain. My understanding is that a shower requires a 2" drain (from a table in a DIY plumbing book). We were ready to change the size of the drain line to 2" when we found out the old tub had a drain separate from the main stack. It is a 1 1/2" pvc pipe that drops straight down to the basement inside an outer wall. In the basement it ties into a long run of 4" drain. There is no way we can replace the 1 1/2" without some major demo.

My questions are - is a 2" drain required by code, if so why the difference between a tub and a shower? Is a 1 1/2" drain adequate for a stand alone shower? The shower base he bought requires a 2" drain, but we can use a reducer.
Old 03-28-07, 11:07 AM
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Code requires a 2" drain for a shower. I suppose that is so you never will be standing in a 'pond' of water. You take it from there.
Old 03-28-07, 04:42 PM
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This really depends on your local plumbing code.

If you have the International Plumbing Code as the code adopted by your local jurisdiction, you can reduce the shower pan drain to 1 1/2" by citing table 709.1 (IPC 2003). There is a requirement of the fixture outlet having the same size trap, but, BUT, table 709.1 makes allowances for this.

The reasoning behind this is that the original drainage fixture units was developed by an arbitrary scale. The lavatory was determined to be about 1/6 of of the requirement of a toilet. How does this relate to a bath tub and shower. A drain size of 1 1/2" drains a bath tub "rapidly" which is code talk for fast. The older codes required a 2" drain on showers to fulfill the "drain rapidly" code requirement. However, the reasoning now follows that water saving devices are used on tubs with or without overhead showers or a single head shower can use the 1 1/2" trap and fulfill the code requirement (IPC 1002.5) of "draining the fixture rapidly".

Hope this helps.

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