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In the US, code dictates that the drain (p-trap and arm) on a shower use 2" pipe, while on a tub you use 1.5" pipe.

I understand that is code, but I am curious as to why it is the way it is. Sometimes code just is what it is, but I was wondering if there is a physics answer to this question. Is there a reason why a shower is larger from a performance perspective?

And, is there any harm in using a 2" drain pipe for the tub as well?

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  • water gets sucked out of the tub by siphoning action ... the shower does not have enough flow to start siphoning
    – jsotola
    Commented Aug 31, 2019 at 6:05

2 Answers 2

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Tubs are big and intend to hold the water; drainage speed is not really important. Showers on the other hand have a low curb and can’t hold much water, so it is important that there is plenty of drain availability to prevent overfilling and flooding.

Some tubs do have 2” wastes, but I think smaller drains are more common as it reduces the possibility of completely using downstream pipes via multiple upstream fixtures draining simultaneously.

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  • That makes sense!
    – DA01
    Commented Aug 31, 2019 at 16:37
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From a performance perspective a slow drain in a tub means it takes a long time to empty. Slow drains in a shower mean that the water can end up sloshing over the curb while the occupant is doing their hair belting out Van Halen's greatest hits. But I suspect the actual answer is that there are way more multi-head showers than multi-tap tubs.

Finding a 2" bath waste might be difficult. Technically you could use a standard one and increase the diameter between the waste and stack, but that would seem to have zero benefit for the overwhelming majority of cases.

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  • 2
    Also, showers do not have an overflow pipe, like tubs. +1 for good answer which includes VH reference. All my future answers will contain a VH reference, maybe hidden or only noticeable by true fan... Commented Aug 31, 2019 at 2:43
  • A very good point. In fact, our shower is going to be a double head shower. I hadn't considered that aspect.
    – DA01
    Commented Aug 31, 2019 at 16:37

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