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My understanding is that the purpose of the overflow holes is twofold--

  1. To allow water to flow down you drain faster.

  2. To prevent overflow if the sink is filling faster than it's draining.

If #1 was true, wouldn't you also see the overflow holes on kitchen sinks?

If #2 was true, wouldn't you also see the overflow holes on bathroom tubs?

So--why do I never see overflow holes on tubs or kitchen sinks?

Edit: To clarify, my familiarity is primarily with sinks/tubs in the United States.

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    The tub usually has one integrated into the drain open/close hardware. – Tyson Aug 7 '16 at 21:44
  • Double basin kitchen sinks will typically overflow into the other basin. – Comintern Aug 7 '16 at 22:41
  • What country are you from? It seems odd to me for sinks and tubs to not have some kind of overflow. – Eborbob Aug 8 '16 at 12:07
  • Every bathtub I've ever seen has a drain hole. Look closer. It's precisely because a tub being filled is the primary use-case for an overflow. I can't say the same for kitchen sinks. But I'm accustomed to double sinks, where the bar separating the sinks is 1/8" lower than the lip around the edge, so each sink is effectively the overflow for the other. – Harper - Reinstate Monica Aug 8 '16 at 13:16
  • My guess (for kitchen sinks) would be some combination of: 1)overflow routing thru a DisposAll unit would be difficult, 2) kitchen sinks in common use rarely have the drain stop installed, 3) most kitchen water disasters are due to a plugged drain (Disposall or trap), so an overflow line wouldn't help. – Carl Witthoft Aug 8 '16 at 16:05
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Most sinks and tubs in North America do have an overflow device, it's simply cleverly hidden.

Bathroom sink overflows (which aren't always present -- ours lack them) are visible as North American bathroom sinks are almost universally single basin. However, North American kitchen sinks are often double basin -- and in a double basin sink, the divider doesn't extend up to the full height of the sink, so the two sinks use each other for an overflow. A rather clever design if you ask me, provided you aren't filling all the basins up that is.

As to the bathtub? There's usually an overflow hiding in the drain-stopper selector mechanism.

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  • I'm changing the answer to yours. I think my original question used flawed logic for a couple reasons. You correctly point out what I missed which is that overflows tend to be hidden in the tub and the kitchen sink (double basin). – spacetyper Dec 11 '19 at 3:19
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In the UK I've never seen a kitchen sink without an overflow. They're universal on bathtubs as well. While they may not get used much in common use, they do come into their own if you get distracted while running washing up water, and distractions are common in kitchens, especially if you're trying to clean as you go.

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  • How common are garbage grinders (Dispos-All, In-Sink-Erator, etc) in the UK? They're pretty popular in the USA – Carl Witthoft Aug 8 '16 at 16:06
  • @CarlWitthoft. I've seen them but rarely. Even when fitted they're not widely used by subsequent owners. Do they interfere with overflows? – Chris H Aug 8 '16 at 17:04
  • Chris, in theory an overflow could be routed to the output side of the grinder. But again, that's a chunk of plumbing (pipes, unions, etc) for very little payback. – Carl Witthoft Aug 8 '16 at 18:46
  • @Carl that would seem reasonable. I've seen a separate pipe connecting the overflow to the trap so it wouldn't be hard. – Chris H Aug 8 '16 at 19:02
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I suspect this is simply because kitchen sinks are rarely operated by two-year-olds, and hence do not need training wheels.

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  • Also, in a similar vein, so I won't make it a separate answer: since the tub takes a while to fill, you may not stand there watching it the whole time. You are less likely to walk away from the kitchen sink while it's filling, I usually stand there doing some preliminary washing. – MAP Aug 8 '16 at 2:15
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    Also, it isn't uncommon to fill the tub, then climb into it, raising the level... Possibly enough to be an issue? ... Which doesn't generally happen with the other kinds of basin. – keshlam Aug 8 '16 at 3:01
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    This is not helpful. Further, the point of overflow drains is to avoid disasters, not to deal with the putative existince of children. – Carl Witthoft Aug 8 '16 at 16:02
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    Point was merely that children are the usual cause of such disasters; adults are capable of anticipating and preventing them. But if you feel it isn't useful, that's what downvoting is for. (Personally I don't find the question especially useful, but I don't object enough to downvote or vote to close.) – keshlam Aug 8 '16 at 16:48
  • I'd like to see statistics on how many overflows are in fact caused by any given age group. I rather doubt that small children are the prime source of such. – Carl Witthoft Aug 8 '16 at 18:47
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My guess (and that's what it is) would be that overflow passages are known to be unsanitary. In an area intended for food preparation, the cultivation of mildew and bacteria would be a more serious concern, where it isn't so much of a concern in handwashing sinks and bathtubs.

This article seems to support my hunch. It also suggests simple economics, as U.S. codes don't require kitchen overflows.

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I don't know many people who shave in their kitchen sinks, I do know a lot of folks that fill their sinks with water when shaving. I also know a lot of folks that fill their tubs, which is why overflows are also typically found on tubs. Some folks also fill the sink when washing up, in an attempt to waste less water.

"But I fill my kitchen sink to do dishes", you might say. That might be true, but the kitchen sink has a much greater volume. So you're less likely to fill it to the point of overflowing. Also, I'm pretty sure kitchen sink overflows were common back before dishwashers (but I could be mistaken). And if you have a double basin sink, they're typically designed so that the basins can overflow into each other.

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It all about profit. It cost more to make a sink with an overflow drain. I am glad to hear that they are on all sinks in Europe.

They are very much needed on all sinks. And, this nonsense about it being unsanitary to have one in kitchen sink is just not true.

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