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Kitchen sinks: why no overflow protec...

Hawk's picture

Why don't kitchen sinks have overflow drains like bathroom sinks?

Seems like the safeguard against overflow is equally imprtant in both locations.

(post #172547, reply #6 of 41)

While we're on this subject, I have a question:
Can one vent the dishwasher air gap outside instead of the kitchen sink?

(post #172547, reply #9 of 41)


The air break in the dishwasher drain is not an overflow device. It is an antisiphon/antibackflow to prevent the disposal from spewing chunks in the wrong direction. Intelligent inspectors will recognize this and allow installation anywhere as long as the height is above the sink rim and will not freeze. The sink usually had a hole to fill as developers were not spending the extra $ to install faucets with sprayer hoses.

BTW, looping the dishwasher drain higher than the trap used to be the accepted method, but guess what! The lack of overflow in the sink would send the water back into the dishwasher......

Cheers; JE

(post #172547, reply #11 of 41)

John-Thanks-you are telling me what I already know but am not sure what an inspector would let me get away with. I am putting in a granite counter and just hate putting any more penetrations in the slab than I absolutely have to. Here in San diego, freezing is not an issue. I know that the dishwasher air gap is meant to prevent backsiphoning into your domestic water supply, so I reason that as long as it is above the sink rim and won't flood the house, it should be ok.

(post #172547, reply #12 of 41)


I live up the coast from you in Seattle. The inspectors here use to allow the airbreak to penetrate the wall. What about mounting in the backsplash?


(post #172547, reply #13 of 41)

But, if I recall correctly, if the drain line behind the air gap is plugged (e.g., a stopped-up disposal), the waste water is returned out the small hole in the air gap.

Not an occurence I would want to see in MY closet -- at ANY height.


(post #172547, reply #15 of 41)

I think I have the answer to the overflow in the tub question. And I've given a hint in my name.


(post #172547, reply #17 of 41)

'Round here (we're out West ya know) the plumbers strap a cu riser to a stand pipe (this is what I call a vert pipe on top of a p-trap, like you might find behind a washing machine), the stand pipe is as tall as it can be under the sink. A notch is cut into the top of this pipe and the cu is directed into it with a couple of elbows at the top of the stand pipe. This provides an anti siphon gap and doesn't require a penetration thru the sink/counter. All of this is exposed in the cabinet, under the sink.

(post #172547, reply #20 of 41)

Well after reading all the replies, I can't resist asking --why don't toilets have overflow protection? Seems that would be a property damage and sanitation issue at least as important as these others we're discussing! Also since the average homeower doesnt seem to know how to stop an overflowing tiolet from running, but most know how to turn off a faucet at sink or tub ....

(post #172547, reply #22 of 41)

DScott-You're right, the sink rim has nothing to do with it, itonly provides a convient location w/ which any overflow can take place.

In respect to what the air gap protects, it protects from waste backflow into your domestic H2O. In the absence of any backflow preventers, all that is necessary for waste backflow is a higher pressure in the dishwasher than the domestic water supply. If your waste drain on DW was clogged, DW filled and this differential pressure existed, you would back siphon into the line and contaminate your domestic water.

I've never seen an air gap under a sink, but if that is allowed, why not place outside (presuming no freeze problems)? And how often have you ever had to service an air gap?

(post #172547, reply #25 of 41)

I can't help but ask... As far as bathroom sinks are concerned. IS IT REALLY FOR OVERFLOW PROTECTION??
The openings in cultured marble sinks are so small as to only protect against a very low input flow. If my toddler turns on the faucet more than halfway..we have overflow. It is my understanding (feel free to correct me..) that this hole is to allow air into the drain line when you open a full sink (i.e. the drain is covered in water) to allow smooth, noiseless draining. Ever try pouring milk fast from a full one-gallon milk jug??

I'm going home tonight and try to drain a full bathroom sink with my finger over the 'overflow hole'.

(post #172547, reply #27 of 41)

Almost True.. You need a long pipe attached to the opening of the milk jug. There is 2-4 feet between the sink and the vent. As water rushes through this pipe a vacuum is created. How might this require additional venting at the sink end. The stopper does not allow enough water into the drain to completely fill the dia. of the pipe (would use of plug act differently?).

The point I was getting at is the gurgling effect you get as air gets back into the milk jug. If the milk only covers half way the air can get into the jug and you have 'smooth' flow. If the milk covers the opening you get a 'glub-glub-glub'.. effect.

I still think is this hole is strictly for overflow, they bettter start making it bigger.....

(post #172547, reply #33 of 41)

Many, if not all English and other European kitchen sinks have overflows. You will find them in the imported sinks sold by A-Ball, Renovators, Waterworks, etc. What do they know that we don't know?

(post #172547, reply #35 of 41)

I'm thinking that a person would be immersed into a full bathtub of water and need the overflow.

A kitchen sink is either full of dishes first and then water is added, or water first and then add a few dishes at a time. Thus the kitchen sink doesnt need the overflow protection.

A bathroom sink might be used to bathe a baby or to wash/soak some delicate clothing items. So an overflow is needed.


(post #172547, reply #37 of 41)


This has turned into a very interesting thread of messages on why kitchen sinks (usually) do not have overflow devices. The messages over at are equally amusing.

The air gap or "Johnson Tee" (what a fitting name!) is in place to prevent a plugged drain or a draining sink from backflowing (siphoning) into the dishwasher.
It is not intended to prevent grey water from accidentally entering the supply water. ALL dishwashers have a residual amount of drain water due to the lower positioning of the pump versus the exit of the drain line. That's why dishwashers have a pump! The proper installation of the air gap is ABOVE the rim of the sink. This assure a plugged drain (probably due to someone making coleslaw in the disposal) will back flow into the sink and the floor if it goes unstopped.

This is the same reason washing machines require the airgap. Not because the plumber undersized the drain, but to prevent a siphoning of the waste water. The pump is lower than the drain line exit point. DUH! or The reason a pump is needed is because the entry into the sewer is HIGHER than the lowest portion of the washer tub.

Does this help?


(post #172547, reply #38 of 41)

O.K., whatever, the place I would really like to see overflow drains is in the laundry tubs. Stop laughing! My LR is next to my kitchen and since there's no drain in the floor - what a mess! Just a tip - my husband took a six inch piece of pvc and perforated it, then stood it inside the drain. It works, but I'd rather have the overflow.

(post #172547, reply #39 of 41)

Why don't kitchen sinks have overflow drains like bathroom sinks?

Seems like the safeguard against overflow is equally imprtant in both locations.

(post #172547, reply #1 of 41)

I always thought the center divider being slightly lower than the perimeter rim was for this reason. The likelyhood of both basins being plugged at the same time is small.

(post #172547, reply #2 of 41)

Gosh Hawk that's a darn good question. It would seem that the overflow might be a place where bacteria could grow - especially having a constant source of things like animal products and such - and create an unsanitary condition.

(post #172547, reply #3 of 41)

Many 2 bowl kitchen sinks (esp SS) have a center divider that is the same elevation as the perimeter rim.

I guess that the solution is to get a sink with the divider lower.

Thanks for your reply.

(post #172547, reply #4 of 41)

Looking at it from the other way, why are there overflows in the bathroom? Also, I've noticed that some current bath lavs don't have overflows. Laundry tubs don't have them either. I can't recall the last time I filled a tub or lav that deeply -- maybe it's a carry-over from earlier life styles when people took more baths and filled the basin to shave.

(post #172547, reply #5 of 41)

For some reason I've also been wondering about this issue lately! I think I'll post it at

(post #172547, reply #7 of 41)

Hawk, you are right about the SS sinks. Yes, it is a fine question, the answer to which lies somewhere out there...

Fred, do you mean like outside, ie, the exterior of the house? If so, the inspectors don't like it when they see plumbing on the exterior, something about sanitation. The sink is convienient though.

(post #172547, reply #8 of 41)


My plumber vents the dishwasher to a closet or some out of the way place.....puts a nice white plastic cap on it and it sits pretty flat against the wall, usually about 4' up.

(post #172547, reply #10 of 41)

From the "For What It's Worth Department"...

Years back when I lived in Wiscoland, I went up to Kohler-ville where they have their big "design studio" or whatever they call it.

I asked the same question and was told the following (definitely paraphrased) by one of the designers: "The only plumbing traditionally found upstairs in a house is in the bathroom. Were these fixtures to overflow, not only would they ruin the bath floor, but the first floor ceiling and the downstairs floor as well. Big damage, so they have overflow protection. Kitchens are typically on the first floor, thus would only ruin the floor. Minimal damage. Laundry was in the basement, so who cared if they overflowed."

Granted, house design and fixture location has come a long way since then, while the fixtures have not changed much at all.

Kind of makes sense, but who knows for sure?

(post #172547, reply #14 of 41)


You are right, I have seen the disposer clog and the dishwasher air vent to spew drain water into the sink. I now run the disposer to clear itself before I run the dishwasher. If the air break was intended to prevent dishwasher dirty water from being sucked back into the clean water line, wouldn't the air break be on the supply line to the dishwasher instead of the drain line?


(post #172547, reply #16 of 41)

This air gap thing has been bugging me too. I've heard the same explanation, but wonder if wastewater were to reach the dishwasher, how work its way backwards up the potable water supply? Or is the theory that it would contaminate the dishes? If the problem is siphonage, aren't there less unsightly ways to break the siphon, say with a line to the roof vent?

As far as the disposal "spewing chunks" (nice and graphic!) I think the code requires the dishwasher waste to enter the line ahead of the disposal. Hence the dishwasher hose attachment on disposals.

The overflow in the tub is designed to prevent you from having a pleasant bath, at least in standard American tubs (cold knees anyone?). Perhaps it was thought of as an anti-drowning measure. I think it is interesting that neither the overflow in the sink or tub is large enough to prevent overflow with the tap full open. My toddler hasn't discovered this ... yet

(post #172547, reply #18 of 41)


As JohnE mentioned here in the Seattle area the air gap can penetrate the wall. The name is a Johnson Tee. No problems if drain clogs and it is also quiet.

Frank N

(post #172547, reply #19 of 41)

I'm a little confused guys.

1) How does an air gap in the DW waste line prevent waste water from getting into the domestic system? What does sink rim have to do with it (unless the sink filled up with waste and the faucet was lower than sink rim)? Or unless the entire DW filled up with waste (displacing all air in the DW) and the waste forced its way into those little tiny spray jets in the DW arms.

2) I suspect its the DW contents that are being protected.

3) Don't know about code requirements all over, but I've always seen the air gap hidden under the sink in the cabinet, so no need to penetrate either counter top or back splash. That still leaves the risk of a big mess under the sink if the drain is blocked.

4) Not every installation has a disposal. In some jursidictions disposals are not allowed by code even if desired. (NYC has only recently allowed test installations in certain areas. Areas with septic systems rather than sewers often prohibit disposals).

5) If the DW and/or sink are not against an outside wall then putting air gap outside poses some piping / servicing / clean it out if it does plug concerns.

(post #172547, reply #21 of 41)

I posted the air gap Q at and am now persuaded that the gap ensures that liquid only flows one way, towards the sewer. One contributor suggested that it was designed to prevent the dishwasher waste pump from pressurizing the waste line, which I imagine could blow the water out of all the traps in the house, breaking the gas seal.

Breaking a siphon is another rationale -- all of a plugged sink's contents could quickly travel down to the dishwasher, then the floor, after a couple of strokes with the plumber's helper.

Take your pick. But it sounds like omitting the air gap or equivalent would be a mistake.

(post #172547, reply #23 of 41)

Be careful. Those plumbers over there are better at the "how" questions than the "why" questions. They fall back on the written code a lot to answer "how come?"