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I would like to install a laundry room tub directly over the floor drain. Can I simply have the tub drain directly into the top of the floor drain?

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    I suppose you could, but it's likely to be messy... – Ecnerwal Apr 17 '15 at 15:12
  • How large is the drain pipe connected to the drain, and how large of a tub did you have in mind? – DarthCaniac Apr 17 '15 at 20:47
  • The floor drain pipe is about 6 inches in diameter, and the tub drain pipe is about 2 inches in diameter. The tub is about 20 x 15 inches large. Thanks! – Todd Apr 20 '15 at 15:55
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You most likely will overflow the floor drain and damage your surrounding walls/floors.

  • Thanks for your answer -- do you think that if the laundry tub drain pipe was smaller in diameter (1-2 inches) compared to the floor drain, the flow would be slow enough to not create an overflow in the floor drain? – Todd Apr 20 '15 at 15:57
  • It's more about venting and the size of the P-trap than the size of the drain. A floor drain might handle it but they were not designed to handle it. You will not know without risking damage. – diceless Apr 20 '15 at 16:18
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This is simple to test.

The typical faucets on one of those tubs let water absolutely gush out; there is no resistance. Likewise, your outside spigot for a hose has no resistance. If we assume the pipe size to your sink and the pipe size to your hose spigot are the same:

  1. Get a 5 gallon bucket and stopwatch
  2. Time how long it takes to fill the bucket up most of the way from your hose spigot
  3. Dump bucket over drain and see how long it takes to drain
  4. Double that time because you could potentially have the hot and cold running full blast at the same time
  5. If your fill time is faster than your drain time doubled, your laundry room could flood. If your doubled drain time is less than your fill time, you should be in the clear

*Don't dump the whole bucket at once. Dump enough to cover the drain with an inch of water and keep dumping enough so that the drain is never exposed to air during this test.

It is highly unlikely that your drain can handle more water than your spigot produces.

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Floor drains should have a grate. Placing your drain pipe so that it pours through the grate will protect your floor drain but the grate will eventually clog. Make sure you can see and remove any gunk buildup. Regardless of your floor drain's capacity (to handle the flow rate from your sink, tub or whatever), consider placing a ball valve in the piping between your tub and floor drain and keep it partially closed if you have any concerns. If there are other sources of water that could be using the same drain simultaneously, testing the flow rate of only your tub may not be useful. A pool store will stock all the drain parts (elbows and such), flexible hose and filter baskets (normally used in skimmers) you may need. If there is still a flooding concern, it may be wise to get a flood sensor. There are one time use sensors, and those who have alarm systems can connect a flood sensor to it to be advised even if no one is home. Some alarm systems can be programmed to close an electric valve, such as your water main for example, when an alarm such as a flood sensor is triggered. Also useful if your hot water tank leaks and many insurance companies will offer a rebate if one is installed.

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