Apparently the garbage disposal drain must be above the drain stub, but I can't find a clear answer for how much height difference is actually required.

Let's use this drawing:

under the sink plumbing

The question is how much higher does the disposal drain (#1) have to be relative to the drain stub (#5)? I've seen answers ranging from "just above it", to 1 inch, to 4 inches, so what's the actual minimum I can get away with?

Also, how do you measure it? Is the important measurement going from the bottom of #1 to the bottom of #5? Or from the bottom of #1 to the top of #5?

  • Where did you hear this requirement? – pdd Jan 12 '16 at 23:41
  • A bunch of sites describing garbage disposal installation says if the GD drain (#1) is below drain (#5) then the disposal will always sit full of water. – Kevin Holt Jan 12 '16 at 23:56
  • Yes, anything below the drain (#5) will be filled with water as that is how a p-trap works. As stated below, installing a sanitary tee on top of the p-trap (#2) is all that is required. – pdd Jan 13 '16 at 20:33

I don't think there's a code imposed minimum, but there's likely a physical limitation based on available fittings.

Between the disposal outlet and the drain, there must be a trap. The bottom of the disposal outlet, must be above the trap weir (the point in the trap at which water starts to drain from the trap).

Therefore, in theory the bottom of the outlet has to be just slightly above the bottom of the drain.

In reality you'll have to use available fittings, so the minimum height would be based on what the fittings physically take up. Bare minimum, you'd exit the disposal with a 90° elbow, and connect directly to the top of the P-trap. So whatever height is taken up by the elbow and P-trap, would be the minimum.

WARNING: I have not searched through plumbing codes, so I could be wrong about a minimum height not existing.

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I'm not aware of a code requirement other than that the connection would have to take place above the trap seal (the portion of the trap that holds water). Therefore installing a sanitary tee on top of the p-trap would be acceptable.

There is a maximum developed length of 1200mm (3'-11") for the fixture outlet pipe (from sink or disposal to p-trap).

Note: The above is based on Canadian code

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