I've got a cabin that had a closet reworked to include a washing machine, by the water heater. After the rework, the drain from the water heater pan and washing machine can freely move around a bit where it goes in the floor drain.

I was under the impression this needed to be airtight to prevent gases from the sewer to come back into the house. There's a p-trap before, but it seems to serve no purpose since any gas could just exit by the floor junction.

Is this a proper setup? If not, how do I fix it? Location is Quebec, Canada.

Diagram below tries to show approximatively how it is setup.

enter image description here

  • 6
    Will need to check if the floor drain has a P-trap itself, it probably does since if it was not used for heater/washer, sewer gas would still come up.
    – crip659
    Sep 5, 2022 at 15:02
  • But then, should I need a p-trap between the washing machine and ground junction?
    – Jeffrey
    Sep 5, 2022 at 15:15
  • 1
    A P-trap between the washer and floor drain seems useless, except for another place for clogs to form.
    – crip659
    Sep 5, 2022 at 15:35

2 Answers 2


If the drain is connected to a sewer system, there must be a P trap somewhere, and the pipe must be sealed from the system to the P trap, but not after the P trap towards the drain or washer. Often floor drains have a built-in trap or a P trap in the concrete slab, and drains can open-flow into them.

Since this is a cabin it is possible that the drain does not connect to a sewer system, and you'd have to check this to confirm. It could possibly flow outside into the open, into a french drain etc... Any P trap would then function as a rodent/insect stop, not a gas stop.

If you see water pooled at the bottom of that drain, and it is the water is ar least 2 inches deep (check with a wooden or cardboard dip stick), you have a strong indication it's a trap.

If the cabin has a drained toilet, you would be sure that there is a sewer system. To test whether your washer drain is connected you can check for this by ear: first remove all water, if any, at the bottom of the drain. Then flush the toilet and listen for rushing water. If you hear water rushing, then this drain is likely connected to the sewer and must be sealed.

A camera inspection or smoke test would be required to confirm this.

  • More of a water hazard than a stop, for any moderately determined rodent/insect.
    – Ecnerwal
    Sep 5, 2022 at 16:09

Every drain that is a permanent part of the building’s plumbing should have a built-in trap. If this was an existing floor drain then it most likely does.

If you can move the appliances’ drain pipe aside temporarily, shine a light down the floor drain and in most plumbing you should be able to see standing water (not just dampness at the bottom of an elbow, but a full circle) if there is a trap present.

  • Though if nothing has flowed through it for a long time the water may have evaporated, so if there isn't standing water try pouring some water down the drain and checking again.
    – Mark
    Sep 6, 2022 at 15:36

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