Plumbing - P-Trap for Tub and Toilet?

I don't think your problem is in the pipes where the water goes. It is most likely in the stack vent pipes that are designed to let air back in and keep the water from getting sucked out of the traps.

I think you have a clog there (maybe a bird's nest or something by the roof keeping air out).

It is definitely your vent. When you flush the toilet it pull the water from the tub with it, due to the large volume of water being flushed and no other vent available except the tub. For water to flow there must be a vent. If the toilet isn't vented properly it will find another vent and with the tub being in line, it doesn't take much to pull that little bit of water out of the trap. I guessing the toilet and tub are side by side. This would explain why you don't get the smell from the other drains. The purpose of the "P" trap is to create a water seal to block the odor.

To test this put your hand over the tub drain and flush the toilet. You'll probably feel the suction, you can also use a ribbon (place the ribbon over the drain and flush, the ribbon will suck down into the pipe so make sure you hold it) Snake the vent and do the same thing. When you can flush the toilet without feeling (or sucking the ribbon) then you've fixed your problem. Hope this helps.

Sounds like the drain system is not properly put together.Every drain needs a trap and every trap needs to be connected to a vent within a certain distance(depending on the diameter of the trap).There should not be any more of an offset than 45 degrees between tub drain and top of trap. This would make the drain S-trapped.This could cause the trap to siphon. Better call a Licensed Plumber that has been around for a while. Smells are the hardest plumbing issues to solve .

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If the tub is standard bath tub with a hole at the top for the overflow and a hole in the bottom for the drain the p-trap has to go below the tee that joins the two holes. It can come as close to the tee as can be properly tightened. The exit from the p-trap is offset and can be turned to allow the pipe it will drain into to miss the joist. Perhaps you can cut just enough of the joist to make it work. If the sewer pipe under the house is low enough you can use an s-trap which keeps going down after the bend. You didn't mention where you were tying in the tub drain so I don't know how to advise beyond this. As far as air in the line if the toilet is nearby it should be sufficiently vented to cover your tub drain.

Vent stack. On the roof, theres a pipe sticking up,probably directly above the toilet,get a flashlite and look down the pipe on the roof. If your not sure which pipe it is, have someone flush the toilet while your on the roof and listen for the pipe with running water in it. But if its got something blocking it, your going to have to listen carefully.Be careful trying to remove the blockage,and don't push it down the pipe, or you will have a whole new problem. The rest of your plumbing has its own vents, like, usually the kitchen sink and laundry share a vent, as do any other bathrooms,the bath sink,toilet and tub/shower share a common vent for each room, meaning you should have at least 3 vents on your roof.

Tub Trap

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Plumbing - P-Trap for Tub and Toilet?

We have been experiencing sewer gas coming back up the bathtub drain, even after being snaked and treated with drain cleaners.

The situation -- when there is water in the P-trap under the bathtub, sewer odor doesn't come in. (I can pour a few cups of water down the drain and stop the...

USA Venting is the problem. John M has a good answer.

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