I have a toilet that backs up through the drain during a heavy rain storm. The toilet does not have a tank on it. The water comes in through pipes in the building. There is a knob to turn off fresh water coming in but I don't believe turning this off will prevent the backup since the water is coming through the toilet drain. Is there some kind of plug or cap to seal the drain with an airtight - water tight seal and where might I find such an object ? Thanks.

More details needed. Is this a condo building and you're on the first floor? A single family dwelling? City sewer or septic tank? There are several ways to fix this but I have to lonow more before I can answer. Regards. Tom

It is a 1st floor apartment and connected to city sewers - the problem affects other apartments at times also

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Is there some kind of plug or cap to seal the drain with an airtight - water tight seal and where might I find such an object ? Thanks.

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Yes, you could put a inflatable test ball, (see image) in the throat of the bowl. But that's just a "stopgap" and who wants to pull a rubber ball out of the toilet bowl every time you want to use it? That's nasty! UGH!!
Nah! That idea's no good. Let's fix the entire building while we're at it and not just one potty.
Here's what I think has happened. Your sewer, and most likely everyone's, is connected to your storm drain and when you have a hard downpour all that water runs out into the city main and backs up in the sewers connected to it. Have you had complaints from the neighbors?
There is a solution to this. It's called a sewer check valve,(see image).
This installs just downstream from the house cleanout and will allow sewage and discharge to pass but if the sewer backs up the check valve will prevent the sewage from entering the building and your toilet. Once the water goes down the check valve opens up allowing you to use the system.
You will have to get the other tennets and petition the landlord to do this bet this will end your backups. Good luck, tom