You can't cap it by code; it has to admit air. This is designed to eliminate sewer gas odor; I don't know how well it would work for stopping back flow, but it's probably as close as your going to legally get.

Fix the problem, not the symptom. Why does your main line keep getting blocked? Fix that.

Thanks for the responses!

Joseph: They make standpipe covers and they sell them here. It can't be against code. And there is a u-trap preventing gasses from coming up through it already. Capping it won't hurt anything at all, except if there is an overflow it might then blow out somewhere else in pipes. I just don't know anything about water pressure in a drain line and know what to expect. If it were capped completely, would it flow back up the pipe, would it go through the 1" tube (ie: is that enough), etc.

Greg: I wish. Talking about a problem that only occasionally happens and I have more pressing issues than spending thousands to replace the line. Additionally, I don't like living with the idea that my neighbors can flush something down their toilets and stop up the mainline, and then it's going to overflow into my basement the next time I use my plumbing. I'd like some peace-of-mind that I'm not the only one on the block with an open sewage overflow into my basement!

If you cap it off, the sewage will just flow somewhere else. Maybe into your washing machine through the drain pipe.

You could keep it open, but 'T' off another branch near the top of the stand pipe that would redirect any overflow to another more convenient location (like your garbage can idea). Not a little 1" tube. Sewage would block that. Keep it as the same size as your standpipe.

I don't mind it flowing into my washing machine... or backing up through to my sinks, tubs, etc. Over the last 20 years no solid sewage has come up through the pipe, it is always water, which is why I was thinking the 1" might work. Doing a bigger one starts to become crazy because of the amount of room it would take up to do the redirect. My biggest concern is building up pressure and blowing out the pipes are a connector somewhere.

Have you looked into a backflow preventer

Hi Jim, I tried but wasn't successful. Could you point me at one that works on 2" pipe?

I do not know your location. can you google: 2” backflow preventer. .

Is this what you're thinking?: ZOELLER Check Valve, 2 in, Single, Inline Swing, PVC, Clamp x Clamp

"Joseph: They make standpipe covers and they sell them here. It can't be against code."

They do make standpipe covers and they have vents because it's against code to not have them.