It's a standpipe for a washing machine discharge. It could also have been used as an illegal sump pump drain. Not a source of sewer backup but certainly a source of sewer gas in the home since the trap is dry.
Where does the pipe from the top of the tee go? Does it vent out the roof or is it the drain from a first floor fixture?
MM
See attached picture.
>
> This is the main waste stack in the basement. Note the odd extension. Why
> is this here? Is this to code?
>
> FYI - the toilets in this home frequently backup. Any relationship? >
>
>
>