Washing Machine Standpipe drain clog

Jul 17, 2014
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United States
I have a Washing Machine installed in the ground floor garage. Every once in 3-6 months the drain gets clogged. The water comes up the standpipe. The standpipe is between sheetrock and an outside wall, so I am not sure how the complete thing looks like. On the LHS of the standpipe is a cleanout.
My undertanding of this arrangement is that the pipe with the cleanout is the air vent to the roof, with a pipe that leads to the outside drain. I cannot get any information from anyone on how the pipes are actually arranged.
The only way I have to snake the drain is through the standpipe. The diameter is quite small.
In fact the last plumber who visited was unable to clean the clog and wants to cut into the shteerock and install a new cleanout.
I cannot see the end of the standpipe, since its behind sheetrock, but I expect it has some sort of a P-trap at the bottom connecting to the drain outside.

What would be the purpose of the existing cleanout on the LHS of the standpipe, if its not for clearing the drain clog ?

Should I cut into the sheetrock to expose the standpipe and see which pipes and how it it connected ? Since the drain pipe runs horizontally, how would I arrange a cleanout ?
While I am not absolutely certain, I am imagining the pipes are arranged as I described.
Is there any place to get the actual piping diagram ? -- house is about 22 years old.


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