Washing Machine Standpipe drain clog

[hitechseeker]

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.

 

[hitechseeker]

Just to complete the post. I got the drain snaked by a plumber who had a 50 ft snake. It had to be pushed to about 40 feet to get the drain flowing again. Anyway, I heard a lot of conflicting information from the HOA and the plumbers, amking everything sound like a much bigger problem than a simple snaking of the tine. Although, I am not certain I think the laundry drain is seperate from the kitchen drain. The P-trap from the washing machine drain is connected to a T piece that drains out to the street and the vertical pipe that provides venting to the roof. This piece also has the cleanout. I guess the only way to know is to use a camera on a long cable or have access to the drain diagram. Other than buying or renting a long snake, any suggestions for keeping this line clean ?