I unscrewed the drain pipe cap that connects to the vertical part of the drain that sticks out of the wall on a TY fitting. The pipe runs straight down to below the floor level bypassing the drain trap for the washer connections and also runs up in the same stud bay venting through the roof. The washer connection connects to the vertical drain pipe on a T fitting and runs over to the next stud bay.

Poured in about 2 gallons of hot water, poured in a half can of drain cleaner, let stand for 1½ hour, poured in about 4 gallons of hot tap water, brought the garden hose in from the back door [the laundry is in the back part of the garage with an exterior door] and run it in the clean out fitting, turned the hose on at a good force and run it for about 10 minutes to flush the drain real good. There was no back up over flow from the garden hose water.

See if it overflows the trap inlet next time laundry is done.

I have cleaned out that washer drain pipe about 12 or more times in the past 20 years. Darn soap scum build up.

I am off to the library to return a couple books and to the gym now for a little leg exercise on the bike and leg curls.