Hello,
I am putting a clothes washer into an existing bathroom. I have space on one side of a wall where I can put the machine. On the opposite side of the wall are the sinks. The drain running under the floor to the main waste vent stack is 2", and the drain running through the wall for the sinks is 1.5".

I understand that 2" is strongly recommended (and is code in some places) for clothes washers. However, the job is significantly easier if I go through the drywall instead of ripping up the tile floor. If I tap into the drain through the wall, the clothes washer waste water would have to run through approx. 3 feet horizontally where the pipe is 1.5" before feeding into the 2" line running the rest of the way to the main waste vent stack.

According to MA state code, I only need a 1.5" standpipe (though I don't see anything about the size of the pipe the standpipe must feed into:

"The standpipe shall extend to a minimum height of 30 inches above the base of the
machine and shall not be less than 1½ inches in diameter."

Is the ~3 ft of 1.5" pipe through which the clothes washer waste water something to be concerned about? It's on the third floor of the building, so I want to do everything I can within reason to avoid flooding.

Thanks in advance!

1. Going through the ceiling of the floor below is much easier and faster than busting out a tile floor.
2. Are you a licensed plumber in the state of Massachusetts?

you are so concerned about MA code???
why not call your local inspector with any and all plumbing related questions?

Codes are a "minimum requirement", and I would NOT even consider using a 1 1/2" drain for a washing machine on the third floor, (overflows can be disastrous). And, if you are thinking about connecting to the 1 1/2" drain from one of the sinks, forget that.

You got room for a washer pan and drain pipe too?