Washing machine drain tie in

I have moved my laundry room and am ready to tie in the washing machine drain. I have figured out the venting, just need to find where i can tie in the drain pipe. There are no sinks near the new drain, but I can get to a cleanout fitting on the main house drain. The drain runs horizontally then turns straight down then turns again to exit the crawl space. The cleanout is in the pipe that runs straight down and comes out the side of the T. Can I tie in right there without adding anything else? I am trying to avoid cutting the main pipe. I dont see anywhere else to connect to. Also will eventually be adding a sink in the same room. Can I increase the drain pipe size to a 3" to allow for this in the future or just run the 2" now.

No, you cannot use the cleanout as a drain.

I hate to be ignorant but why not? It is inside the crawl space, on the down side of the main drain before it turns to exit the house. It comes out one side of a wye and has a threaded cap. Why cant you tie in there and leave another outlet for cleanout?

It might be permitted to install another wye at that location, but without seeing the exact configuration is would be wrong to say otherwise.

AFter many years this is the first time I've seen a plumber agree that an existing cleanout might be the right place to start a new branch or arm (with venting). In my case, I intend to add another clean out a few inches away. Hope this helps.

I agree, just wish someone could tell me what harm it does.? It really is no different then the other drains that are tied into the line. Just that it is on a downward pipe instead of the horizontal pipe. I know it does away with the cleanout but does it effect the draining. I know I can add another cleanout on the line, but why would I have to? House has been here 26 years without ever touching the fitting. I am on sewer not septic. Not arguing just want to know why?

I'm not a plumber, but I have a great interest in doing things right.
I understand the desire to understand.

There are things that will work and there are things that will meet the code requirements. Meeting the code requirements is our minimum goal. Not doing so increases the likelihood of future problems.

Cleanouts are required by the plumbing code. They are often specifically required to be located in certain locations. Altering the existing cleanout could cause it to no longer meet the requirement.

Each plumbing fitting has requirements for the minimum radius of a bend. When flow turns from horizontal to vertical or vice-versa, only certain fittings can be used to initiate a change in direction. Draining into a cleanout Tee is not permitted.

If the main drain you are looking at is the stack for the bathroom, tying in below the WC on the vertical would not be permitted.

On of the few places it would be ok to use a cleanout would be at the end of a main line, where a wye could be installed with a new cleanout installed at the end of the line.

Never in a cleanout tee.

I just got a call today asking about a cleanout on a home built in 1950. They haven't snaked it before, maybe. Point being; now they are looking for a cleanout.
Don't put your self in the same position as them. One day, a tree will grow, and it could be stealing water from your sewer line out to the public sewer. Add to that, the fittings needs to be a proper bend if used as a waste line. You should be able to cut something in there that will do both things. Cutting pipe is not that big a deal.

Thanks to everyone for all the help and advice. I see why I may eventually need the cleanout. I will cut the line and put in a new fitting. Sometimes I really just want to understand why I should or shouldnt do something rather than just NO. Thanks again