How to drain 2 washers into laundry tub drain?

[jpmaddog]

Our laundry tub drains directly into the slab. Instead of drain two washers into the laundry tub, my wife wants to connect them under the laundry tub and have all 3 go into one p-trap.

How can this be done?

 

[Cacher_Chick]

The purpose of the laundry tub is to act as a "reservoir" so the water can drain as quickly as the trap and piping allows. Because of the discharge capacity of the 3 combined, it would require each washer to have an individual standpipe and trap draining into a 3" or larger drain.

 

[Sponsor]

 

[Furd]

In other words, what you want to do cannot be done, for several reasons. The first reason is that each washing machine AND the tub require their own vented trap. The second reason is that it is highly unlikely the drain piping through and under the slab has the capacity to handle the load. As Cacher stated, the laundry tub serves as a temporary reservoir to hold the high influx of drain water and allow the drain to handle it on a slower discharge basis.

 

[jpmaddog]

Additional details:
The drain in the floor is 3" and it's right next to a 4" cast iron pipe that goes straigt up to the roof.

The new washer needs to handle 4.2 gal/min.

It all drains well and fast right now. Can't I just tie two standpipes and the laundry tub into one pipe and the have one big trap for it to go through?

 

[Gary Swart]

No, each standpipe must have its own vented trap.

 

[Cacher_Chick]

Also, if the 4" stack has any other fixtures using it as a drain, the vents would have to be connected at least 6" above the flood rim of the highest fixture draining into that stack.