Can a vent come before a p-trap?

We are adding a washing machine hook up on our 2nd floor. We would like to make the hook up as shown in the attached drawing. The washer drain hose would be tightly connected to PVC pipe, with the vent rising directly after this connection, and the p-trap following immediately. After the p-trap the pipe will go 7 feet and join the main soil stack, which, of course, has a vent. We were told that 7 ft is too long for a washing machine drain line to travel to vent, which is why we are putting in the additional vent. However, we are a little bit confused as we have read that a the vent should always be AFTER the p-trap, but our proposed setup (vent before trap) would be much easier to fit in our given space.

What to do you think? Is this a reasonable set up?


Thanks for your help!!

Installing a vent in that way will do nothing to vent the trap. If the drain pipe leading to the trap is 2", you need to allow at least 4" in between the trap and the vent, after the trap.

Change the p-trap in your drawing to a 90 and bring 2" pipe up through floor.
Add a 2" santee on the vertical pipe with a p-trap.
Then put a 3' to 4' stand pipe in the p-trap for the laundry hose.
Now just extend the top of the santee through the roof.