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Vent stack for washing machine

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Need to install a washer. Lots of issues that make it impractical to tie into any existing vent stack system. The plumbing inspector says a new stack must be 3". That's going to mean copper rather than PVC to get it inside 2x4 stud bays.

A plumber suggests only the roof penetration needs to be 3", that the run through the wall can be 2" and transition to 3" in the space above the ceiling. Anybody with experience in Westchester County, NY want to comment?

Any concerns about the stack going up through a flat roof at a lower elevation than intersecting gables or gable ends. I plan on staying 10' away from a window in the gable end.


(post #178483, reply #1 of 10)

I saw a thin wall DWV system in PVC a while back that would fit in a 2x4 wall. I think that it came from Genova.

(post #178483, reply #2 of 10)

Not a plumber, but I'm certain that 3" is way way way beyond code (only the clothes washer on this vent?). According to the table i have here, 1 1/2" should be enough.

Goofy solution: how about two small vent pipes in parallel? I challenge your inspector to explain why this wouldn't work.

Flat roof penentrations make me nervous. Also, do you want to look out the window at a vent stack? Don't forget termination must be above window 2/3' as well as horiz. (do both spacings apply at once? not sure).

Good luck!

(post #178483, reply #3 of 10)

Out in SoCal, Code says 2". I go with whatever the inspector says. Argue with him if you must, but he won't permit it unless you satisfy him.

There is no way I would vent this washer to a low, flat roof under intersecting gables. That flat roof will get a ton of water. Assuming that is the only place to put the vent (this can't be true, can it? Why not just run the vent into the attic with a couple 90's and tie into the cast iron stack?), then I'd re-roof the flat roof, adding a rubber seal or fire and ice over the whole flat roof; or hot mopping it with flashings in place.

(post #178483, reply #4 of 10)

From Taunton's "Plumbing a House" by Peter Hemp, page 50, (adapted from UPC):

A 1-1/2" vent is good for max of 8 fixture units for up to 60 feet (horz+vert). A 2" vent is good for a max of 24 fixture units up to 120 feet long. A washing machine counts as 2 fixture units. A 3" vent cold handle up to 42 washing machines.

The 3" rule probably comes from this (page 114): "my major code states that in areas where snow or frost may clog the vent, each vent extension through the roof shall be 3" in diameter and the the change in diameter shall be made inside the building at least 1 foot below the roof and terminate not less than 10 inches above the roof."
So take 1-1/2" to within a foot of the roof and then adapt to 3". And get that book, it's got a lot of good info and good diagrams. Happy Plumbing! -David

(post #178483, reply #5 of 10)

Peter Hemp's book is $24 at

(post #178483, reply #6 of 10)

I believe what the inspector meant is that the part of the vent near and above to roof penetration needs to be 3". Someone somewhere plugged their 2" vent with snow and ever since the powers that be have decreed 3" stacks through the roof. A 2" vent inside the wall will be ample. At least that's how the UBC (national code) reads here in the wilds of Montana.

(post #178483, reply #7 of 10)

Following David's quote would meet code here in WI with no problem. We can even terminate the vent in 2" as long as there is at least one other 3" vent through the roof. 1 1/2" should be more than enough vent to handle one washing machine as long as you meet the anti-clogging requirements when terminating through the roof. I'd sure try to change directions in the attic if I could and run the vent through the roof on one of the gables instead of the flat area.

(post #178483, reply #8 of 10)

Thank you both. Combined the vents into one roof penetration will save hassle & leak risk, plus make the insulation nuts (no names) happier!

(post #178483, reply #9 of 10)

Thanks guys. The 3" roof penetration seems to be the deal regardless of where it's placed (flat or gable). The under roof transition to a smaller size seems to make sense and I'll check with the inspector. Would any one venture a guess whether "one foot below the roof" means vertical distance or pipe distance? If it's pipe distance the 3" elbow and X" run of 3" pipe will fit in a joist bay and transition to 2" before turning through a plate and into a wall stud bay.

The positioning constraints are that to the North and South running to the eave space under the gable will require ripping open ceilings in rooms that were not going to be disturbed. East is entirely blocked by a solid masonry 2 flue chimney. West is all flat roof.

The flat roof is scheduled to be redone. How would a neoprene boot with a metal flashing layered into a double coverage asphalt foor hold up?

Had looked for Hemp's book this morning but my supplier only had his fixture book in stock. This one's on my list.

(post #178483, reply #10 of 10)


As I noted earlier, has "Plumbing a House" in stock at $6.00 less than list. Typical order-receipt time for Amazon is two days. Once for me was next morning. Your supplier is maybe headed to obsolescence?