I am putting a metal roof on our cabin. I presently have two pipe vents coming up from the bathroom through the roof. Less is more and I would like to consolidate the two pipes into one vent. One vent has a toilet, washing machine and sink tied together and going up through the attic and through the roof. The second vent only vents the shower. The two vents are about 5' - 6' away from each other. Can these two vents be tied together in the attic so I can eliminate one hole in my roof? If it's ok to do, does it matter if I bring the shower over to the toilet vent or visa versa? Do I need to use larger diameter pipe for the horizontal tie-in pipe and for the one going up through the roof giving more possibility of greater vent gas flow? Does the horizontal tie-in pipe need to be angled one way or the other or is horizontal (flat) ok for the gas air flow? Thanks very much for your comments and help.

Not a professional plumber, but your questions are easy. Of course they can be tied together, just check that you have enough units of venting, check your local codes, requirements vary. I would tie the shower over to the main stack, just easier. Of course you slope the horizontal runs, it can rain in your vent, and they do have to drain. I didn't want any breaks in my metal roof, so I ran my vents out the gables. There are rules on that also, again check your local codes. You can also call up your inspector and ask, it is my experience that most are more than willing to help you do it right, rather than force you to correct it later.

Current plumbing code requires at least one 3" vent through the roof. You can combine the individual vents in the attic. Maintain 1/4" slope/foot on the horizontal vent pipes in the attic down to the vented components. Use sanitary Tees turned upside down for connection to the main vent stack (upside down relative to installing them for waste flow).

This page shows a sanitary tee installed incorrectly for a vent (it needs to be flipped over to allow free flow of gasses into and out of the vent stack): https://www.nachi.org/bbsystem/viewtopic.php?t=7724

Posted By arkie6 on 22 Jun 2016 07:58 AM
Current plumbing code requires at least one 3" vent through the roof. You can combine the individual vents in the attic. Maintain 1/4" slope/foot on the horizontal vent pipes in the attic down to the vented components. Use sanitary Tees turned upside down for connection to the main vent stack (upside down relative to installing them for waste flow).

This page shows a sanitary tee installed incorrectly for a vent (it needs to be flipped over to allow free flow of gasses into and out of the vent stack): https://www.nachi.org/bbsystem/viewtopic.php?t=7724

The present pipe coming up & through the roof from my shower is 1.5" diameter. The other, with the toilet, laundry & sink is only 2.5". If I attach a sanitary tee with 2.5" x 3" x 3" & attached the 2.5"to the 2.5" vent pipe, and going up through the roof with 3" and over from the 1.5" with a 3" line coming over from the shower, would that vent everything ok? Also, about the 1/4" slope/foot on the horizontal vent pipe... does it slope back to the 2.5" vent, so I would cut the 1.5" pipe a little shorter?

I would install a 3"x3"x1.5" sanitary T on top of your existing 2.5" vent with a reducer bushing to mate the 3" fitting to the 2.5" pipe. Then run 1.5" pipe sloped down at 1/4" per foot to your existing 1.5" vent from the shower. Vents slope down to the fixture. 1.5" horizontal vent is sufficient for a shower unless the run is very long. Insure that you put the 3"x3"x1.5" sanitary T high enough to accommodate the 1/4" down slope on the 1.5" horizontal vent pipe run. Install the sanitary T correctly - visualize water coming down the vent stack and draining to the shower- air will be doing the same thing and you want the T oriented to allow that free flow downhill.

Posted By arkie6 on 25 Jun 2016 02:55 PM
I would install a 3"x3"x1.5" sanitary T on top of your existing 2.5" vent with a reducer bushing to mate the 3" fitting to the 2.5" pipe.

That was my plan, but I am having a hard time locating a 3" to 2.5" reducer at Home Depot. 3" to 2" no problem, but 2.5" seems to be an odd ball size. I thought I'd try a plumbers supply. The present vent pipes are copper. How do I adhere the ABS or PVC to copper?

Yeah, that is an odd one. My first thought was a Fernco coupling, but I couldn't find one that would one that would fit the outside of 2.5" copper pipe.