Can I combine multiple vent pipes into a single vent pipe in the attic?

I am in the process of installing a metal roof. Currently there are 5 vent pipes exiting through the roof and I would like to combine the pipes into a single large pipe that will exit through the roof. Don't want multiple pipes exiting through the new roof due to possibility of leaking and aesthetics.

In the attic, I have clear access to all of the pipes and they are evenly spaced about 5-6 feet apart. Is there a problem with running a horizontal pipes to combine them? All of the horizontal pipes will be minimum 8 feet above the highest plumbing fixture.

Thank you for your help.

Allen

Hi Allen:

What size pipes are we talking about connecting together? If say, you have a bunch of 2 inch pipes then you certainly can connect them altogether and then make just one penetration through the roof.

Be sure, however, that none of these vents are from a sewage ejector in the basement. If one of these vents is from an ejector then technically speaking this ejector vent needs to penetrate the roof individually and cannot connect into the other vents.

When you connect all the pipes together you will most likely need to increase the pipe size up to 3" or even 4" depending on the number of fixtures connected altogether.

Finally, be sure to INVERT the connecting tee fittings you use to connect all together. Here, the idea is for rain water that gets in the pipe through the roof vent to flow down the pipes by gravity... by inverting the fittings you end up with a more natural flow for water to follow.

All pipes need to be secured every 4 feet or sagging will occur.

Back to you...

MARK

Hello Mark,

The pipes are 1.5 to 2 inches. I was thinking about connecting them in series. Essentially a long horizontal 2 inch pipe, which 4 vent pipes will terminate into to T connections. Length of this horizontal run should be about 15 feet in total.

This horizontal pipe will tie into an existing vent. I think this vent is 1.5 to 1.75". I can enlarge the vent to 3" from the joint to the roof penetration.

Total amount of fixtures involved are: 2 sinks, 2 toilets, 2 showers, and 2 laundry. Kitchen is not involved in this part of the house. House does not have a sewage ejector. It would be easier to make the horizontal run without any slope. Any rain that comes through the roof penetration should go down to the existing vent pipe and not through the tie-in. What do you think?

Thank you very much for your help.

Allen.

Allen:

Plumbing code will require that you enlarge the vent to 3" and pick up a couple of these vents with 3" as well... reduce to 2" to finish the horizontal run. If you can, put a little pitch in the horizontal piping... again, code required.

Don't forget to invert the tee fittings... ;)

MARK