Plumbing Question - Can you mate black PVC to steel pipes

[Tidybuoy]

I'm putting on a new roof and I want to merge three 1 1/2 steel pipes with a single 2" pipe coming out of the roof. These are all plumbing vents and based on my research this is OK to do via my local codes.

The one 2" Pipe that will remain is a non-removable steel pipe (i.e., it goes down the wall into the house plumbing and there are no joints in the attic).

My plan was to cut the 2" pipe in the attic and install one black PVC 4-way connector and then re-install a portion of the steel pipe to exit the roof.

Can I mate smooth PVC connectors to a cut pipe or must I connect using threaded pipe. I can use threaded connections to all of the 1.5" pipes because they all have joints in the attic but the 2" pipe will need to be cut and then mated to the PVC. I don't have access to a 2" threader.

I figured I could just put it all together like tinker-toys and not worry as these are just vent pipes and won't have any water pressure in them but I thought I would check to see if anyone knows the correct methods for doing this. All of my pipes will be downward flowing to prevent any rain water from sitting in them. I'm asuming the PVC cement won't work with the steel but I will check at the hardware store.

Thanks.....Vern

[nota]

thread the pipe is best and use a threaded fittings
but most just use a rubber connector with hose clamps

but is black plastic even PVC
most of the black pipe is flexible and other chemical content

the other way is force the plastic pipe up the existing iron pipe a few feet
and use epoxy or seam compound to seal any small gaps

[Icemaster]

You sure it's steel and not cast iron? So you're trying to sandwich the PVC between the existing pipes?

[scottmandue]

For a vent a rubber clamp should be fine.

I have a section of 4" sewer line under my house black plastic to old black iron with rubber sleeve clamps... been no problem for many years.

Steal/iron to copper is the big no-no.

[Tim L]

I would cut the steel pipe and change to PVC via a Fernco (rubber coupling) then just run the rest in PVC. Even though it’s just a vent you still don't want any leeks.

[Porsche-O-Phile]

Shouldn't be a problem - as said the issue is typically galvanic action when dissimilar metals (iron/copper) are used.

Keep in mind that if any PVC is being used for the vent pipe portion extending above the roof line that PVC does get deteriorated over time by UV exposure. For this reason you may want to consider a protective finish coat/paint, cladding it or sleeving it, or ensure that you can yank the weathered portion out and plug a new one in easily every few years (would require re-flashing) - make the connection below the roof simple and easy to get at so you don't have to bust half the roof or ceiling apart to get at it. Or just use black pipe for the portions above the roof. A rubber connector is a good idea, it will also create a thermal break (iron/metal will conduct heat) so put the fitting reasonably close to the underside of the roof deck and/or insulate the exposed portion below the roof.


An access panel might make sense if you can stick one in a closet or other non-obtrusive location so you can access, undo the connection from inside, go up on the roof, yank the old one out, plug the new one in, reflash and go back inside to connect it up.

[vash]

no hub.

[dad911]

Black is ABS not PVC, make sure you use the right glues.

For vents & drains, you can couple dissimilar pipes with fernco connectors (rubber with hose clamps)

[Zeke]

There are special band couplers for this purpose and they might not be found at a home store. Mating ABS and PVC to iron and steel plumbing is common in remodeling.