Combining vent pipes in attic to reduce roof penetrations?

[makethatkerdistick]

I currently have six plumbing vent pipes penetrating my roof. When reroofing becomes necessary, I'd like to reduce the number of roof penetrations in order to reduce leak potential and for aesthetic purposes.

Is it possible to combine these into two or even one vent roof termination? My plan was to cut the stacks (a mix of cast iron, galvanized steel and copper pipe) in the attic and then via Fernco add a PVC combiner system to it that I can attach to the underside of the attic rafters.
I assume a minimum slope is required for every run so that condensation doesn't gather? If so, is there a prescribed minimum slope? Also, what diameter pipe should the termination piece have? Is 3 inches enough?
Naturally, I want to keep the vent termination away from my fresh-air intake. However, is there a regulation that stipulates a minimum distance from the separate intake for combustion air for my furnace?

Here are the existing vent stacks and their thicknesses:
kitchen: 1.5 in galvanized steel
laundry: 1.5 in copper
bathroom I: 2 in cast iron
bathroom II: 3 in cast iron & 1.5 in galvanized steel
bathroom III: 2 in cast iron

Thanks for all the information you can provide!

 

[Terry]

Normally for a three bath home they require a 3" vent through the roof or equal by combing vents.
The equivalent for 3" would be two 2" vents and one 1.5" vent.
For a 4" needed for four bathrooms, it's four 2" vents.

Grade is 1/4" per foot and if the horizontal is more than 1/3 the vertical, it's bumped up a size on the venting.

 

[Sponsor]

 

[makethatkerdistick]

Thanks, Terry. In this case, I will combine the six vents into three terminations as the horizontal run for tying together both sides of the house is at least 40 ft. That way, I can still reduce the penetrations by half without having to worry about too long a run and having to increase pipe diameter too much.

It would be nice to have it all together in one spot but this might be hard to do (large one-story ranch house). I have one vent stack that runs inside an exterior wall. I just realized this would be almost impossible to cut off due to roof slope and ensuing lack of access in the attic.

 

[wwhitney]

I have one vent stack that runs inside an exterior wall. I just realized this would be almost impossible to cut off due to roof slope and ensuing lack of access in the attic.

If you're doing this while re-roofing, you can lift up the roof sheathing around the current vent penetration, so that you have access from above. The next most upstream connection can be in a part of the attic where you have adequate head room already.

Cheers, Wayne