Your vent size will be mandated by local code but below is a link to a great website that should hekp you get the info you need and a reasonable understanding about vent sizes. I hope it helps.

Here is a link that might be useful: Hometips.com

Thanks for the link. I found it very helpful and it corrected a couple of misunderstandings I had.

Venting is without question the most difficult aspect of plumbing to understand, and to complicate it even further there are dramatically different requirements between the UPC (Uniform Plumbing Code) and the IRC (International Residential Code.)

I reviewed the website and although it appears to be based upon the UPC the information presented is far too vague to rely upon it for a final design.

In addition, there are many rules which are not even touched upon in that website, by example, under both the UPC & IRC you may not have a horizontal offset in a vent line until the line reaches an elevation which is at least 6" higher than the flood level rim of the highest fixture served by that vent.

Under the IRC each structure is required to have one MAIN Vent, which must run undiminished in size from the main drain, through the roof. After that is accomplished additional auxiliary vents may be reduced to 1/2 the diameter of the line they serve, but not less than 1-1/4" and if the TDL (total developed length) of the vent line exceeds 40' the line must be increased by one nominal trade size. When computing TDL you must compute both the physical length of the pipe and the fitting insertion loss length of the fittings.

Under the UPC we do not have to install a main vent, however all vents must terminate through the roof and they be sized by the number of DFU's served by that vent as specified on table UPC-904.1 In addition, the combined total aggregate cross sectional area of all vents must be equal to or greater than the cross sectional area of the main drain and if any vent has a horizontal offset greater than 3' the line must be increased one nominal trade size.

Vents may not be connected to a vertical stack until he vent line reaches an elevation at least 6' higher than the flood level rim of the highest fixture served by the stack.

Under both the IRC & UPC a 1-1'2" vent would be sufficient for the DFU load of the fixtures listed, however my concern is how are you venting the tub? You may not have a horizontal vent line under the floor and if you run a vertical vent up from the point where the line from the trap passes under the wall at the head of the tub, you must have a minimum distance of 2x the drain line diameter from the trap weir to the vent opening. Assuming the tub drain to be 1-1'2" that means you must have a 6" section of horizontal pipe from the trap weir to the vent riser.

If you could post a drawing of your layout, or email a drawing to me, i will be glad to work it out and post the reply.

I would also need to know which code you are under. If you don't know, tell me what city, county and state you live in and i will find out which code your local code is based upon.

Lazypup, I'm indebted to you for your generous and definitive reply.

I have more legwork to do locally before I take advantage of your kind offer, however. I expect to hire a local independent plumber with whom I had worked before and he will be able to guide me; I had hoped to figure some things out before calling him, and maybe do some of the work (roof penetration, if necessary, for example) before he started.

I think I understand most of what you explained, but what is a "DFU"?

Also, are there any conditions under either code which allow air admittance valves? This would certainly be the easy answer to my tub problem, which is going to be difficult by any measure.

Thanks again,

Mark

Just looked up "dfu" and see that it means "drain fixture unit".

Regarding my tub it may be feasible to wet vent it; it is allowed in the UPC, which governs our area:

908.4.1 BATHROOM WET VENTING
Where permitted. Any combination of fixtures within one or two bathrooms located on the same
floor level in dwellings and guest rooms shall be permitted to be vented by a wet vent. The wet
vent shall be considered the vent for the fixtures and shall extend from the connection of the dry
vent along the direction of the flow in the drain pipe to the most downstream fixture drain
connection to the horizontal branch drain. Only fixtures within the bathrooms shall connect to the
wet vented horizontal branch drain. Any additional fixtures shall discharge downstream on the wet
vent system and be conventionally vented.
908.4.2 VENT CONNECTION
The dry vent connection to the wet vent shall be an individual vent or common vent for the
lavatory, bidet, shower, or bath tub.
908.4.3 SIZING WET VENTS
The wet vent shall be sized based on the fixture unit discharge into the wet vent. The wet vent
shall be a minimum size of 2 inches for 4 dfus or less, and 3 inches for more than 4 dfus.

So that may come in handy.

Thanks again for your help.