The structure is separate from the house, therefore it must be vented as a separate structure.

Under the International Residential Code ALL STRUCTURES MUST HAVE a "Main Vent" that runs undiminished in size from the building "Main Drain" through the roof.

Under the Uniform Plumbing Code vents ALL VENTS must terminate through the roof. Individual vents may be reduced in size, however the total cross sectional area of all vents must be equal too or greater than the cross sectional area of the building "Main Drain".

Under both codes a toilet with a 1.6gal/flush is required to have a 3" drain therefore you would be required to have a 3" vent through the roof under either code.

Both the shower and the watercloset should be set up to "Wet Vent" to the "Main Drain"

A vent may not be run horizontal until it reaches an elevation which is 6" above the flood level rim of the highest fixture served by that vent, which in this case it would be 6" higher than the top rim of the lavatory bowl.

Depending upon how the fixtures are laid out, the minimum venting you would be required is to have a 3" line from the main drain to the wall behind the lavatory, then a 3" riser up to the roof.

Make an illustration showing dimensions of your layout and email it to me and i may be able to confirm your layout.

The bottom line, no matter how you do it, you will be required to put a vent through the roof.

LazyPup@yahoo.com

And the other bottom line is that your plumber didn't do the full job of roughing in for fixtures. Roughing in plumbing correctly also involves venting. Every single thing that has to be calculated for a system to work has to be taken into account for the rough in stage. The finish work is the fancy pretty stuff that's far less important. If you specifically paid for that structure to be rough plumbed, then give the plumber a call and ask him about the venting issue that you're now dealing with.

lazypup-
Thanks for providing good advice (as you always do) and for the kind offer. We've had a power outage since yesterday, so I just saw your message. I'll email you tonight.

live_wire_oak-
I agree with you, but the house was 4 years old when I bought it, so, unfortunately, I don't have recourse to the original plumber.

QUOTE--"And the other bottom line is that your plumber didn't do the full job of roughing in for fixtures."

Not True...Although this is a separate structure on a separate slab it still could have been connected to the primary structure by means of a vestibule, roof deck or other means. If that were the case it would all be one structure and would not require the "Main Vent" or a vent equal to the diameter of the main drain. It would only require an "Auxiliary vent".

Under the IRC an auxiliary vent may be reduced to 1/2 the diameter of the line it serves, which in this case would be a 3" line, so the auxiliary vent would only need to be a 1-1/2" pipe. In this case the 1-1/2" riser to the lavatory would be extended up to the roof and it would provide the vent for the entire bathroom group.

Under the UPC vents must be sized by the number of DFU's on the line the vent serves.

A watercloset with a 1.6gpf or less is 3DFU's. A shower stall is 2DFU's and a lavatory is 1DFU for a combined total of 6DFU's. Under the UPC a 1-1/2" line is rated for up to 8DFU's however we may not vent a watercloset with a 1-1/2" line so the line from the main drain to the wall and up to the lavatory would need to be a 2" line, with 2" extending up through the roof.

Because this is a separate structure it requires a 3" vent through the roof.

I can confirm that my roughed-in drains do connect with the main house drain since I can hear water flowing in the studio drains when someone flushes inside the house. (The studio and house are on separate slabs, but he buildings are only about 6' apart, and the roof edges almost touch.) I re-measured the drains...there is one that's 1.5" where I'll put the shower, and two 3" drains, so I'm assuming I can put the toilet in either of these two locations.

Even though this structure is within 6" of the primary structure and the drain lines are connected, none the less, it is a separate structure and will require the 3" vent through the roof.

I received your drawing and have reviewed it. The dimensions are close enough that you will only require one vent for the bathroom group, that being a 3" vent through the roof as mentioned above.

You have two 3" openings however a watercloset may only be installed on the end of the line, it may not be installed over a TEE. You would need to look down in the pipes to determine which one is on a tee and which has a long radius elbow. The toilet MUST BE installed on the one with the elbow.

The drain for this structure is tied into the main structures DWV system so the position of the watercloset should be the 3" which is furthest away from the main structure.

When you install the 3" vent riser on the opposite 3" opening you could then install a 3"x3"x1-1/2" Sanitary Tee on that riser and run a horizontal line to the lavatory. If you are under the International Residential Code that horizontal line may be up to 6' long when measured along the top of the pipe from the vent riser to the fixture trap. If you are under the Uniform Plumbing Code the maximum length of that horizontal waste arm would be 3'6". Remember, that is the actual developed length of the pipe, not the horizontal distance from the vent to the fixture.

From the placement of the 1-1/2" line in the floor it is obvious that it was intended to be for a shower or tub. If you install a tub/shower you may use the 1-1/2" line but if you install a shower stall the code now requires that the drain line for a shower stall must be a minimum of 2", in which case you would need to change that line to 2".

lazypup-
Thanks very much. Your advice was very thorough and clear and I understand exactly what I have to do now. I appreciate all the thought and time you put into this "consult."