We are putting a small apartment in our barn. I am trying to figure out how to vent the drains from the various fixtures.

There will be a single 4-inch drain picking up the following fixtures (starting at the most upstream part of the drain): kitchen sink; bathtub; water closet; lavatory; washing machine. The lavatory and washing machine will probably share a drain.

I know how to vent the drains from the sink, bathtub, lavatory, and washing machine, but I don't know how to vent the drain from the water closet. Do I just hook a vent into the drain at some point downstream of the elbow coming out of the bottom of the water closet?

the main drain should be a vertical stackpipe at the minimumventing either thru the roof or at 90 degrees as high as possible. All your fixture,water closet,basin,shower...etc,will have their own traps before they drain into the stackpipe.The more fixtures,the larger the main stack or drain.

I've always done them this way rather than the troublesome horizontal vents.

If you're using PVC you can clamp a saddle onto the stack as an afterthought or just cut it and installa proper tee.

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The water closet is self trapping.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

on a side note, I would install something like this

http://www.homedepot.ca/webapp/wcs/stores/servlet/CatalogSearchResultView?D=938837&Ntt=938837&catalogId=10051&langId=-15&storeId=10051&Dx=mode+matchallpartial&Ntx=mode+matchall&recN=0&N=0&Ntk=P_PartNumber

It seems a bit pricey, but copper prices have come down a lot lately so you might find it cheaper. Water heating accounts for a large portion of a homes energy use. Very little of the heat is actually used in a shower, most of it goes down the drain. This can recapture it.