Proposed Plumbing Layout Question for Utility Building - shower/sink/toilet

Question

I have two questions regarding the plumbing layout for my utility building:

1. I would like to run the plumbing from the inside of the utility building to a currently existing main four-inch line running parallel to the building and onto a septic tank. In order to do this, I would like to connect 4 inch sanitary tees from the 4 inch line to under the building using a 90 degree Long sweep elbow that will Then go up into the flooring. There will be three tees connecting a toilet, a sink, and a shower. See view number one. The question is will this configuration cause any problems for drainage?

2. Secondly, I have a question about a vent line. I would like to run a single line above the 4 inch line as a vent for all plumbing in the building. My question is will this be acceptable for ventilation?. The maximum distance between the shower and the sink will be 10 feet. I don’t understand the need to have ventilation at each drain for the shower and sink as well as a vent at the toilet, if the pressure is ventilated out of the main line.

There aren’t really any building codes to be concerned about in this area. And this is only a utility building, there won’t be high use throughout the day.

Answer 1

I think that you are describing connecting each fixture back to the 4" main separately. This would require each fixture to be vented individually. A simpler solution would be to run a single brach from the existing line and drain all the fixtures to that single connection via a wet vent:

Regarding the use of long sweep 90º elbows to transition from vertical to horizontal: allowable in some jurisdictions and not in others (we have to use two 45º elbows (even in the radius is tighter than a long sweep 90º elbow) where I live).

Regarding connecting to the existing main with tees: I'm not aware of anywhere that permits that. This would be typical be made with a wye and 45º elbow or a combo wye with 45º. Tees are used in the vertical orientation or on the venting in either orientation.