Help with rough-in of drain system

To all,

I’m planning on building a detached garage/shop in the near future. I would like to rough-in the drain system for a sink and a toilet prior to the contractor pouring the concrete slabs. My building site has approximately 7 foot of slope running from one end of the building to the other (52†total). To lessen the amount of excavation and the amount of fill, I’ve decided to pour one slab 40 inches higher then the other. In other words, the garage portion of the building (the right side) will be 40 inches higher than the shop portion of the building (the left side). I’ve enclosed a picture of the layout. If it’s not obvious, the slope of the land is from right to left.

I need some help in designing the drain system given that my septic system will be approximately 44 feet from these fixtures. The toilet will sit on the upper slab and the sink on the lower slab. The elevation difference between the toilet and the septic system is almost 7 feet.

My initial idea is to plumb the sink drain through the block wall (to the right) and tie into the drain for the toilet. I will then drop vertical to a point where I can begin my slope under the lower shop slab to the septic system. I realize that all the books say that the slope of the drain must be at least ¼ inch of drop for every horizontal foot. Given that formula, my total drop is 11 inches. Due to the distance I have to go to my septic system (~44 feet), is there any advantage to increasing the slope of the drain or does that cause other problems? Is there an advantage of using a 4†main drain versus a 3�

Also, since my two fixtures are within 4 feet of each other, will one 2 inch vent be sufficient? If not, I can vent the sink at the block wall and vent the toilet just above where it connects to the main.

Sorry about all the questions but I’m trying not to make any big mistakes. Thanks in advance for any advice I can get.

Regards,

Reid

You can slope the drain as much as is needed. When you install the toilet piping, vent it right away in the wall serving behind it and drop the vertical pipe down low enough to follow the gradual slope of the drain in the lower level floor. You do not exactly have to tie the drain to the toilet in the close proximity, catch it in the floor as it heads toward the septic tank.

Tie your sink vent into you toilet 3" main vent stack @ 42" or higher and run one pipe through the roof.

Running 4" inside the building is overkill but it should be 4" before it passes through the building wall. So..............4" through the building wall into a 4" tee with a 4" by 3" bushing out the front to catch the sink and toilet. Run 2" instead of the minimum requirements of 1.5" for the sink; it's well worth it depending on what you plan on sending down the drain. Come out of the top of that tee with 4" and use a recessed 4" cap for your cleanout to the septic tank.

Run 3" continuous to the roof. That is minimum requirements in most areas if that is the only bathroom facility in the structure.

Thanks very much Rugged. I understand the bulk of your suggestions but I'm somewhat confused on your 3rd paragraph. You're suggesting I use a 4" tee after coming through the wall. I assume you're referring to the wall adjacent to the toilet and not the wall closer to the septic system. I'm not sure what you mean by using a 4" by 3" bushing to connect the sink and toilet? If I've got a tee, I connect one end of the top of the tee to the outgoing 4" drain. The other end of the tee reduce to a 3" and connects to the toilet drain. The lower end of the tee reduces to a 2" and connects to the sink drain. Is that correct?

Also, thanks for the suggestion of using 3" vent - I'll take your advice on that as well.

Thanks very much,

Reid

I believe he intends for that tee to provide a cleanout to the main line - not as the junction of the 2 fixture drain lines.

The fixtures would be joined via a second junction (a wye) further upstream, closer to the bathroom. No need to run 2 pipes all that distance as separate drains.

Thanks Prashster,

Ok - that make more sense. Sounds like if I'm runnning 3" drain pipe inside the building and I need 4" to go through the building wall, the tee must end up just inside the building. Right?

Is there are value adding another clean out close to the toilet?

Reid

rlmaier said:

Thanks Prashster,

Ok - that make more sense. Sounds like if I'm runnning 3" drain pipe inside the building and I need 4" to go through the building wall, the tee must end up just inside the building. Right?

Is there are value adding another clean out close to the toilet?

Reid

Click to expand...

Yes. This allows an easier access to unclog the drain through a 4" opening and somewhere you have reduce to 3", might as well do it with a 4" tee and come off the front with a 4X3 bushing.