Sewer Drain fall question - not enough slope

J-Rod · Nov 17, 2013

Howdy,

I live in SE Texas near the coast. I recently built a detached garage behind my house. Before we put in the pad for the shop I discussed with my contractor about connecting it into my existing septic

The garage is on the left side of the house, and the septic on the right. He looked at it, and had us build up the pad an additional 2 feet above the house to have enough fall.

At the time the garage was built we just roughed in the drains and left them capped and buried outside.

I am now trying to run the sewer line and I have run into an issue. The drain behind the garage is buried about 4-6" in the ground. The drain comes out of the back of the garage, goes over about 5', makes a 45º turn and then runs across my backyard. When it gets even with the end of the house it makes another 45º turn and turns towards the main drain of the house which empties into my septic system.

I've dug my trench and shot it with a laser last night to set my "level". The total length of the system is 163'. In checking the drop I only have 16.5" of drop from one end to the other. I had planned to use 4" pipe, but the recommend fall is 1/4" to a minimum of 1/8" per foot.

By my calculations I have .083% fall currently 1/12". It looks like I need at least 8" more fall for the total system to even be at the minimum of 1/8" (.125%)

I believe my options to be:
Install a lift pump.
Install a new septic system behind my detached garage

I wanted to see if there were other options I was not considering. I had seen that if I went up to a larger diameter pipe that I could get away with less slope (6" or 8"), but I was concerned that the pipe would then be too large to keep the proper velocity.

Anyway, I'm just looking for options on what would be the best course of action.

Thank you.

camaroderrick73 · Nov 17, 2013

Those are your only options. Lift station is probably your least costly option.

phishfood · Nov 17, 2013

If you are going to install as is, I would back down to 3" pipe to keep the velocity up, and install a few extra cleanouts. You are 3 7/8" short on your fall to make 1/8" per foot, which is the required (IPC) fall on 3" and larger pipe.

Quick but precise way to calculate necessary inches of fall for a given run. Take the length of run and divide it by the lower number in your 1/8" per foot or 1/4" per foot requirement.

163 / 8 = 20.375

20.375 - 16.5 = 3.825 = 3 7/8"

As close as you are to making it, is there any way you can shorten the distance by rerouting the pipe?

J-Rod · Nov 17, 2013

Thank you everyone for the input so far.

phishfood

I have cleanouts set up now for every 40'. I was under the impression (perhaps incorrectly) that if I ran 3" I needed a minimum of 1/4" per foot. I understand that shrinking the diameter should keep velocity up, but if I go to 3" can I run it at 1/8" per foot. The area I live in is not subject to any inspections, I'm simply trying to make sure I do it right and don't run into issues.

I do have another question. I've seen the required fall expressed as 1/4" and 1/8", but I have also seen it expressed as a percentage (2%). 1/4" would be 2.5% and 1/8" would be 1.25%. I was wondering if one was "more correct" than the other.

Also, thank you for the calculations. The formula I found was to take the length of the run, and add the fall into it. Then divide the fall into the length to calculate the percentage of fall. So, I converted feet to inches first.

(163'*12") = 1956"
1956" + 16.50"(fall in inches) = 1972.50"

Then divide the fall into the length to calculate the percentage of fall.

16.50/1972.50 = .008365

Backing into the equation the other way. Length of the run * (1/8" in decimal )

1972.50" *.0125" = 24.656" of fall needed over the length of the run

I like the formula you used, I'm just trying to figure out if the fact that I am converting to inches is adding to my drop.

But, if I back into your formula, and I take 16.5" * 8 = 132'

So, I need to take approximately 30' of the run. I don't think I can do that, But I will check it tomorrow to see what I come up with. Thank you again for the input.

Chris · Nov 17, 2013

I have installed many city mains in 8" pipe at 1% fall with no ill affects. It is not recommended but that is what we have done when we can not get slope under direction of city or district officials.

I would do what Phish says and go to 3" pipe and run it with the cleanouts like you plan at 40'.

phishfood · Nov 17, 2013

Under International Plumbing Code, 3" pipe can be installed at 1/8" per foot of fall.

So for every 8 feet of pipe, you need to drop 1 inch. Therefore, my equation.