How deep below a slab would a sewer line (to the street) be?

[wallbobby]

I have a 1952 house - daylight basement with a slab. House was converted to sewer in 1966, but they didn't bother to hook up the basement bathroom to the sewer and it's still going to the original septic tank (other parts of the house are going to sewer. I'm in the process of trying to fix that.

Today I broke the slab in what would be a straight line from the main stack to the street. I have dug down 2 feet below the slab and still see no line (and stuck a probe another 6-8 inches past where I've dug). I have the original permit filed in 66 and it says 4" cip, 12 ft at the curb line. Depending on what they call the curb, I've either dug 10 ft below the curb line or I'm right at 12 ft. It's about 26 ft to the curb, so if there's 1/4" per foot this thing has to start a minimum of 6-7" higher...

Anyway, in the pacific northwest, of a house of this age, what's the deepest you've ever seen a sewer line under a basement slab? I naively assumed they would make it as shallow as they could get away with.

 

[Reach4]

I think you are saying that you are digging down along side of a 4 inch vertical sewer pipe that decends through the basement floor. There is a cleanout in that pipe, and I hope that cleanout is low. Try probing down the cleanout, and feel the change in direction.

 

[Sponsor]

 

[wallbobby]

My hole is 3 ft from where the main stack enters the slab (and the cleanout is there). I did stick a tape measure in the cleanout and got 1about 16" before resistance (I'm assuming the base of the stack) which is why I'm surprised I got to 24" and didn't find anything. I've stuck a camera in there before but couldn't tell much. My hole is 3 ft wide so I'm sort of perplexed. Not looking forward to going another 2 ft. I will see if I can get my arm in the cleanout.

 

[Bluebinky]

Perhaps the sewer "hookup" might have been made just upstream of where the drain from the house entered the septic tank, as that would be a sure find. Just a guess, though...

 

[wallbobby]

Yeah unfortunately they are opposite directions - septic in backyard, sewer goes towards front. The city was even out coincidentally and marked the sewer line on the street (they marked everybody's) and I followed that line - it lines up with where I thought it was and am digging. I'm still scratching my head why they didn't just do it right when they put in the sewer - I suspect they would have had to redo all the vents (I am doing that) but compared to the work of actually putting in 30 feet of 4 inch cast iron at least 3 ft under the slab, it seems like small potatoes. If it's actually just deeper (and I'm not missing it) then at least I don't have to worry about having enough depth to connect the rest of the fixtures.

 

[Bluebinky]

Are you saying that the septic tank is in the back yard and the sewer hookup is at the street in front? If so, I would think that the hookup meandering around to the back yard is more likely than they tore up the basement floor to turn the plumbing around. Just a guess...

 

[wallbobby]

I've scoped the sewer, there's no meandering, I think it's just much deeper than I thought. It's a daylight basement, they had zero choice but to go straight under the house. Anyway, they left this drawing on page 2 of the permit. To the right is the back yard, to the left is the front yard and street.

 

[Reach4]

Can you borrow a metal detector that can detect big iron at a depth?? If you dug through steel mesh, that would probably mess up trying to detect the cast iron.

Where is the stack? Under N in the marked-u sketch? Near S?

Where are you digging? Under K?

 

[wallbobby]

Yes, that's right, digging under k exactly and the stack is under N. I think the other line is referring to perhaps an upstairs toilet, or actually, probably to the upstairs tub which is the only other thing they hooked to the sewer. No mesh, it's just a 3-4 inch slab. About an inch or 2 of sand, then I'm down 2 feet into dirt. Might try metal detector.

 

[hj]

I am trying to figure out WHY they would have run the sewer through the building and NOT connected the basement. AND, why the city allowed them to make the connection and NOT "abandon" the septic tank, (every city I have ever heard of insists on the septic tank being abandoned and filled with material).

 

[Reach4]

Does the basement floor show signs of the newer concrete work? Suppose they decided to go out the back wall with the new sewer and around the house. That would have meant they would not have had to dig much in the basement floor.

Maybe they decided to intercept the pipe to the septic tank and run that around the house. Do you have an indication that anything is going to the septic tank now? How many times have you had the septic tank pumped? Did the sellers represent that they had had the septic tank pumped? For the line to the septic to originally be under the basement floor, the back yard altitude would be significantly lower than the front.

You will think that those things I have suggested are not in accord with the permit sketch. It seems to me that you have established that what you have is not in accord with the permit sketch.

Could you use a small mirror and a flashlight to see which way the elbow at the bottom of that stack is pointing? Could you use some kind of asymmetric probe to feel which way the elbow is pointing? How about a periscope to look into the cleanout? If you can get enough light from a flashlight around the periscope, that would be good. If there is a vent on the roof immediately above that stack, somebody could reflect the sun down the pipe. You might find an inspection camera. The cheap lighted USB flex camera that I bought was disappointing.

 

[wallbobby]

I am trying to figure out WHY they would have run the sewer through the building and NOT connected the basement. AND, why the city allowed them to make the connection and NOT "abandon" the septic tank, (every city I have ever heard of insists on the septic tank being abandoned and filled with material).

It's a good question, HJ, we've been trying to figure it out too. The obvious explanation was that the sewer line was too shallow, but that doesn't seem to be the case. The only thing I can think of is that they just didn't want to do the work. Those people are no doubt long dead or very elderly so it would be hard to find out. We didn't question it when we bought the house because it was clearly on record that the house was converted 50 years ago. The seller knew, I am pretty sure, but did not disclose it.

As to all the other questions: a) I have scoped both systems myself. The sewer goes to the street, the septic system is still active and I scoped it right into the tank too. There is no meandering on either. b) I pumped the septic tank last summer after I found out that we had one. The seller did not disclose that there was any septic on the property. We bought the house 12 years ago, so it got pumped once in 12 years and I don't know how often before that. I dug the thing out and watched it working - it's clearly never leaked and is still functioning as the scum line is hard and never higher than the outlet to the d box which is somewhere in our lower yard. They didn't intercept anything because the tank is still working and the pipe is still coming from the house to it! The back yard where the septic is is at least 15 feet lower than the front.

Septic tank had 1)all the sinks in the house which is 2 kitchen, 2 upstairs lav, 2 downstairs lav, downstairs toilet and downstairs shower, so very considerable volume going to it since 1952 when it went in (that permit is here too, 500 gal concrete). Sewer had upstairs toilet, upstairs shower and laundry (downstairs) going to it. I say had because I am redoing everything to fix this and rerouted all the upstairs sinks to the sewer already, and demolished the downstairs shower, so now all that the septic is serving is a toilet and 2 downstairs lavs, digging this hole and tying into the sewer is the last step, moving those over with all new piping.

I talked to an uncle who has built a few hundred houses since 1940 and he mentioned they probably wanted to keep that optimal 1/4 slope out to the street, so if it's 12 feet at the curb, minus the 8 feet to get to the basement floor, minus the 8 inches or so for the slope, my pipe is probably just deeper under the house, I need to dig another foot or so.

 

[Reach4]

I am trying to figure out WHY they would have run the sewer through the building and NOT connected the basement.

Maybe they were concerned about a city sewer backup flooding the basement? A flooded basement filled with other people's sewage is memorable and motivating, even if it just happens to an acquaintance.

If that is a concern, you could put in a vented sewage pit. Then pump the sewage into your sewer stack above ground level. The problem is that your basement plumbing drains would not work in a power outage, and there is increased system cost. Another problem would be if your line to the sewer got blocked, your pump would pressurize your sewer line and it could come out on an upper floor.

 

[wallbobby]

That doesn't make sense - a sewer backup would be the same regardless, and the laundry (in the basement) was hooked up to sewer. Again, it's not a true basement, only the back wall is foundation, it's a daylight basement.

 

[Reach4]

That doesn't make sense - a sewer backup would be the same regardless, and the laundry (in the basement) was hooked up to sewer. Again, it's not a true basement, only the back wall is foundation, it's a daylight basement.

Is any toilet feeding to the septic tank right now? If not, yes, you could have a blockage. But you would not get anybody's poop coming into your shower or basement etc.

A city sewer can back up with other people's sewage coming in under pressure. If you are at the bottom of a hill, that is more likely than if you are at the top of a hill. After a big rain, notice if you see a bunch of stuff at the curb of the low-altitude houses. If you see that, look into protection. The people who got flooded basements will work to not accept sewage into the basement next time. So the next time, the problem moves uphill to the houses that did not flood last time. Any sewer could get blocked up and refuse to accept your own sewage.

 

[wallbobby]

Yes, there is one toilet going to the sewer and one toilet going to the septic. Again, I've sent a camera through the septic cleanout right into the tank (and watched the tank fill with the lid off), that path is clear, and I've sent a camera 50 feet down range into the city sewer, I've seen with my own eyes every sq inch of both systems (they are not connected together). Anyway, the sewer main on the street is 8 inch. I don't think this is the issue, the neighborhood sewer has been working fine for 80+ years - we can see every service request online, as well as all the sewer and water assets. Mine is interestingly only the second house on the line as it goes downhill - so if there was a blockage ever in the sewer, it could only come from our house or the house to our left.


It's kind of crazy, I went out and measured the locate line at the street again and I think it's about a foot to the left of where I'm digging. I'm not sure why they wouldn't run a straight line from the stack though, and when I scoped it there weren't any turns like that.

 

[Reach4]

Mine is interestingly only the second house on the line as it goes downhill - so if there was a blockage ever in the sewer, it could only come from our house or the house to our left.

That would seem to indicate you would be OK.

I did stick a tape measure in the cleanout and got 1about 16" before resistance (I'm assuming the base of the stack) which is why I'm surprised I got to 24" and didn't find anything.

At that depth in your hole, I would try probing or digging toward the stack.

 

[wallbobby]

I'm starting to wonder - and this sounds absurd, I know) about putting in a sewer ejector system. There is a secondary 2 inch cleanout at the base of the stack (unused) and I could put everything into that with a pump. I'm not sure if anybody's ever put one of those in not because their line is too shallow, but rather too deep. It's just going to be a huge amount of work to dig this out if it's really as deep as I suspect.

 

[wallbobby]

I found it - a couple feet to the left of where I was digging, definitely not in line with the stack. It's only about 12 inches deep, hopefully that's enough, distance to the toilet is not too far - 8 ft at most, could do a diagonal and make it 6. If I only need 2 inches of drop, seems like it's doable - not sure how much space the fittings for the toilet take up.

 

[Themp]

Not sure how your sewer bill is calculated but ours is based on the amount of water used. Since you had a large portion of your water going to the spectic tank for 11 years and the city passed the original inspection for the sewer hookup, you might have a case for a sewer usage adjustment for 11 years.