Measuring slope of main drain lines

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Mitchell-DIY-Guy

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While the plumbing in my home works well, my next door neighbor (with an identical home and identical plumbing system) has had one major sewage blockage, and a second minor one in the past few months. He claims no normal offense: no unsavory or inappropriate things tossed in the toilet. So, he suspects that perhaps the slope of the main drain isn't right.

So, this is a very long run in the crawl space of the home, probably about 70' or more from the furthest toilet to the exit. The main drain line is 3" Sched 40 PVC. Is the slope supposed to be .25" per foot? That would mean a drop of 17.5" or so from the start of the main at the furthest toilet, to the connection at the front of the house.

Any considerations here? The toilets are new meaning low flow. There are I believe, two bends probably totally 45 degrees. All the other drains eventually tap into the main.

Any advice on investigation this with my neighbor before we put on the spelunking hats and go measuring?
 

JG plumbing

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I would just look for sags and non optimal fittings. Short 90°'s or double drainage fittings. I wouldn't expect a full 2% slope either. 1% is common and fine as long as load accounts correctly.
 

Jeff Handy

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Same house same drains but different occupants.

Neighbor’s house might have people who overuse TP.
Or who take massive dumps.
Or who flush toilet wipes, tampons, q tips, paper towels.
 

Twowaxhack

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Find out how much water your neighbors toilet uses compared to your toilets. That may or may not be a factor.
 

Jeff Handy

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Regarding my earlier “massive dumps” comment.
I had a client who had recurring clogs in his master bath toilet, that needed a toilet auger to unclog.
I was going to pull the toilet to investigate, but first I suggested they try an experiment and have husband and wife use different toilets for pooping.
Husband moved to basement toilet, problem followed him there.
He was constipated and only pooped about three times a week.
Massive python turds with loads of paper.
Metamucil was suggested and his toilet routine improved, and clogs went away.
Sorry if TMI but real life story.
 

Jeff Handy

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Find out how much water your neighbors toilet uses compared to your toilets. That may or may not be a factor.
Add a second flush after a big poop.
A little water waste beats paying to clean the drain line.
Unless this is out west with the water crisis.
Then they should start replumbing toilets to flush with gray water from shower or laundry somehow.
 

Tom the Elder

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I wouldn't expect a full 2% slope either. 1% is common and fine as long as load accounts correctly.
My 1951 ranch style house has two laterals, both originally orangeburg pipe so I had to replace them. Mine is the first house on the upper end of the sewer main so VERY shallow. Pipe under house has slope but between house and main the 30-40 ft long laterals are essentially flat (local standing water) but the inspector passed the replacements anyway because there is no practical alternative. 10+ years later I have never had a clog in the replaced laterals. I attribute that to high-flow toilets and daily use of bathtub. When I replaced a toilet a couple months back, it took two days of driving around to used toilet sources before I finally found a used high-flow replacement 70 miles away. Well worth the effort if that is what is keeping the line clear.
 

Mitchell-DIY-Guy

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Find out how much water your neighbors toilet uses compared to your toilets. That may or may not be a factor.
Our homes are nearly identical; same essential floor plan, mostly the same fixtures. Gerber toilets. No issues over here. I gave the neighbor some things to look for in the main line regarding slope, dips, and too-tight turns. Hopefully he'll put on his hard hat with light and go looking and measuring...
 

Tom the Elder

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A little water waste beats paying to clean the drain line.
Unless this is out west with the water crisis.
Then they should start replumbing toilets to flush with gray water from shower or laundry somehow.
I live in CA- embarrassed I didn't think of this. - will look into it.
 

Jeff Handy

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Probably much easier and maybe even locally approved to route your gray water right out into your landscaping, unless they are making you rip out everything but desert type plantings.
You can also route gray water into a holding tank, then add a small pump with a garden hose fitting.
I think in a very few years, homes out west in areas with water crises will be collecting rain water also, in tanks or maybe some kind of neighborhood co-op tank.
 

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