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 Under Slab Sewer Belly/Dip Repair
Author: John S (PA)

Recent video inspection shows that my below concrete slab sanitary sewer line is 30% blocked with standing water due to a belly/dip in the line. Location of the belly begins about 25' in from the outside wall. Length of the belly is about 10'. Symptoms of the problem are gurgling and sometimes toilet overflows when higher demands are placed on the drain line.

The building is about 35 years old and was built on top of a sand bar along coast of Florida. Other similar age buildings nearby have had sewer lines replaced.

The plumber has recommended excavating from the inside by cutting through the concrete slab and correcting the slope. Depending on conditions found, the plumber says that the belly/dip problem may extend further than the 10'.

Given the age of the building, and corrosive effect of salt water on the existing cast iron pipe and steel pipe hangers, should the entire sewer line be replaced by tunnelling under the slab? I also am concerned about the mess of demolition and cutting through ceramic floors in my furnished home.

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 Re: Under Slab Sewer Belly/Dip Repair
Author: e-plumber (NY)

Call around and get a few estimates, it seem that this problem is common in your area. Be prepared to have more of the floor opened than where the belly is, several feet before it and several feet after it.

A video camera inspection when it's complete to confirm that it was done correctly may be a good idea.
Having the work filed and inspected by the local Building Dept. is recommended as well.


"The society which scorns excellence in Plumbing as a humble activity and tolerates shoddiness in philosophy because it is an
exalted activity will have neither good Plumbing nor good philosophy: neither its pipes nor its theories will hold water." -
John William Gardner 10/8/1912 - 2/16/2002

Repair your leaking Plumbing fixtures ASAP [www.theplumber.com]
This slow drip will waste 7+ gallons of water per day.

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 Re: Under Slab Sewer Belly/Dip Repair
Author: hj (AZ)

The only way to fix the belly in the pipe is to excavate from a location before the belly to a location after it that is the proper depth to give adequate slope. Those two points could be separated by much more distance than the length of the belly, and could entail replacing the entire pipe line. Until you have "exact" dimensions as to the depth of the pipe and the parameters for repairing it, you cannot really make a good decision.

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 Re: Under Slab Sewer Belly/Dip Repair
Author: John S (PA)

Thanks for the valuable suggestions. What information would you need to determine whether 1) to cut the slab and excavate from above or 2) tunnel from below the slab?

If the cause of the problem is due to rusted out pipe hangers that were attached to the bottom of the slab, should all of the adjoining pipe hangers be replaced?

This is a furnished condo. If the excavation is made from above, What steps should be taken to taken to avoid dust, dirt, and sewage matter from contaminating the interior of the condo?

My main concern is that due to the 35-year old age of the sewer pipe system in salt water conditons, the entire sewer line should be replaced. Is there any way to determine if this is the case without excavating a portion of the sewer line? The condo was built on top of a sand bar with the Gulf of Mexico on one side and a bay on the other--with only one road down the middle and condos on both sides.


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 Re: Under Slab Sewer Belly/Dip Repair
Author: redwood (CT)

Determining whether to dig straight down or tunnel is determined by obstructions to either.

Underground there will not be hangers. The soil is what supports the pipe.

The area will have to be cleared, tarps an plastic sheets used to protect the area.

You are probably correct in assuming the effcts of salt water on the pipes.

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