Backup, bellies, flat spots, odor

QUESTION: I purchased a new home in Rocklin in June 2002. In February 2004, the laundry room flooded. In other rooms, sinks, tubs and toilets were filled with backed-up water. A plumber cleared the system and said he found what he referred to as a “belly” in the pipes. We called the builders. They came out and determined the system to be clear but found what they called a “flat spot.” I fear that this mess could happen again.
Also, from the time I moved in, I’ve noticed a musty odor coming from the kitchen sink. When I finally got the attention of the service rep, I told him the problem was getting more pronounced. He sent a plumber, who said everything is draining okay and that the odor is not unusual because garbage disposals are made differently now and they collect food particles under the rubber gasket. I feel they are not addressing the drainage or odor problems.
Natalie Malone, Rocklin
ANSWER: “Building codes require that all drainage systems have a minimum grade of ¼ inch per foot,” says Vince McDonald of McDonald Plumbing, Heating and Air Conditioning in Sacramento. “This means that for every foot the drainage pipe travels, it drops in grade ¼ inch.”
“A ‘flat spot’ is a section of drain line that has no grade, up or down. A ‘belly’ in the line means that the line dips down and then back up before it resumes the proper grade downhill. The belly will hold water in the line at all times. Both situations can cause line stoppages. In a home as new as yours, both are caused by improper piping installations.”
“More than likely, the plumbing contractor failed to install the pipe with adequate grade and the building inspector failed to discover the mistake. There is also a chance that the pipe was installed properly but at the time of the backfill of the piping trench, the pipe was pushed down and is now out of grade. Often, installation mistakes like this go unnoticed because the line never stops up. With the advent of TV cameras for drain and sewer line inspections, we can now accurately diagnose bad drain lines. In your case, it sounds like there is a possibility the installation error is now causing your stoppage problem.”
“Unfortunately, the only way to fix the problem is to open up the floor and properly install a new line. This is a difficult and disruptive process, especially since you have concrete floors.” McDonald says you should talk with your home builder. “It is my belief that they are still responsible for the repairs. When in question, you can always contact the State Contractors License Board (800/321-2752 or and your local building department. Even if they agree to take care of the repairs, you need to have them thoroughly explain the impact of the repairs and remediation on your life and home. You may choose to continue to clean and monitor the line occasionally instead of the repair.”
“As far as the odor problem, it is unlikely that it is related to the piping problem under the floor. The odor could be caused by:
 A water leak in the walls or vicinity of the kitchen sink,
 An improper trap in the drain line under the sink,
 An improper venting system that permits the water to drain out of the trap under the sink, causing a sewer gas smell,
 A broken drain or venting pipe in the wall, or
 Improper installation of the dishwasher drain line hose

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