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What is the code requirement on minimum depth on a 4" sewer line leaving a residential house?
What city, county, state are you in and when was the line installed?
Montgomery county, PA. This is for an addition to an existing house and an existing 4" sewer main which is approximately 4 1/2 feet deep at house. 20 feet away from the house is lateral which is 6 1/2 feet deep and is also a 6" lateral. We want to connect onto existing 4" main just before lateral using a 4 x 3 "Y" and continuing 3" schedule 40 piping approximately 25 feet into new addition. This 3" line will pick up first and second floor bathroom. We want to be above footer of new addition. The footer for the addition is 3 feet deep. We want it to be roughly 18 - 20 inches deep coming into the addition which will put our new sewer line 4" off of floor slab.
The inspector won't pass it because he claims that the sewer line will freeze if it's not below frost level (3feet) which is correct for a water main (code) not a gravity fed sewer line with slope of 1/4" per foot or slightly more.
Our question is does a sewer line have to be by code 3 feet deep? I have never heard of this and I have been in business 22 years and I have never been told or turned down from any municipality or inspector for having a sewer line enter or leave a house less then 3 feet deep.
I believe you fall under the 2006 International Plumbing Code and whatever local amendments your area may have. In doing some research it is now fairly common that sewer lines are required to be installed below the frost line. There are some options in some areas where they allow the pipe to be insulated with a minimum 2" thick foam pipe insulation when the pipe falls above the frost line when exiting the residence.
I believe your inspector has you on this one. You may want to discuss the option of using solid foam non hydroscopic pipe insulation where you fall above the 3 foot depth. This would be a reasonable option to present for your condition.