Septic Inlet Pipe Size

mburke121 · Oct 22, 2010

I removed an old mobile home from my property, and now I'd like to cap the septic tank inlet to protect it for potential future use. The pipe is flared at the end (90 degree elbow with coupling), and is just an inch or so above ground level. I "eyeballed" the size, and went to Home Depot and purchased a 4" PVC street elbow, with a cap. Problem is, the four inch coupling is about a 1/2" too big for the inlet pipe. While I haven't put a caliper to the pipe, it appears to be 3 1/2". The wall is about half the thickness of the PVC coupling I purchased.

Does anyone know if that was a standard size about 30 years ago, and where I might be able to find the right size coupling?

There's no place like TBN to get answers to obscure questions like this. Thanks in advance for your help.

Kays Supply · Oct 22, 2010

3 1/2" OD is 3" pipe sch 40. In Illinois, that is too small for a sewer line outside the house. 4" is the min. size. There is a 4" SDR and a 4" S+D pipe sometimes used for sewer line. It has a smaller OD than 4" sch 40. A 4" S+D coupling will fit inside a 4" sch 4o fitting.