I completed a repair to a leaking sewer line under my house. This required replacing rusted cast iron pipe with PVC. For safe measure, I ran a pipe snake down the sewer pipe to ensure that a clog was not backing up sewage. Material cost was only less than $10 for this fix.

To answer a few questions that professional plumbers have asked:

I reduced pipe diameter in order to fit the distal PVC pipe end into the inner diameter of the original corroded iron pipe. Connection to the outer diameter would not have been reliable and could have more easily leaked.

As mentioned in the video, this drain is for the kitchen sink and clothes washer only. Large material is unlikely to enter this drain pipe. Therefore, I am not too concerned with the transition to a lesser pipe diameter causing a clog. If a clog does occur, I can easily snake the pipe from the distal end of the PVC under the house, by removing the PVC from the ID of the cast iron pipe.

Yes, the pipe does tilt downward to allow gravity to drain into the septic tank.

While use of DWV would have been the more traditional route instead of pressure fittings, I'm not so sure why one can't use pressure fittings instead of DWV. Certainly one should not use DWV where pressure fittings should be used (due to high pressure), but using pipe that can handle high pressure in low pressure scenarios should not be a problem. Pressure fittings have less gluing surface, but the surface area is still sufficient for a glue joint that does not have any applied load.

As mentioned in the video, this was a fix to delay digging up of the entire septic tank and system, which may be necessary in the next 10-30 years. The fix does not need to be perfect, but only functional. When the septic tank requires replacement, the remaining steel pipe can be removed and replaced with PVC. Then a fully proper fix can be made. In the meantime, my repair has not had any issues the last three years, and I don't expect any issues for years to come. I have seen zero leaks and zero clogs.