I am start a new thread because the old Sewer Get/Check Valve thread is from 2009. The problem is the same (City sewage line gets block causing sewage to come out the basement toilets and showers). This is the 3 time in 25 years at this address. The end townhouse added a check valve the second time this happened and home was not flooded. The plumber that did the work told the owner that the pipes were not "right" and he could not find the outside line. He had to snake a camera down the line from inside the house to find the "right" location and basically jackhammer through the basement floor and install it. My questions are as to inserting a valve.

1). Is that really the right way to do it? Or the laze way?
2). Normally, where are the valves installed, inside or outside?
3). If outside, can I dig the hole myself to save money? I hear that controller don't want that people doing that, but it’s an idea. Someone of course would have to show somewhere to dig, so its probably not workable.
4). What’s the best valve to put in? Any brands/designs that are better to use? The owner is concerned what would happen if it go stuck closed because of sewage mud/junk after a backup or over time.
5). What type of contractor should she look for: plumber, general contractor, or other type of person?