The home I just bought was built in the late 50's and it does not have a clean out valve for the sewer. My realtor is saying that is very common. Is that true. Should the person who did my home inspection have noticed this and informed me. I have only been in the home for a few weeks and have already had major sewer issues. Since I don't have the clean out valve they need to access my plumbing from my roof or pull the toilet.

A 50s house should have a cleanout. A much older one should. Send a letter to the inspector holding him responsible for overlooking it and for work about to be done. He may jump at the chance to do it himself (many can) so as to not be sued. Whether you would win or not (small claims) I don't know.

Cast iron cleanouts tend to rust out first so it may have been there originally I would guess, and someone later cheated.

On slab houses code dictates that the plumber install a clean out not over 18" off the foundation and brought up to grade. On older homes with basements we installed a cleanout just before exiting the house. In time a cleanout may be built over and hidden with a addition or buried under the ground. When that happens we do one of two things to locating a lost cleanout. On slab houses we take a rod with a pointy end called a probe and began to poke around the section of the house that we feel faces the street sewer or the septic tank. Since the sewer's up near the ground level we will poke into a sewer pipe sooner or later.
In basement homes we determine where the main stack comes down from up stairs. Since we attempt to keep our drainage in a straight line we then establish where the city connects to the house sewer or where the septic tank is and plot a straight line from the basement stack to the terminal point and that will give us a pretty good idea of where to dig a lateral trench to locate the pipe or cleanout. All of the homes I've ever worked on have sewer cleanouts. It's not only good plumbing practice but just plain common sense. Good luck, Tom