I'm about to start a basement rough-in for a 4 piece bathroom. While planning, it struck me that there is a potential problem in the event of the toilet backing up.
Although upstream and not accounting for grade,the waste lines for the tub and shower will connect to the main at the same elevation as the tiolet. Both connnections are approx. 5 foot runs, 8 feet from the WC connection. The main and WC will be 4" the tub and the shower 1-1/2" (would you recommend 2"?) Grade will be between 1/8" to 3/16" / ft. If at some point the toilet were to backup, it wouldn't take much before waste would enter the fixtures. What can be used to prevent waste entering these fixtures? Is ther I understand backwater valves prevent solid waste only? What other valve, device or method can be used to control flow direction. The grade I've mentioned is the best I can get under the circumstances. Is this a valid concern?