I can't seem to find any reference to grease waste pipe slope requirements in IPC 2009, only for sanitary. Does IPC 2009 consider grease waste the same as sanitary? the grease interceptors section doesn't seem to address the issue either. thanks in advance
My understanding is there is no code language mandating a greater slope for grease waste lines to get the grease to the interceptor quicker. Sizing grease waste system interceptors is a complicated issue and the grease interceptor sizing and grease waste requirements change from municipality to municipality, depending on which code or AHJ is involved in the project.
International Plumbing Code makes no special distinction about the slope of the grease pipe.
And he is also corrrect that how the grease interceptor is designed and/or located is very dependant on the local code authority - but be careful in that the plumbing inspector may not be the code authority for the grease interceptor design.