I have a common drain for three sinks the clogs 1-2x per year. All three sinks back up at the same time and slowly drain. Total distance between sinks is about 25ft. I suspect that the sinks share a common lateral drain pipe and a T comes off that (hidden in walls) to a visible diagonal drain (several bends) and then down. All interior piping. If I snake down a sink, it just goes across to the other sink.
I have cleared the clog by snaking up the diagonal drain pipe (~5ft from a removable coupling). But as stated in other posts, this is probably only creating a hole in the clog. The suspect T is not visibly accessible. The T is probably ~10ft from the closest sink. There's a roof vent somewhere along the lateral pipe.
I have tried running hot water down sinks on both sides of the drain, but that does not seem to help.
I don't want to damage the cast iron pipes or replace them at this time.
Any suggestions for clearing the bigger clog? Anything safe to pour down the sinks that would dissolve hair, scum, etc? I can poor from both sides with or without water in the pipes (~10ft to the T). It will sit there for a while and slowly drain.