What if sewerage backs up into tub?

I just wanted to see what you guys think on this. I've been doing a pile of remodeling and repairs on an old river cottage. The toilet is slow to drain. The tub taps in past the toilet and the tub is not vented... everything else is vented properly (tub trap gurgles when toilet is flushed). The question is... what is to keep that tub from backing up with sewerage if the sewer line to the septic tank stops up? How would you plumb a drain to prevent this without using a back flow preventer? Is this addressed in a code somewhere?

If the tub is the lowest fixture it's going to happen unless a backwater valve is used.

John

You can't stop it. Back water valves are illegal in most places without permission

If the toilet is slow to drain, it may be in the lower trapway of the bowl.
If you have a main line backup, then yes, the tub can start filling, and before that you may see water seeping out from under the toilet bowl.
If that is the case, then clearing the main line may be an answer.

It's easy enough to pull the toilet and bring it outside and see how it works when it's not connected to the house plumbing. And then while you have it out, it's a good time to throw it in the back the truck and dispose of it. If nothing else, you can take a good look at the lower trapway and see if that needs clearing and take a garden hose to it before reinstalling.