Drainage overflow

[HomeOwner53890]

Hello all, new to the forums but have been reading for a while. I'm dealing with a previous install and the toilet is causing the shower to overflow. I've attached an image of what the plumbing looks like, best of my knowledge. I've been pulling apart this bathroom as the shower was rotting cardboard....

Anyways the toilet seems to cause the shower drain to overflow whenever the toilet is flushed two to three times in a row, doesnt even have to be used....

I was wondering if capping the shower drain might just remove this problem, with out causing others. It kind of seems the shower drain isnt properly vented (sinks either?) to me but not a plumber.

I've already had a plumber snake the drain when it clogged through use and it started working again after but the problem is returning a month later and seems to occur when flushing in rapid succession.

Thanks for any help on this, it is much appreciated.

 

[Terry]

No venting on the sinks.
You can't tie a vent back into a waste line for upstairs.

No way of knowing how that was run below the slab as to what fittings they used.

 

[Sponsor]

 

[HomeOwner53890]

No venting on the sinks.
You can't tie a vent back into a waste line for upstairs.

No way of knowing how that was run below the slab as to what fittings they used.

Great to know it isnt up to code, didnt even think of the venting into the main waste line. I plan on having the bathroom scrapped at a later date. Shower is already unuseable. The upstairs bathroom isnt functional atm though, so for the meantime as it is the only functioning toilet, or fixture besides the sinks downstairs. Would capping the shower drain prevent the backflow?