Bubbling through shower plug hole when toilet is flushed

[cotopaxi]

I rent out a flat and when the toilet is flushed water is bubbling up through the plug hole in the shower cubicle.
I have tried treating it with caustic soda and my tenant has tried other shower unblockers but to no avail.
Does anyone have any ideas what the problem might be and how it could be fixed? I would prefer to fix it myself if possible.

Any advice much appreciated.

 

[Hugh Jaleak]

Usually a sign the drain is blocked downstream. If a discharge is made (i.e. W.C. flushed) and this water cannot drain away, then the equivalent volume of air then needs to be displaced from the pipework. This will take the path of least resistance to vent to atmosphere, usually the shallowest trap.

Check drains downstream for possible blockage. If the drain is blocked caustic soda is as much use as a chocolate teapot. Needs rodding or jetting out to clear any debris and/or fat build up.

 

[Elkato531]

If its already had caustic and other chemicals down it, be careful to wear protective gear if and when the blockage is located. If you decide to get someone in, be sure to warn them of any chemicals used.