Toilet using shower as vent?

[Atropical1]

Hi, I discovered that a toilet in a home I rcntly purchased, when flushed, will draw the water out of the shower trap. What would cause this and is the fix worse than the problem? Thanks for any ideas.
(On 4 inch concrete slab, in South Florida)

 

[Reach4]

The shower trap trap arm should be vented before it joins the toilet waste.

Who knows what the fix is, because we don't know the pipe configuration. Ideally a bird built a nest on the vent pipe stopping the air. Not likely something as easy to fix.

 

[Sponsor]

 

[WorthFlorida]

With a slab it is doubtful that a DIYer did any additions to the home. If you have a home built 50 years ago, typical in the old areas of Ft Lauderdale, I would guess the vent pipe for the bathroom or toilet is is clogged. AAV's were not used back then so all venting is through the roof and a trip to the roof may be necessary to check on the vent pipe. If the vent is clogged you would have other drain issues throughout the house unless there are two or more vents on the roof. You can start by running a garden hose into the vent pipe and see if there is any backup. If none then you may need to call on a plumber with a camera to scope out the drains.