The roof vent for the toilets is shared. It is the pipe (on the roof) that lets water in the drain system. When clogged, it causes what you have. A garden hose placed in the roof vent (and turned on) may clear the blockage (could be leafs or pine straw or a dead bird). Needs to be cleared... I have used a plumbers snake with 100% success....but you may not have one...so the garden hose suggestion. Be careful on the roof.
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The roof vent for the toilets is shared. It is the pipe (on the roof) that lets water in the drain system. When clogged, it causes what you have. A garden hose placed in the roof vent (and turned on) may clear the blockage (could be leafs or pine straw or a dead bird). Needs to be cleared... I have used a plumbers snake with 100% success....but you may not have one...so the garden hose suggestion. Be careful on the roof.