My toilet starting overflowing, but no one had used it for at least 30 minutes. The water overflowing was clear, so I don't think it's a sewer back up issue (this is not on a septic tank). What could be the cause?
Edit: I found out that the washing in an adjacent unit was running at the same time this happened. I'm not sure if it's related, but more details might help.
It's definitely a backup from a source higher up than the toilet. It could still be on the same floor of the house/building. i.e. The washing machine drainpipe is likely higher than the toilet.
The backup can be clear if the source is something like a shower, dishwasher, washing machine.
Since you are in an apartment, you should first contact your landlord before trying to clear the blockage yourself.
Is water going over the holding tank or toilet bowl?
If the toilet bowl; then there is waste line blockage either at your toilet (in the bend) or further down line, this coupled with a leakage of seal in tank allowing water to continue to enter bowl would cause an over flow. If you do not hear any water flowing into your toilet and the supply valve is turned off, then the drain is plugged up and adjacent units water is backing up and out your toilet. Since it is clear it could come from shower or sink.
If over flowing tank; float valve is not closing.
