When we flush one high efficiency American standard toilet it backs the water up from the p trap on the lacy that is about 5 foot from the toilet. Is there a problem with the venting all of a sudden, it just started the day before.

Hi Johhny1

I'm not sure what a "lacy" is, but figure that it is a tub or shower and if that is the case and flushing the toilet causes water to back up into the lacy then the branch drain that feed the entire bathroom group is clogged and needs to be snaked to clear the drain.

If the lacy is the lowest fixture in the home then your MAIN DRAIN is clogged and the main drain will need to be snaked to clear the drain system.

Let us know how you make out!

Mark