Why Does the Floor Drain Back Up When You Shower?

It feels like the floor drain is the problem, since that's where the water shows up. Structure Tech, a Twin Cities home inspection company, puts it directly: "when water backs up out of a floor drain, it usually means there is a clog in the drain line, not the floor drain itself."

The clog is somewhere downstream, past the point where the floor drain's own line joins the rest of the house's plumbing. Water from an upper fixture runs down, hits that blockage, and has nowhere left to go but back up through whatever opening sits lowest.

Structure Tech explains that when a clog sits in the main building drain, water run from an upper fixture backs up until it finds somewhere else to come out, and "that'll be the lowest fixture," which in most homes is the floor drain.

A shower is one of the higher-volume fixtures in a typical house, so it's often the one that pushes enough water through the shared line to surface the problem. That doesn't mean the shower caused the clog. It just means the shower is what's sending water toward it right now.

Try running a few other fixtures one at a time, not all at once, and watch the floor drain each time. If the kitchen sink or a different bathroom also brings water up, the blockage is further down the shared line, past where several branches join.

If only the shower does it, the clog is more likely close to where the shower's own branch meets the rest of the line. Either way, this is useful information to give whoever looks at it, since it narrows down where to start.

Not every basement has a floor drain, and the same backup still happens somewhere. Structure Tech notes that the lowest fixture in a home without a floor drain might be "a basement shower, bathtub, standpipe, or a sink," and that fixture will take on the same role a floor drain usually plays.

Clearing the current clog solves today's backup, but it doesn't stop a future one from doing the same thing. Family Handyman prices a basic backflow preventer, sometimes called a backwater valve, at $30 to $75, with about 15 minutes of installation time in an existing floor drain.

The valve uses a floating ball that, as Family Handyman describes it, "seats tightly against the insert and keeps sewage from flowing out onto the floor" once sewage levels rise. It's not a fix for the clog itself, but it limits what a future backup can do to the room.

Stop running water in the house until the floor drain goes back down. If it's actually sewage rather than clean water, stay out of it and don't try to clean it up yourself.

Call to get connected with a local plumber who serves Eden Prairie to find and clear the blockage, especially if more than one fixture brings the floor drain up.