Why Does the Floor Drain Back Up After Heavy Rain?

A floor drain that behaves normally all year and only backs up during or right after a heavy rain is showing a different problem than a clogged pipe. The City of Eden Prairie has a name for this: inflow and infiltration, or I&I, and has run a program to reduce it since 2008.

The sanitary sewer system under your street is built to carry wastewater from toilets, sinks and drains, not rain. When rain or groundwater gets into that system anyway, it's competing for the same limited pipe space as every other house's wastewater.

The city is direct about the mechanism: "during major rain events, this additional surge of water can consume the capacity of sewer pipes, resulting in home backups and overflows into creeks and wetlands, in addition to the Mississippi River."

That's a citywide volume problem, not something specific to your plumbing. A floor drain, as the lowest opening in most basements, is usually the first place that extra volume shows up.

One of the biggest contributors is something a lot of homeowners don't realize is a problem. The city explains that "inflow is usually traced to sump pumps... roof downspouts and foundation drains that are connected to the City's sanitary sewer system," and that "sump pumps are often connected to the sanitary sewer system via a hose leading from the sump to a laundry tub or a floor drain."

If a sump pump anywhere on the line is discharging into the sanitary system instead of outside, it adds clean groundwater to the same pipes during the rain events that strain the system most.

The fix starts with checking where a sump pump, foundation drain or downspout actually discharges. The city states plainly that "state law requires sump pumps, foundation drains and roof down spouts to be connected directly to a storm sewer or onto the ground, not to the sanitary sewer system."

If your own sump pump's hose runs to a floor drain or a laundry tub instead of outside, rerouting it removes a direct source of inflow, and it's worth checking even if your floor drain backup seems minor.

Not every case involves a sump pump connected the wrong way. The city also describes infiltration, which "occurs when groundwater seeps into sewer pipes through cracks, leaky joints or deteriorated manholes," with no hose or connection involved at all.

That version tends to show up during a high water table, which heavy rain and spring thaw both raise. Either way, it's a sign of the shared system struggling, not your own plumbing failing.

Stop using water in the house while the drain is backed up, since every sink, shower or load of laundry adds to the same system that's already overloaded. Avoid standing water in the area if it looks or smells like sewage.

Call to get connected with a local plumber who serves Eden Prairie if the backup doesn't clear once the rain stops, or if it happens with every heavy rain rather than just once.