Basics

Sanitary sewer
Carries wastewater from homes and businesses to treatment
Storm sewer
Carries street runoff to drainageways and streams, untreated
Inflow
Rainwater entering the sanitary system directly, through open cleanouts, connected downspouts or sump discharge
Infiltration
Groundwater seeping into sanitary pipes through cracks and leaky joints

How a storm becomes a sewage problem

Most cities along the Front Range, including Thornton, run separate sanitary and storm sewer systems, so rain is not meant to enter the pipes that carry household wastewater. Over decades, though, small leaks add up. A lateral with cracked joints under a soggy lawn lets water in. A cleanout cap that was knocked off by a mower becomes an open funnel. A sump pump or downspout tied into a floor drain sends rain straight into the sanitary line.

During an intense summer thunderstorm, that extra water can surcharge a sanitary main for a short time. Homes with the lowest drain openings feel it first. The pattern looks like a main line backup, but it may clear on its own as the storm passes. Report it to the sewer provider anyway; repeated storm-related backups help them find problem areas.

Is the water from the storm or from the sewer?

  • IfWater is entering along walls or window wells during the stormThenIt is surface or groundwater. Look at gutters, grading and window well covers. Not a sewer issue, though still needs drying.
  • IfWater is rising from the floor drain during or right after heavy rainThenLikely a sanitary surcharge or a sump or downspout tied into the drain. Treat it as contaminated until the source is known.
  • IfBoth are happeningThenTreat the mixed water as contaminated. Report the drain backup to the provider and document both sources for your insurer.

Owner-side fixes

  • Check that the outdoor cleanout has a secure, intact cap
  • Make sure downspouts discharge onto the ground away from the foundation, not into any drain
  • Confirm a sump pump discharges outside rather than into a laundry sink or floor drain; ask the city what is permitted
  • Have a camera check for cracked joints if backups follow storms
  • Ask a plumber about a backwater valve if the home is on a low section of street

General information for Thornton, not a quote or professional advice. In an emergency, call 911.