Two common approaches

A licensed plumber should decide which suits your plumbing; this is only an overview.

Mainline backwater valve

  • Installed in the building drain or lateral, usually with an access box
  • Protects every fixture downstream of the valve
  • Requires cutting into the line and often breaking concrete
  • Needs a permit in most cities; ask the building division

Floor drain plug or check

  • A floating ball or flap that seats in a single floor drain
  • Inexpensive and simple to fit
  • Protects only that one opening
  • Sewage may simply rise at the next lowest fixture instead

What happens when the valve closes

A backwater valve has a flap that swings open with outgoing flow and is pushed shut by water moving the wrong way. During a public main surcharge, that closed flap keeps sewage in the street from entering the house. The catch is that while the flap is closed, nothing from inside the house can leave either. Anyone who keeps running the washer, showering or flushing fills the house side of the pipe, and that water has nowhere to go but up through the lowest drain.

That is why households with a valve need to know it is there and what to do when the street backs up: stop water use until the provider clears the main. A valve also does nothing if the blockage is in your own lateral between the valve and the house, because the problem is on the house side.

Maintenance over a year

Every few months
Open the access cover and check that the flap moves freely and nothing is caught on it.
Before spring storms
Clear any debris and confirm the cover seals.
After any backup
Have it inspected; solids may have lodged in the seat.
When the line is cabled
Tell the plumber the valve is there so it is not damaged and so they use the correct access point.

Questions for the plumber

  • Where on the line would the valve go, and will it protect every lower-level fixture?
  • Will upstairs fixtures still drain through the valve, or bypass it?
  • How will I reach the valve for cleaning, and how often should I check it?
  • Is a permit required, and will the work be inspected?
  • Does the line need cleaning or repair before a valve makes sense?

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