When heavy rain overloads a municipal sewer, the water in the main can rise above the level of the drains in a basement. With nothing to stop it, sewage flows the wrong way up the house's own sewer line and comes out of the lowest opening it can find. That is usually a basement floor drain, toilet or shower. A backwater valve is there to stop exactly that.
What a backwater valve does
It is a one-way valve on the main sewer line, usually under the basement floor near where the line leaves the house. Wastewater from the house flows out through it as normal. When water starts to flow back in from the street side, a flap inside closes and seals the line.
The device itself is simple. For a home with a bathroom, laundry or floor drain sitting below the level of the street sewer, it can decide whether a storm stays outside or ends up on the basement floor.
What it protects against, and what it doesn't
This is the part that catches people out. A backwater valve guards against backflow from the municipal side. It does nothing about a blockage in your own pipe.
- It helps when the city main is overloaded, for example during a heavy storm
- It does not help when roots, grease or a collapsed section block your own sewer lateral, because then it is your own wastewater that can't get out
- It does not stop groundwater seeping through the foundation; that is a sump pump and drainage question
- It only works if the flap can close, which is why maintenance matters
If your floor drain backs up on a dry day, or every time the washing machine drains, the problem is far more likely to be in your own line. Our guide to a basement floor drain backing up explains how to tell the difference.
Go easy on water while the valve is closed
When the valve shuts, nothing from the house can get out either. If someone upstairs runs a bath or starts a load of laundry while the street sewer is overloaded, that water has nowhere to go and can come up through a basement drain on the house side of the valve. If you know the city sewer is backing up, keep water use to a minimum until it clears.
How to tell if you already have one
- Look for an access cover set into the basement floor near where the main sewer line leaves the house, often in a utility or laundry room
- Newer homes are more likely to have one. Many older Lower Mainland homes don't.
- Your home inspection report or the original permit drawings may show it
If you can't find one, a sewer camera inspection will show whether there is a valve in the line and what condition it is in.
Is a backwater valve required?
BC's plumbing code and local bylaws set out when backwater protection is required, mainly for new construction and for fixtures that sit below the level of the street sewer. Requirements differ between municipalities and have changed over the years, so a house built decades ago may not meet today's rules. Adding a valve to an existing line is plumbing work that normally needs a permit, so check with your municipality's building department before work starts. Our guide to BC plumbing permits explains how that usually works.
Who is responsible for a sewer backup: you or the city?
In general, the homeowner looks after the plumbing inside the house and the sewer connection on their own property, and the municipality looks after the sewer main in the street. Where one ends and the other begins varies. Some cities draw the line at the property line, some at an inspection chamber, and the rules on what the city will clear or repair are set locally.
If the problem looks like it is in the main, for instance several homes on the street are affected at once, call your municipality's public works or sewer line first. Most cities list an after-hours number for this on their website. If the blockage turns out to be on your side, that is the point to bring in a plumber.
The maintenance most owners skip
A backwater valve that hasn't been opened since it was installed may not close when it needs to. Debris can hold the flap open, and the seal can stiffen or foul over time.
- Open the access cover once a year, ideally in early autumn before the rains
- Check that the flap moves freely and closes fully
- Clear any debris from the valve body and the seal
- Keep the cover accessible. Don't tile over it or stack storage on top of it.
- If the cover is damaged or the valve is hard to reach, have it looked at before storm season
Some valves are easy to check yourself. Others sit deep enough, or are dirty enough, that you would rather hand the job over. Either is fine, as long as someone checks it.
What installing one involves
Adding a valve to an existing home generally means locating the main line under the basement floor, cutting out a section of concrete, installing the valve with an access point, and restoring the floor. Before any of that, it is worth running a camera through the sewer line. A new valve won't help a lateral that is cracked, root-bound or sagging, and it is far better to find that out first. Where the line itself needs work, our sewer line repair page covers the options.
The insurance question
Sewer backup coverage is often an optional add-on to a home insurance policy rather than part of the standard cover, and some insurers ask whether a home has a backwater valve. Policies differ and change over time, so we won't guess what yours covers. Ask your broker directly, and keep any paperwork from installation or servicing, because that is what they will ask to see.
Dealing with this right now? Mr. Plunger Plumbing & Drainage answers 24/7 across Surrey, Delta, Langley, White Rock, Richmond, Maple Ridge, Pitt Meadows, Vancouver and the rest of the Lower Mainland. Call (604) 870-1442 to talk to a real plumber, get an ETA before anyone is dispatched, and get a quote before any work starts.