Everything you need to know about backflow valves

21 October 2021

Backflow valves (also known as backwater valves) protect your home from sewer water and rainwater backing up in the basement. Having one installed by a professional plumber helps protect both your basement and the health of your household members.

What’s a sewer backup?

Sewer backups occur when waste or storm water flows back into a property. This can happen in 3 situations:

  • When a lateral sewer gets blocked (grease, paper, household waste, tree roots, collapse or other structural issues)
  • When the municipal sewer main becomes overloaded, obstructed or damaged
  • When rivers flood or there are heavy rains

What’s the danger?

If your sewer backs up, your basement will fill with dirty water. This creates a high risk of viruses and bacteria that can cause infection. Once dry, your basement may also show signs of mould.

What does the backflow valve do?

Backflow or backwater valves are installed on piping to control the direction of water flow. Each valve contains a stop (a kind of door) that prevents wastewater from overloaded municipal sewers from backing up into your property. There are four types of backflow valves:

  • A normally closed valve is installed on each plumbing fixture drainpipe located below street level. However, if there are multiple plumbing fixtures connected to the same drainpipe, the valve can be installed on the drainpipe alone.
  • A normally open valve allows water and air movement at all times. When sewers back up, the valve rises and prevents the sewer water from flowing up.
  • An insertion valve is installed in a floor drain. It is inserted into the pipe and positioned at the mouth with its elastomeric rings.
  • An inflatable valve is controlled electronically. It has a ball that inflates and seals the pipe as soon as it detects a backflow. It deflates when there’s no more water. The advantage of this device is that it requires no maintenance.

How much does it cost?

The cost of a backflow valve ranges from $40 to $200 depending on type. Add to this the cost of installation by a plumber and miscellaneous costs to access the pipe that the valve will be installed on. Allow for $1,000 to $2,000 (and up) for general contracting work such as cutting the basement floor, drilling into the concrete slab, digging around the drain, cutting the drainpipe, installing the valve, redoing the concrete and patching everything back up. The cost also depends on the number of valves to be installed. For larger buildings, it’s not uncommon to see bills of $3,000 to $8,000, not counting excavation.