What is a trap primer and should my Toronto basement floor drain have one?

Question

Toronto Basement Remodeling · Accepted Answer

A trap primer is a small device that automatically adds water to floor drain traps to prevent sewer gases from entering your basement. Most Toronto basement floor drains should have trap primers, especially if the drain rarely receives water use, but many older GTA homes lack them entirely.

What Is a Trap Primer?

A trap primer is essentially an automatic water supply system for floor drains. Every floor drain has a P-trap underneath — a curved section of pipe that holds water to create a seal preventing sewer gases, methane, and odours from backing up through the drain into your basement. The problem is that if water doesn't flow through the drain regularly, this trap water evaporates over time, breaking the seal and allowing dangerous gases to enter your home.

Trap primers solve this by automatically supplying small amounts of water to the floor drain whenever other plumbing fixtures in the house are used. They're typically connected to a nearby water supply line (often the laundry sink or utility sink) and release a few ounces of water into the floor drain each time water flows through the connected fixture. This keeps the P-trap filled and the sewer gas seal intact.

GTA Basement Floor Drain Reality

Most Toronto-area homes built before 1990 have basement floor drains without trap primers, and many homeowners only discover this when they start smelling sewer odours in their finished basement. This is particularly common in Scarborough, North York, and Etobicoke homes from the 1950s-1970s where the basement floor drain connects to the main sewer line but receives no regular water flow.

In older GTA homes, the floor drain often connects directly to the foundation weeping tile system rather than the sanitary sewer, which means it may not have a proper P-trap at all. These drains were designed to handle groundwater infiltration and minor flooding, not to prevent sewer gas infiltration. If your basement occasionally smells like sewage, especially during dry periods or after extended time away from home, an unprimed floor drain trap is likely the culprit.

Modern Ontario Building Code requires trap primers on floor drains that don't receive regular use, but this wasn't always the case. If you're finishing your basement or installing a new floor drain as part of waterproofing work, a trap primer should absolutely be included.

When You Need a Trap Primer

Your Toronto basement floor drain needs a trap primer if:
- The drain rarely or never receives water flow (most basement floor drains fall into this category)
- You smell sewer odours in the basement, especially during dry weather
- The drain connects to the sanitary sewer system rather than just foundation drainage
- You're finishing the basement and want to eliminate any possibility of sewer gas infiltration
- You're installing a new floor drain as part of interior waterproofing work

You may not need a trap primer if:
- The floor drain receives regular water flow from a nearby laundry sink, utility sink, or dehumidifier discharge
- The drain connects only to foundation weeping tile and storm sewer (though this should be verified by a plumber)
- You're in a newer home (post-2000) where trap primers were likely installed during construction

Installation and Cost

Trap primer installation requires a licensed plumber and costs approximately $300-$800 in the GTA, depending on the complexity of connecting to a nearby water supply line. The most common installation connects the trap primer to the cold water supply line feeding your laundry sink or utility sink.

The primer itself costs $50-$150, but the labour involves cutting into existing plumbing lines, installing the primer valve, and running a small supply line to the floor drain. This work requires a plumbing permit and inspection in most GTA municipalities.

If you're already having plumbing work done — such as installing a basement bathroom, interior waterproofing system, or backwater valve — adding a trap primer is relatively inexpensive since the plumber is already on-site and working with the existing plumbing.

Alternative Solutions

If a trap primer isn't feasible, you can manually maintain the floor drain trap by pouring a gallon of water down the drain every few months. However, this requires remembering to do it consistently, and most homeowners forget until the sewer smell returns.

Some homeowners use mineral oil instead of water in the trap, as it evaporates much more slowly. Pour about a cup of mineral oil down the drain after filling the trap with water — the oil floats on top and significantly slows evaporation.

For basement finishing projects, addressing floor drain odours before installing flooring and drywall is essential. Nothing ruins a beautiful finished basement like persistent sewer gas smells, and accessing the drain becomes much more difficult once the space is finished.

Need help finding a plumber to install a trap primer? Toronto Basement Remodeling can match you with