What causes the sewer smell from my Calgary basement floor drain?

That sewer smell coming from your basement floor drain is almost always caused by a dry P-trap — the curved section of pipe beneath the drain that normally holds water to block sewer gases from entering your home. It's one of the most common and easily fixed plumbing complaints in Calgary basements, though there are a few other causes worth ruling out.

How the P-Trap Works (and Why It Dries Out)

Every floor drain has a P-trap — a U-shaped bend in the pipe that stays filled with water, acting as a seal against hydrogen sulfide and methane gases rising from the municipal sewer system. In Calgary basements, floor drains often sit unused for months or years. Because our climate is so dry (especially during long winters with forced-air heating running constantly), that water evaporates faster than you'd expect — sometimes in as little as 4-6 weeks of non-use. Once the trap is dry, there's nothing between your living space and the sewer line.

The fix is simple: slowly pour about 4 litres (one gallon) of water down the drain. Add a small amount of cooking oil on top — the oil floats on the water and dramatically slows evaporation. Make this part of your seasonal routine, especially heading into winter. Some homeowners install a trap primer (an automatic device that periodically adds water to the trap), which is worth considering if you're constantly battling this issue.

Other Causes to Rule Out

If adding water doesn't solve the problem within a day or two, a few other culprits are worth investigating. A damaged or missing trap seal — where the trap itself is cracked, corroded, or was never properly installed — will let gas through regardless of water level. Older Calgary homes from the 1960s and 1970s sometimes have cast iron floor drains that have corroded badly enough to lose their seal.

Biofilm buildup inside the drain or trap can also produce a persistent rotten-egg smell even when the trap has water in it. A thorough cleaning with a drain brush and an enzyme-based drain cleaner (not harsh chemical drain openers, which can damage older pipes) often resolves this.

In rarer cases, the smell indicates a cracked or deteriorated sewer line beneath your basement slab — particularly relevant in Calgary homes built on expansive clay soils, where ground movement over decades can stress and crack buried drain lines. If the smell is strong, persistent, and present in multiple areas of the basement, that's worth having a plumber investigate with a camera inspection.

When to Call a Professional

If the simple P-trap refill doesn't work, or if you notice the smell is strongest near walls or multiple drains, call a licensed plumber. A camera inspection of your sewer lateral runs roughly $200–$400 in the Calgary market and can definitively identify cracks, root intrusion, or collapsed sections. Any repairs to buried sewer lines require a plumbing permit from the City of Calgary (call 311 or visit calgary.ca/development) and must be inspected by a Safety Codes Officer — this is not DIY territory.

Your next step: Start with the free fix — pour a gallon of water and a splash of oil down the drain today. If the smell returns within a week or two, or doesn't improve at all, it's time to browse licensed plumbers in the Calgary Construction Network directory and get a professional assessment before the problem gets worse.