1. Tread worn to the Tread-Wear Indicator (≈1.5 mm / 2/32″)
According to Transport Canada, a tire must be replaced when the tread is flush with the built-in tread-wear indicators—visible across the grooves—usually at about 1.5 mm depth. Driving on tires worn beyond this reduces traction, especially in wet or snowy conditions.

2. Damage: Cuts, Cracks, Bulges
Transport Canada also advises replacing tires that show signs of deep cuts, slits, cracks, blisters, or bulges in the sidewall or tread. Even minor damage can lead to tire failure.

3. Tread Depth Should Be ≥4 mm for Winter
For safe driving in winter, Transport Canada recommends maintaining a tread depth of at least 4 mm (5/32″)—well above the minimum—to ensure proper traction on snow-covered roads. Tire performance decreases significantly below this threshold.

4. Uneven or Rapid Wear
Causes like poor alignment or imbalance can lead to uneven wear, diminishing handling and safety. A yearly alignment and balance check—or sooner if uneven wear appears—is strongly advised.

5. Age, Usage, or Seasonal Changes
Although Transport Canada emphasizes tread depth and physical condition over age, tires degrade over time. Many manufacturers recommend replacements after 6–10 years, even if the tread remains, due to age-related risk. Keep this in mind as part of tire replacement planning.

6. After Significant Impact or Incident
Any serious impact with potholes or curbs can damage internal tire components. Inspection and possible replacement are essential after such occurrences.