Contributory negligence: the fault lines in personal injury law

When an accident happens, the fault lines are not always where you expect. In Canadian personal injury law, contributory negligence recognises that an injured person can share some of the blame for their own injuries, and when that happens, their compensation is reduced accordingly. For personal injury lawyers, knowing how courts draw those lines can help every damage negotiation from the very start.

How do courts apportion fault for contributory negligence in Canada?

Every province in Canada has laws governing how fault is divided when both the plaintiff and defendant contributed to an injury. For instance, Ontario’s Negligence Act has two provisions which carry the most weight when it comes to contributory negligence in practice:

• Section 3: requires courts to apportion damages in proportion to each party’s degree of fault, where the plaintiff’s own negligence contributed to the loss
• Section 4: sets a fallback rule, where a court cannot determine separate degrees of fault, both parties are treated as equally responsible

Every province follows a similar proportionate reduction model, though important differences apply across jurisdictions.

Contributory negligence is just one of the defenses to the tort of negligence, among many other defenses:

Learn about the provincial laws and deadlines that lawyers and litigants should not miss when it comes to personal injuries proceedings. Check out our article on “Personal injuries proceedings act: Limits, notices, and traps” to find out more.

How provincial laws on contributory negligence differ

Here’s how laws regarding this defence may vary from one province to another:

• Ontario: Under ss. 1 and 3 of Ontario’s Negligence Act, defendants remain jointly and severally liable even where the plaintiff is contributorily negligent. The plaintiff can collect the full reduced award from any one defendant, who then seeks contribution from the others.
• British Columbia: Under ss. 1 and 2(c) of BC’s Negligence Act, defendants are severally liable only where a plaintiff is also contributorily negligent. Each defendant pays only their proportionate share of the loss, which can leave a plaintiff short if one cannot pay.
• Saskatchewan: Under s. 3.1 of Saskatchewan’s Contributory Negligence Act, in force from January 1, 2005, a contributorily negligent plaintiff shares part of the shortfall when a defendant cannot pay their portion, in proportion to their own degree of fault.

Still, common law plays a huge role. For Ontario, this regime of joint liability on contributory negligence was confirmed by the Supreme Court in Ingles v. Tutkaluk Construction Ltd., 2000 SCC 12.

The governing test on contributory negligence

The Supreme Court of Canada elaborated the governing test in Bow Valley Husky (Bermuda) Ltd. v. Saint John Shipbuilding Ltd., [1997] 3 SCR 1210. It said that contributory negligence requires foreseeability, not a duty of care between the parties. Further, the Court said that:

• a person is contributorily negligent if they ought reasonably to have foreseen that, by not acting as a reasonable and prudent person, they might get hurt
• the defendant bears the onus of proving this and of showing that the plaintiff’s negligence caused or contributed to the loss

Apportionment is not a causation exercise, but rather, courts measure relative blameworthiness. As stated in Cempel v. Harrison Hot Springs Hotel Ltd., 1997 CanLII 2374 (BC CA), courts will look at “the amount by which each proximate and effective causative agent fell short of the standard of care that was required of that person in all the circumstances.”

Here’s another video explaining this type of defence for litigants in personal injury cases:

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What are the instances that contributory negligence applies?

Contributory negligence is a defence raised by defendants, and not a claim brought by plaintiffs. As such, it can arise across many types of personal injury cases in Canada, but three situations come up most often in practice:

• non-wearing of seatbelts in motor vehicle accidents
• refusing medical treatment as failure to mitigate damages
• slip-and-falls and premises liability cases

We’ll discuss these situations below.

Not wearing seatbelts in motor vehicle accidents

Failing to wear a seatbelt is among the most common contributory negligence arguments raised in motor vehicle claims.

Courts will reduce damages for seatbelt non-use, but only where two conditions are met:

• the plaintiff was not wearing a seatbelt at the time of the accident, and
• proper seatbelt use would have prevented or reduced the specific injuries claimed

Non-use alone is not enough. The defendant must connect the seatbelt choice directly to the injuries being claimed. The Ontario Court of Appeal addressed this directly in Snushall v. Fulsang, 2005 CanLII 34561 (ON CA), holding that reductions for seatbelt non-use fall within a range of 5 to 25 percent of the total award. In Snushall, the court found a jury’s finding of 35 percent contributory negligence unreasonable and reduced it to 5 percent, confirming that the range has a firm ceiling and a floor that defendants should not underestimate.

This range reflects the fact that seatbelt non-use did not cause the collision, and that the defendant’s negligence remains the primary cause. The reduction accounts only for the plaintiff’s contribution to the severity of their own injuries, but not for any role in bringing about the accident itself.

Refusing treatment as failure to mitigate damages

Treatment refusal operates under a different legal principle from contributory negligence, and the distinction matters. The Supreme Court in Janiak v. Ippolito, [1985] 1 SCR 146, was explicit on this point: “[m]itigation has to do with post-accident events,” whereas contributory negligence concerns the plaintiff’s conduct before or at the time of the accident.

In Janiak, it was held that a plaintiff cannot recover damages they could have avoided if they took reasonable steps. That is why when a plaintiff unreasonably refuses recommended treatment and their condition worsens as a result, courts can reduce the award by the amount attributable to that worsening.

However, the reduction does not extend to the underlying injury itself. It means that even after a finding of unreasonable refusal, unavoidable losses remain compensable.

In assessing whether a refusal is unreasonable, Janiak sets out three factors the trier of fact must weigh:

• the degree of risk from the proposed treatment,
• the gravity of the consequences of refusing it, and
• the potential benefits to be derived from it

The defendant bears the burden of establishing both:

• that the refusal was unreasonable, and
• that the plaintiff’s losses would have been smaller had they complied

Where medical opinions conflict, a plaintiff who follows any one of several courses recommended by their advisers cannot be found to have acted unreasonably.

Note that there’s one important limit on this doctrine: where a plaintiff has a pre-existing psychological condition that prevents them from making a rational choice about treatment, Janiak places them within the “thin skull category.” In those circumstances, the objective standard of reasonableness does not apply.

Slip-and-falls and premises liability cases

In slip-and-fall and occupier’s liability claims, a plaintiff’s choice of footwear is regularly raised as a basis for contributory negligence. Courts weigh the plaintiff’s conduct against what the occupier did to address the hazard on their premises.

• Chettri v. Brunet, 2026 SKCA 71: the Saskatchewan Court of Appeal upheld a 15 percent contributory negligence reduction for a visitor who wore slippers on outdoor steps she knew had no handrail
• Jansen v. J.M. Reynolds Pharmacy Limited, 2026 NSSC 79: the Nova Scotia Supreme Court found a plaintiff 25 percent at fault after she caught her foot on a planter bed lip she had passed multiple times before, in daylight
• Merkley v. St. Lawrence College of Applied Arts and Technology, 2025 ONSC 4368: by contrast, the Ontario Superior Court found no contributory negligence where the plaintiff encountered black ice that was not visible to a reasonable visitor because the sidewalk’s salt treatment had become ineffective below –10°C

What is the effect of pre-existing conditions on contributory negligence?

Pre-existing conditions are not the same as contributory negligence. A defendant cannot argue that a plaintiff was partly at fault simply because they had a health condition before the accident. In other words, defendants take their plaintiffs as they find them under the law.

Thin skull and crumbling skull

The Supreme Court of Canada confirmed this in Athey v. Leonati, [1996] 3 SCR 458. Where a defendant’s negligence caused or contributed to an injury, the defendant is responsible for the full extent of the consequences, even where those consequences are more serious because of the plaintiff’s pre-existing condition. This is the “thin skull rule.”

A separate principle applies where the plaintiff’s condition was already deteriorating before the accident. This is the “crumbling skull rule,” where:

• the defendant is not responsible for losses the plaintiff would have suffered regardless of the accident
• compensation is limited to the additional harm the accident actually caused

The British Columbia Court of Appeal applied this distinction in McGuigan Estate v. Pevach, 2024 BCCA 106, a case where the trial court had awarded future care costs without sufficiently accounting for the probability that some of those costs would have arisen from the plaintiff’s pre-existing health conditions anyway.

The appellate court reduced:

• extended residential care costs by 15 percent
• community support and taxi service costs by 20 percent

These reductions had nothing to do with fault. They instead reflected the realistic likelihood that some of those future care needs would have developed regardless of the accident.

In sum, pre-existing conditions belong to the quantum analysis, not the fault analysis. They affect what the accident actually cost the plaintiff, not how the blame is divided between the parties. A court will not reduce a plaintiff’s award for contributory negligence on the basis of a pre-existing condition alone.

Contributory negligence: reading the fault lines

The fault lines in a personal injury case are worth reading early. Contributory negligence applies across Canada, but the rules shift with the province, the accident type, and the conduct at issue. Evidence about a client’s footwear, seatbelt use, and treatment compliance can even shape the final award far more than many clients expect. Knowing where those lines fall is one of the most practical tools a personal injury lawyer can bring to any file.

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