The Discoverability Rule in Ontario: When Does the Limitation Clock Actually Start?

You slipped and fell in a parking lot. You hired a contractor who did shoddy work. A professional gave you advice that turned out to be wrong. In each case, you know something went wrong — but when exactly did your legal right to sue begin? The answer is rarely as simple as the date the event occurred.

Ontario's discoverability rule is the legal principle that determines when the two-year limitation period actually starts running. It is one of the most important and frequently misunderstood aspects of civil litigation in this province. Get it wrong, and you may either miss your deadline or mistakenly believe your claim has expired when it has not.

The Core Principle

Ontario's Limitations Act, 2002 does not simply measure two years from the harmful event. Instead, the clock starts on the day the claim was discovered — or the day it ought to have been discovered by a person exercising reasonable diligence.

"Discovery" has a specific legal meaning. A claim is discovered when you first knew, or reasonably ought to have known:

1. That the loss, injury, or damage occurred
2. That the loss was caused by an act or omission of a specific person
3. That the act or omission was negligent, wrongful, or otherwise actionable
4. That a proceeding against that person would be an appropriate remedy

All four elements matter. Even if you knew you were injured, the clock may not have started if you did not yet know — and could not reasonably have known — that someone else caused it or that legal action was an option.

Why This Rule Exists

Before discoverability was recognized, limitation periods could create deeply unjust outcomes. A patient who received negligently performed surgery might not discover the negligence for years — often only when a later diagnosis or complication revealed what the original doctor had done. Under a rigid "from the date of the act" rule, a patient could lose their right to sue before they ever knew they had been wronged.

The discoverability principle corrects this. It gives plaintiffs a fair opportunity to learn of their claims before the clock starts running against them.

What "Ought Reasonably to Have Known" Actually Means

The rule is not purely subjective. You cannot close your eyes to obvious warning signs and later claim ignorance to revive an old claim. Courts ask what a reasonable person in your circumstances would have known or discovered if they had been paying attention.

This is measured objectively. If:

• You had symptoms consistent with a medical complication and never sought a second opinion
• You saw obvious signs of water damage but never investigated the cause
• You received a letter from a contractor explaining deficiencies and took no action

…courts may find you ought to have known about your claim earlier, even if you say you did not.

The interplay between what you actually knew and what you should have known is one of the most fact-intensive questions in limitation law.

Discovery vs. Damage: They Are Not the Same

A common misconception is that the clock starts when the damage first appears. But damage alone is not enough. You also need to have reason to believe the damage was caused by someone's wrongful act — and that suing them would be appropriate.

Example: Hidden Mould

You buy a home and two years later discover significant mould. You learn the previous owner was aware of a chronic leak but patched it cosmetically before the sale. The clock for a misrepresentation or disclosure claim may not start from when the mould appeared — it may start from when you discovered, or should have discovered, the seller's knowledge and concealment.

Example: Professional Negligence

An accountant filed your returns incorrectly five years ago. The error only produced a CRA reassessment this year. You may have a claim, and the clock may run from when you reasonably understood the reassessment was the result of the accountant's error — not from when the return was filed.

Deliberate Concealment and Fraudulent Acts

When a defendant actively conceals their wrongdoing — covers up evidence, makes false assurances, or takes steps to prevent you from discovering the truth — courts will generally hold that the discovery clock does not run until you could reasonably have found out despite the concealment.

However, deliberate concealment does not give you an unlimited window. The ultimate 15-year limitation period (as of writing) can still apply. If the underlying act occurred more than 15 years ago, you may be time-barred regardless of concealment — with narrow exceptions.

When You Are Not Sure Whether the Clock Has Started

This uncertainty is one of the most important reasons to consult a lawyer early. Some key questions your lawyer will analyze:

• What exactly did you know, and when? Reconstruct a timeline.
• What would a reasonable person in your position have investigated? This shapes the "ought to have known" analysis.
• Did any defendant take active steps to conceal the harm? This may extend the discovery date.
• Does the ultimate 15-year period apply? If the underlying act is old, both periods must be considered.

The Danger of Waiting

People sometimes assume that because they only "recently" discovered a problem, they have plenty of time. This thinking has two risks:

1. Courts may find you discovered — or ought to have discovered — your claim earlier than you think.
2. The ultimate 15-year period may already have expired, making the discovery date irrelevant.

Waiting to consult a lawyer compounds both risks. Every month of delay narrows your options.

Frequently asked questions