When does the limitation period start for a latent defect I couldn't have discovered right away in Ontario?

For a hidden, or latent, defect that isn't obvious at the time something is built, sold, or delivered, Ontario's discoverability rule under the Limitations Act, 2002 generally means the two-year limitation clock doesn't start running until the problem was actually discovered, or reasonably should have been discovered, through the exercise of reasonable diligence. This is different from claims involving an obvious, immediately apparent problem, where the clock generally starts close to when the underlying event happened, since there's nothing hidden to delay discovery.

The practical difficulty with latent defect claims is proving exactly when discovery genuinely occurred, or when it reasonably should have, since this becomes a factual question that can be disputed. A defendant may argue the claimant should have noticed warning signs earlier than they claim to have, while a claimant may point to how genuinely hidden or hard to detect the defect actually was. Because this analysis is fact-specific and courts look closely at what a reasonably diligent person in the claimant's position would have noticed and when, keeping records of when problems first became apparent, and what efforts were made to investigate them, meaningfully strengthens a later claim about when the clock actually started.