Can I sue a home inspector who missed a major defect before I bought my house in Ontario?

Potentially, yes. A home inspection is both a contract and a professional service, so a claim against an inspector who missed an obvious, visible defect can be framed as breach of contract, negligence, or both. The key question is whether a reasonably competent inspector, doing a visual, non-invasive inspection consistent with the scope agreed to, should have caught the problem. Defects that were hidden behind finished walls, underground, or otherwise not reasonably discoverable during a standard visual inspection generally won't support a claim, even if serious.

Read your inspection contract closely before assuming you have a claim. Most inspection agreements include limitation-of-liability clauses that cap what you can recover, and disclaimers about the scope of what was actually inspected - courts will enforce clearly worded limits unless they're unconscionable or the inspector was clearly negligent in a way the clause doesn't cover. You'll want the original inspection report, photos, and often a second inspector's opinion on whether the defect should have been visible and noted at the time. Damages are typically the cost to repair the missed defect, not a full refund of your purchase price.