Can an insurer deny a claim by arguing my condition existed before the policy started in Ontario?

Yes, insurers can and do deny claims on the basis that the damage, defect, or condition being claimed actually existed, or began, before the policy took effect - since a policy generally only covers losses that occur, or begin, during its coverage period, not pre-existing problems the policy simply inherited. This comes up in property claims (a defect the insurer says predates the policy), and in other contexts where timing of onset is genuinely disputed.

Because these disputes usually turn on exactly when the condition actually started or worsened, rather than on the policy's wording itself, they're heavily fact- and expert-dependent - an engineer, contractor, or other technical expert is often needed to give an opinion on timing based on physical evidence, maintenance history, or prior inspection records. If you believe a pre-existing condition denial is wrong, gathering your own history of the property or asset - prior inspections, maintenance records, and any documentation from before the policy started - is usually the most effective way to challenge the insurer's timing conclusion rather than disputing the policy's terms directly.