In Ontario, we have a buyer beware real estate system: generally, the seller does not have to disclose anything about the house. The exception to that rule, of course, is that the seller cannot actively conceal anything about the house. If you had a leaky roof and fixed the problem, and haven’t had a leak in five years, that is one thing; if, instead, you simply painted over the stain in the ceiling, without fixing the problem, that is quite another.
If you actively conceal an issue, you can be required to pay. A lot. Not only could you be required to cover the cost to repair the damage, you could also be required to cover any additional expenses of the buyer. What if the buyer has asthma and you actively hid mold? What if the damage you hid reduces the value of the home significantly?
If there are problems with the house, even if they have been fixed, it is always best to be upfront about them rather than run the risk of a major lawsuit in the future.