1. Continue with the purchase as-is, with the same purchase price and accepting the current condition;
2. Renegotiate with the seller, stating that their false statement led you to offer a certain price. Have the seller either reduce the price, repair/replace the roof, or provide you with an allowance to repair/replace the roof;
3. Walk away and purchase another house instead.
If the seller was truly deceptive in stating that the house had a new roof rather than making a mistake (assuming this person has a lot of properties listed), then you could consider filing a claim for false representation. However, I'm not sure what you'd gain from this other than a lot of hours lost in preparing the complaint. Real estate boards won't take any action without sufficient proof that the statement was intentionally false and/or misleading.
There is a complaint process to be followed in this case. Contact the Real Estate Board in the area and pursue the resolution of this contract with that body. They will advise you of the proper orderly steps to take to resolve this matter.