Now a roof can do more than cover a house; it can help clean the environment. Smog-Eating Tile uses a photocatalytic agent, titanium dioxide, to neutralize the exhaust from cars by speeding up oxidation. As a result, it breaks down nitrogen oxide and reduces its impact on the environment. Cost: $800 upgrade on a 2,000-square foot house. MonierLifetile by Boral Roofing. 800-571-8453.