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The answer is yes, you can lay new roof shingles over old ones. But before we get started, one thing must be clear: adding new roofing over old (sometimes called a "re-roof") is possible only with asphalt shingles (also called "composition shingles").

Average New Roof Cost. The average cost of a new roof is between $4,707 and $10,460 depending on the roof size and materials used. A 2,000 sqft. asphalt roof costs from $3,600 to $10,800 on average, while a metal roof costs between $8,000 to $24,000.

Tile and Asphalt are the most used materials for roofing, and they're manufactured with the usage of concrete, clay or oil. The rise of oil prices can directly impacted asphalt shingles to become more costly. Furthermore, the cost of disposing old and damaged materials has also increased in the recent years.

According to leading consumer and trade magazines, the US Department of Energy, the heating and cooling contractor you choose does make a difference. The same equipment can be installed in the same home by two different contractors, and there can be a difference in comfort, equipment efficiency, and overall life. 

For instance, a typical asphalt shingle roof lasts 20 to 25 years, while a roof installed over an existing layer of shingles should be replaced after 20 years.

Most experts agree that a typical roof will last between 20 and 25 years. It also depends on whether the old roof was removed and you only have one layer of shingles, and if it is properly ventilated.