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").
Why is roofing so expensive?
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.
What type of plywood do you use for roofing?
Before lumber companies began selling OSB, the standard choice for sheathing a roof with a 24-inch rafter spacing was 1/2-inch CDX plywood - the letters CDX indicate that it is construction-grade material intended for exterior applications.