On the basis that a roof is at or above 15 degrees, it is not good practice to overlay existing roof systems.
Even those that appear to be in good condition, may have suffered moisture ingress in the past and affect the life expectancy of the new shingle system.
On a practical level, lap build up and existing felt detailing at the eaves, verges and ridge positions may impede the installation of the shingle, causing them to not sit flat on the roof.
Issues, when this occurs, can range from unsightly appearance to the uneven seating of the shingle which are then susceptible to wind damage and water ingress.
