All roof coverings require a good solid substrate. The roof deck has to be strong and thick enough to hold the fasteners to ensure the roof system does not blow off in severe winds. For some roof materials, it’s extremely important to ensure that the roof substrate is smooth, even, and is not susceptible to large seasonal movements. For example, thin plywood decks can move under snow loads during the winter. Such movement can tear materials that become brittle in the cold, such as shingle roofs. Metal roofs, cedar shakes, and rubber tiles are less susceptible to breakage from flexing substrates, but they are also significantly more expensive to begin with.