01
Different layers perform different jobs
Project specifications may use underlayment for temporary dry-in, separation, cushioning or secondary water management. Eaves, valleys and other vulnerable areas may receive different materials than the general field. Compatibility with the deck, metal and installation sequence matters.
02
Do not design a hidden primary roof
A durable slate roof should not depend on exposed water routinely passing through inadequate laps to reach a membrane below. If the slate layout or flashing geometry is wrong, a premium underlayment does not make the visible roof detail correct.
03
Exposure during construction matters
Underlayments have limits for ultraviolet exposure, temperature, foot traffic and attachment. Scheduling should minimize unnecessary exposure before slate installation, and damaged areas should be repaired according to the product and project requirements.