Final answer:
Concrete and clay tile roofs with a slope of less than 4:12 need two layers of underlayment for adequate waterproofing.
Step-by-step explanation:
Concrete and clay tile roofs with a slope of less than 4:12 require two layers of underlayment. This is because a lower slope may allow for more water accumulation, hence additional layers are important for waterproofing and protecting the roof structure. This standard practice is critical in roofing to ensure the longevity and effectiveness of the roof's waterproofing system against various weather conditions.