01
Ask what each layer does
Slate forms the exposed roof covering. Underlayment is installed beneath it as part of the specified assembly and construction sequence. Product selection and installation must account for the actual roof design. A claim about an underlayment's water resistance does not establish that poor slate layout or deficient flashing is acceptable.
02
Review the complete assembly
Request a section identifying deck, underlayment, slate, and interfaces at edges and penetrations. For a proprietary system, include its specific components and instructions. Ask how the work remains protected before completion and how concealed defects would be addressed. A sound specification explains how the layers work together rather than relying on the strongest marketing claim for one layer.