What additional layer thickness is required for the top layer of valley lining when using mineral-surfaced roll roofing?

The correct additional layer thickness for the top layer of valley lining when using mineral-surfaced roll roofing is 36 inches. In roofing applications, particularly with roll roofing materials, it's essential to provide adequate coverage and water shedding capabilities, especially at critical points like valleys. Valleys are areas where water can accumulate and are more prone to leaks if not properly covered.

By specifying a thickness of 36 inches for the top layer of valley lining, it ensures that there is sufficient overlap and protection against water intrusion. This thickness allows for proper installation and helps to prevent any potential pooling of water at the valley, which can occur if the material coverage is insufficient. This guideline is based on industry best practices that aim at enhancing durability and effectiveness in waterproofing areas prone to weather exposure.