For open valleys, valley lining of two plies of mineral-surfaced roll roofing complying with ASTM D 3909 or ASTM D 6380 shall be permitted. the bottom layer shall be _______ and the top layer a minimum of ______ wide.

For open valleys, valley lining of two plies of mineral-surfaced roll roofing complying with ASTM D 3909 or ASTM D 6380 shall be permitted. the bottom layer shall be _______ and the top layer a minimum of ______ wide.

• A. 36 inches, 18 inches
• B. 18 inches, 18 inches
• C. 18 inches, 36 inches
• D. 36 inches, 36 inches

Answer: (C)

In open valleys using two plies of mineral-surfaced roll roofing, the valley lining is designed to create a robust, overlapping seal that directs water away from the seam. The bottom layer acts as the base along the valley, while the top layer provides extended coverage up both slopes to protect the seam and edges. Having the bottom layer at 18 inches wide gives enough width to anchor securely in the valley without overdoing waste or bulk. The top layer being at least 36 inches wide ensures substantial overlap over the bottom ply and extends a long way up each slope, which helps prevent water from reaching the seam and provides redundancy against movement or shrinkage. This combination balances proper coverage with practical installation. Wider bottom coverage or a narrower top would either waste material or leave insufficient overlap and protection along the slopes, increasing the risk of leaks. The 18-inch bottom and 36-inch top widths align with the standards for mineral-surfaced roll roofing in open valleys.

In open valleys using two plies of mineral-surfaced roll roofing, the valley lining is designed to create a robust, overlapping seal that directs water away from the seam. The bottom layer acts as the base along the valley, while the top layer provides extended coverage up both slopes to protect the seam and edges.

Having the bottom layer at 18 inches wide gives enough width to anchor securely in the valley without overdoing waste or bulk. The top layer being at least 36 inches wide ensures substantial overlap over the bottom ply and extends a long way up each slope, which helps prevent water from reaching the seam and provides redundancy against movement or shrinkage. This combination balances proper coverage with practical installation.

Wider bottom coverage or a narrower top would either waste material or leave insufficient overlap and protection along the slopes, increasing the risk of leaks. The 18-inch bottom and 36-inch top widths align with the standards for mineral-surfaced roll roofing in open valleys.