Modified bitumen membrane roofs shall have a design slope of not less than:

Modified bitumen membrane roofs shall have a design slope of not less than:

• A. 1/8:12
• B. 1/4:12
• C. 1:12
• D. 2:12

Answer: (B)

Modified bitumen membranes need enough drainage to prevent standing water, which can lead to premature aging and leaks at seams or flashings. To ensure reliable drainage and performance, there is a stated minimum slope for these roofs. The design slope shall not be less than 1/4 inch per foot (1/4:12), which is about a 2% grade. This threshold helps keep water moving toward drains and reduces the risk of ponding that can compromise the membrane. Slopes shallower than this would increase the chance of water accumulation, while steeper slopes still satisfy the requirement but are not the minimum.

Modified bitumen membranes need enough drainage to prevent standing water, which can lead to premature aging and leaks at seams or flashings. To ensure reliable drainage and performance, there is a stated minimum slope for these roofs. The design slope shall not be less than 1/4 inch per foot (1/4:12), which is about a 2% grade. This threshold helps keep water moving toward drains and reduces the risk of ponding that can compromise the membrane. Slopes shallower than this would increase the chance of water accumulation, while steeper slopes still satisfy the requirement but are not the minimum.