Plan a Roof Configuration for Unobstructed Drainage

In modern construction, adding complexity to roof plans is commonly done to improve curb appeal. But adding complexity to the drainage pattern of a roof can also create excessively concentrated or obstructed roof drainage patterns as shown in Figure 4–3. If not avoided or properly addressed, these conditions often lead to moisture intrusion and durability problems. The following design actions should be considered:

• Strive for easily drained roof geometries with minimal obstructions to roof water flow. Balance the desire for a roofline with curb appeal with the drainage performance.
• Specify an adhered waterproofing membrane underlayment applied to roof valleys and adjacent vertical surfaces where flow concentrations occur as shown in Figure 4–3.
• Where concentrated flows discharge from the roof into gutters, use a gutter deflector or splash guard to prevent the water from “over-shooting” the gutter, and size gutters and downspouts accordingly (see Section 4.2.6).