The Role of Gutters and Downspouts in Metal Buildings

Gutters and downspouts are essential for managing rainwater runoff in metal buildings, preventing:

• Water infiltration at doors, walls, and foundations
• Soil erosion and structural foundation damage
• Ice buildup and safety hazards in colder climates
• Premature deterioration of metal panels due to prolonged water exposure

MBMA Guidelines for Gutter and Downspout Design

The Metal Building Manufacturers Association (MBMA) provides the following recommendations for drainage design:

• Eave Gutters & External Downspouts – Designed for a 5-year recurrence interval with a five-minute duration rainfall intensity
• Interior Gutters, Valleys & Parapet Gutters – Designed for a 25-year recurrence interval with a five-minute duration rainfall intensity

Additionally, MBMA notes that:

• Gutters, downspouts, and valley drainage systems are not automatically included with a metal building system and must be specified in the order documents.
• The customer or their designated design professional is responsible for ensuring proper foundation design and water management beyond the provided building system.

Types of Gutters for Metal Buildings

Depending on the building design and water management needs, common gutter types include:

• Eave Gutters – External gutters installed along the roof edge to collect runoff from the roof panels.
• Valley Gutters – Internal gutters located where two roof planes meet, requiring higher drainage capacity.
• Parapet Gutters – Used in buildings with parapet walls, these gutters are typically concealed behind the parapet and collect water that drains through internal or external downspouts.
• Box Gutters – Larger-capacity gutters, often integrated into the roof system, used where high-volume drainage is needed.

Downspout Design Considerations

A successful gutter system depends on proper gutter sizing and selecting the right combination of downspout size and spacing. Key factors include:

• Gutter and Downspout Balance – A properly sized gutter can still overflow if the downspouts are undersized or spaced too far apart. The gutter’s capacity must be matched with downspouts that can drain water efficiently without creating bottlenecks.
• Spacing of Downspouts – Typically placed every 20-40 feet along the gutter system. If downspouts are spaced too far apart, gutters will have excessive pooling and may overflow during peak rainfall.
• Number of Downspouts – Increasing the number of downspouts can reduce the required size of each downspout while maintaining proper flow.
• Typical Downspout Locations—While not a requirement, it is common to locate downspouts at frame-line locations to prevent interference with potential wall openings, such as doors and windows.
• Discharge Method – To prevent erosion and water damage, water must be directed away from the foundation using splash blocks, underground drains, or extended drainage pipes.

Secondary Drainage for Valley and Parapet Gutters

Because valley and parapet gutters handle water from multiple roof surfaces, they require careful design and often include secondary drainage systems to prevent overflow and water damage.

Common Secondary Drainage Solutions

• Scuppers – Openings in parapet walls or sidewalls allow excess water to drain directly from the system. Scuppers act as emergency overflow points in case the primary drainage system becomes blocked or overwhelmed.
• Emergency Overflow Drains – Additional drainage openings placed at a slightly higher elevation than primary drains provide backup drainage if the system reaches capacity.
• Redundant Downspouts – Secondary downspouts dedicated to overflow conditions help prevent excessive water buildup in valley and parapet gutters.
• Grated Drainage Openings—Interior grated openings redirect excess water before it overflows into critical building areas.

Secondary drainage is especially important in large-span metal buildings where a clogged or undersized valley or parapet gutter system could lead to ponding, structural stress, or water intrusion.

Additional Design Considerations

• Gutter Hangers & Brackets – Provide support, especially for long gutter runs.
• Snow Retention – Snow guards prevent ice and snow from damaging gutters in colder climates.

Come back for part 2, where we’ll discuss how we handle your gutters and downspouts at Whirlwind.