When designing roof curb height, there are several considerations to keep in mind. A roof curb is a raised frame attached to a building’s roof to support roof-mounted equipment and features such as HVAC units, skylights, and hatches.
Proper roof curb height ensures weather protection, effective drainage, and compatibility with the mounted components. Below are common guidelines used in architectural and construction practice.
Roof-mounted mechanical units must be installed on curbs raised a minimum of 8 inches (203 mm) above the roof surface.
When roofing materials extend beneath the unit, use raised equipment supports to maintain at least the minimum required clearance height in accordance with project specifications.
This clearance helps protect the roofing assembly from water infiltration and allows for adequate flashing and maintenance access.
For unit skylights installed on roofs with a slope of less than three units vertical in 12 units horizontal (25% slope), the curb must extend not less than 4 inches (102 mm) above the roof plane — unless otherwise specified by the manufacturer’s installation guidelines.
This minimum height allows for proper water shedding and ensures that the flashing system performs effectively.
For low-slope conditions, skylights must be mounted on curbs tall enough to provide at least an 8-inch vertical flashing height. This height helps prevent ponding and water intrusion during heavy rainfall.
On steep-slope roofs, water drains rapidly. Therefore, flashing heights can be lower than 8 inches while still maintaining proper water protection. The specific height should follow manufacturer recommendations and local building codes.
Determining the correct roof curb height requires evaluating slope, climate, roofing system type, and equipment weight. Coordination among architects, roofing manufacturers, and mechanical engineers ensures curbs meet structural and waterproofing requirements.
Proper design and installation of roof curbs protect both the roofing system and the mounted equipment over the life of the building.
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