Manufacturer and supplier of roof products and accessories for the metal building industry since 1964

Installing Roof Curbs on Metal Roofs

February 9th, 2023 | Eric McClung

Most commercial buildings have a good amount of equipment on their roofs such as condensers, fans, skylights, hatches, and vents. In most cases these items must penetrate the membrane or metal roof panel and always add weight to the overall structure. Roofing curbs are utilized to protect these investments, ensuring their efficient, safe operation while preventing damage associated with leaks. There are several details to consider when installing rooftop curbs which we will cover below. At R&S Roof Products, we offer roof curbs for commercial buildings in addition to roof hatches that allow for the safe installation and access to roof equipment.

What Are Roof Curbs?

Roofing curbs consist of elevated metal frames designed to safely mount equipment and their accessories onto roofs. They are used for roof top units that penetrate the roof surface as well as those that mount on the rooftop. These roof curbs ensure the equipment can be mounted on a level surface. They always include flashing to protect the structure from water getting through the membrane of the roof, which can lead to damaging interior water leaks.

Obtain Installation Instructions

All curb manufacturers must provide installation instructions for their curbs. In addition, most Pre-engineered Metal Building (PEMB) Manufacturers will also have installation details in their literature. We recommend cross-referencing the two to ensure compatibility.

Some manufacturers offer an optional extended Weathertightness Warranty on their roof systems. These may require a specific curb design sourced from a list of approved roof curb manufacturers. Talk with your metal building representative to gather this information ahead of time and supply this detail to the curb manufacturer prior to fabrication.

Structural Openings by PEMB Manufacturer

Building plans supplied by the building manufacturer should include framed openings for rooftop equipment in their design. Take time to confirm that the sizes of these openings match the mechanical schedule for the project. This will ensure that the roof has been engineered to tolerate equipment loads.

Curb Placement on Roof – Structural Openings

Roof Curb Standing Seam
Curb with fixed symmetrical contours

The placement of structural openings in relation to the seam pattern of the roof must also be considered. Roof curbs for metal buildings include flanges that nest with the corrugations of the roof panel profile. These are often referred to as the panel highs or seams. If the curb has welded rib cups and closures (we refer to them as contours), they are typically arranged in a symmetrical pattern based on the curb size. This means that the curb installation is “fixed” with respect to left/right orientation.

For small lightweight equipment like fans or skylights, this may not be an issue as there is flexibility as to where the equipment can be installed on the roof. However, if the placement is fixed due to the structural opening location, there are two primary options to consider:

  1. Fixed Contour Option: The required dimensions for contour placement can be provided ahead of time to the curb manufacturer. This allows the placement of the contours to be customized according to the precise curb location. This saves time in the field but carries some risk should actual panel modulation vary from the plans.
  2. Loose Contour Option: The curb can be fabricated with “loose” contours. In this case the curb flashing is left flat and is notched in the field where the panel highs and curb flanges intersect. This slightly increases installation times but provides the flexibility to adjust the curb location as it is being installed.

CAD Drawing of Curb to Confirm Size and Design

Curb showing factory installed vibration isolation rails.
Curb factory-installed vibration rails

All curb manufacturers should also provide a CAD drawing of the curb for each rooftop unit. This should be reviewed for accuracy by the mechanical contractor. Here is a quick list of design features to check for:

  • Does the curb require a damper shelf for exhaust fans or partitions (duct support) for HVAC equipment?
  • Are there other required components such as vibration isolation rails or seismic provisions for your geographical location?
  • Be sure to confirm that pitch of the roof is provided to the curb manufacturer so a level top can be included in the design when required.

Proper Backer-Channel is Required

Backer-Channel (or sub-framing) should also be offered as an option by the curb manufacturer. This material is used as a substrate to accept fasteners during the installation procedure. Metal roofs are made of very thin material, typically not adequate for accepting fasteners. Sub-framing is installed beneath the roof and is a critical element to draw the curb flange tight to the metal roof for a proper seal.

Mastic, Sealants and Fasteners

Self drilling fastener with zinc cast washered head.
Zinc Cast Head Screw

To seal the curb to the metal roof you must also use quality sealants and fasteners. We recommend using both a triple bead mastic along with a 1/8″ bead of polyurethane sealant. The sealant helps to ensure a complete bond between the mastic and the curb flange. It also acts as a lubricant as you are setting the curb into place. Lastly, the sealant is a great visual queue to confirm a full seal as you will see some of this material squeeze out from beneath the curb flange once fully seated.

For fasteners we highly recommend a cast washered-head fastener versus a standard fastener with a loose washer. These fasteners help to ensure that the rubber sealing washer is not over-tightened which can cause it to split and lose its sealing capacity.

Work With a Knowledgeable Curb Manufacturer

The process of determining the best roof curbs for your building does not have to be difficult. At R&S Roof Products, we can help you find the best roof curb solution for your application. To learn about how R&S Roof Products can help you, call us today at 800.423.1619 or submit a contact form.


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