Metal Roof Curb Design and Installation Tips and Guidelines

A metal roof curb is a crucial component in metal roofing systems, providing a smooth transition between the roof and the wall. It's essential to design and install it correctly to ensure a watertight seal.

The curb's height and width can vary depending on the roof's pitch and the type of metal roofing material being used. Typically, it ranges from 2 to 4 inches in height and 2 to 6 inches in width.

A well-designed metal roof curb should be able to withstand various weather conditions, including heavy rainfall and strong winds. This is achieved by ensuring the curb is securely attached to the roof deck.

The curb's material should be compatible with the roof's material, such as aluminum or galvanized steel, to prevent corrosion and ensure a long-lasting seal.

Installing roof curbs rib to rib can eliminate fasteners down both sides of the curb in the pan of the roof panels, making it a more efficient installation method.

To ensure a weathertight condition, it's essential to use the proper material for the curb, such as continuously welded 3003 H14 aluminum or 18 gauge stainless steel, as recommended by the MCA.

A proper installation incorporates the over/under technique, also known as "shingling", where the metal roofing panels go over the curb's flange on the upside of the roof slope and the curb's flange goes over the metal roofing panels on the downslope side.

Building specifications are a crucial part of any construction project. The metal building roof curb specifications can be broken down into several key components.

Material options include 0.080 aluminum, 18 GA, 14 GA, or 16 GA aluminized steel, and 1″ Rigid Insulation R6.5.

The construction process involves full penetration welds, thoroughly tested to ensure quality and durability. Mitered corners are used to achieve a true square top.

Curb upslope involves a flange that installs under the panel, with closures welded or shipped loose. An integrated 4″ high water cricket is also included.

A minimum of 9″ is required between the up-slope curb wall and closures.

Curb downslope involves a flange that installs over the panel, with caps (straps) welded or shipped loose. A minimum of 3″ is required for the down-slope flange.

Curb side flanges are seam-to-seam, integrating with the roof panel, and a minimum of 3″ is required between the curb sidewall and panel seam.

The curb top is designed to be level, suitable for mechanical equipment or as specified. Skylight curbs slope with the roof.

Dimensions for the curb height on up-slope are typically 6″ – 8″, with custom heights available.

Finish options include aluminum mill finish, galvanized finish, powder coat (standard colors or custom), and Kynar (exact match).

Accessories such as 14 GA backer-channel, sealants, and fasteners are also available.

Certifications include engineering calcs and stamped submittals, with additional costs applying.

Warranty options are 20 Years for aluminum and 1 Year for steel, with an MBCI 20 Year Weathertightness Warranty Curb also available.

Sub-Framing is a crucial aspect of any construction project, and it's essential to get it right. You'll need to consider the type of sub-framing you'll be using, which can include backer channels.

A backer channel is a type of sub-framing that provides support for the curb. It's available in various models, including SUB-MBS-T, SUB-MBS-V, SUB-MBS-R, and SUB-MBS-F.

These models are designed for specific installation requirements, such as trapezoidal seam clips and flat side flanges. You can find more information on these models in the specifications section.

To ensure proper installation, it's essential to follow the manufacturer's guidelines. This can be found in the installation section of the sub-framing accessories.

Water Flow Clearance is crucial to ensure proper drainage on your roof. A curb with a minimum clearance of 12 inches between it and any other object on the upslope end will give water enough room to flow around the curb easily.

Water seeks the path of least resistance, so creating pockets of resistance is a major no-no. A curb with at least 6 inches of free clearance on its two sides is needed to allow water to keep going and not back up to create a water head at the upslope end of the curb.

Clearance means free-flowing drainage; lack of clearance can mean water buildup and leaks.

Proper installation is crucial to ensure a well-installed, weathertight metal roofing system. Installing roof curbs rib to rib is a good practice as it eliminates fasteners down both sides of the curb in the pan of the roof panels.

This type of curb also allows better transitioning from under the roofing on the upslope end to cover the roof on the downslope end. The curb can be installed either during the roof installation or after the roof is finished.

The MCA recommends using either continuously welded 3003 H14 aluminum or 18 gauge stainless steel for curb construction. Aluminum isn't affected by heat-warp and spot and joint welds won't damage its long-term corrosion resistance.

A proper installation incorporates the over/under technique, also known as "shingling." On the upside of the roof slope, the metal roofing panels go over the curb's flange. On the downslope side, the curb's flange goes over the metal roofing panels.

Here are some recommended curb materials:

Installing a metal roof curb can be a bit tricky, but with the right approach, you can ensure a watertight installation that lasts just as long as the metal roofing system itself. This is achieved by coordinating the curb size with the rib spacing to provide a rib-to-rib curb.

The MCA recommends using continuously welded 3003 H14 aluminum (0.080" minimum thickness) or 18 gauge stainless steel for curb construction. This material choice is crucial in preventing heat-warp and corrosion issues.

Proper curb installation involves using the over/under technique, also known as "shingling." On the upside of the roof slope, the metal roofing panels go over the curb's flange, while on the downslope side, the curb's flange goes over the metal roofing panels.

Correct Procedures are key to a successful roof curb installation. Reasor believes that if an erector understands and applies the same fundamental details of a panel lap and an eave termination, then they have a high likelihood of sealing out a roof curb on a metal roof.

A shingled curb style is superior, so the up-slope connection should emulate a panel lap with a backer plate for fasteners, uninterrupted mastic band and inside closures installed just like the eave condition. This is in line with the Metal Construction Industry's recommendations.

Proper placement and amounts of sealants and fasteners are also imperative. Conrad claims that most procedural errors result from unprofessional workmanship, and that using butyl tape sealant, urethane tube sealant and self-drilling fasteners is a common practice.

Fasteners must be placed through the mastic field similarly to a panel lap, and the spacing of fasteners should be prescribed by the curb installation details. A band of tape mastic must be continuous along the perimeter of the curb flanges to create a complete long-term weathertight gasket.

Properly installed roof curbs can float with the expansion and contraction of the panels, which is only accomplished via specially designed sub-framing systems that incorporate metal building fundamentals. This is crucial for avoiding issues that develop after a few thermal cycles on the roof.

The location of a roof curb on a building determines its style, the roof installation procedure, and the coordination of installation details. This is especially important for gable buildings, where a roof curb placed on the ridge requires coordination of ridge details to ensure a weathertight transition.

Roof installation procedures should be modified to accommodate a floating low-eave condition with the ridge pinned, as saddled curb placement on the ridge can restrict thermal movement. This is crucial for buildings where thermal movement is a concern.

For standing seam roof systems, curbs must be installed to allow thermal movement. This means that curbs installed on these systems should be designed to accommodate the building's thermal expansion and contraction.

It's essential to ensure that roof panels can support the weight of the selected curb and unit while still operating at the desired design. This means that all support framing should be installed prior to the installation of roof panels.