Roofing curbs provide a watertight solution for a wide variety of roof penetration scenarios such as access hatches, skylights, and mechanical equipment.
A properly functioning roof curb has two distinct functions:
The guidelines we discuss in this article are specific to low slope (>2:12 pitch) metal roof systems.
Depending on the curb's size and purpose, there are several options from which to choose. The ultimate goal is to raise the penetration connection above the roof surface, where leaks are the most likely to occur.
One-piece curbs are the most popular option for new construction. The curb gets installed with the roofing panel, which minimizes the potential for leaks.
The two-piece curb works well on retrofit applications involving existing chimneys or equipment locations. This option allows for a weathertight seal to be installed (or replaced) around penetrations previously installed on the building.
A double curb works well for exceptionally large or heavy rooftop equipment. The first curb may be a structural curb to support the equipment load, and the second curb provides the required weathertight seal.
The first step of the installation process is to use the proper material to construct the curb. The MCA (Metal Construction Industry) recommends that curbs utilize either continuously welded 3003 H14 aluminum (0.080" minimum thickness) or 18 gauge stainless steel.
Aluminum isn't affected by heat-warp like hot-dipped galvanized steel or Galvalume®. Spot and joint welds can also damage these metallic coatings and diminish their 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.
The curb should have a diverter device on the curb's upside to efficiently direct water around the penetration.
In addition to managing the water runoff, the diverter also protects the curb's upside from corrosion and leaks because water can't collect and create a pool of stagnant water.
There must also be a minimum of 12" from the tip of the diverter to the next roof panel seam.
And there should be a minimum of 6" of clearance between the curb and the adjoining panel seam.
These dimensions provide a good rule of thumb to follow. But it's important to remember that larger curbs can require even more clearance to work effectively.
You should always consult with the roof system manufacturer for additional requirements you need to fulfill (based on your specific needs or design) to not void their weather-tightness warranty. Many curb manufacturers offer curbs in various sizes that match common industry roofing panels, making curbs even simpler to install.
Perhaps the best curb solution of all is to utilize curbs that run from "rib-to-rib." These curbs' sides extend to the adjoining roof panels, eliminating the need for a seam where the water flows down the roof panel surface to the gutters and they are quicker to install than other options.
Click here for more information from the Metal Construction Association (MCA) about low slope roofing curbs and their installation.
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