January 25, 2017
Metal roofs made from galvalume-coated steel provide great corrosion resistance and can readily satisfy a 20-year weather-tightness warranty. However, when a large penetration in the roof is needed, such as a large exhaust fan or other equipment, the integrity of the roof can be compromised if not addressed properly. The common method of dealing with large penetrations (i.e., spanning over one or more standing seams) is to install roof curbs that form the transition between the roof and the equipment being installed.
Of course, like most aspects of building construction, there are choices available in materials, methods, techniques, and styles of installing a roof curb. When the key objective is to provide a curb that will perform for the entire life of the standing seam roof, there are four key points to keep in mind.
A galvalume-coated roof doesn’t mean that a galvanized steel roof curb is the best thing to use – in fact, galvanized roof curbs are known to rust, corrode, and leak, particularly along weld joints, as soon as a year after installation. Instead, a curb made from aluminum (preferred) or stainless steel should be used to prevent premature corrosion. To put any concerns about dissimilar materials and galvanic corrosion to rest, keep in mind that galvalume is approximately 80 percent aluminum by volume, so they are highly compatible.
It is not uncommon for a roofer to choose a curb type referred to an an “over/over” curb, meaning that, after the opening is cut, the curb is installed over the roofing on both the upslope and the downslope sides. This might be the easiest to install during construction, but it will very likely create more work and callbacks when the upslope side starts to get water into or under the joint, and leaks. Instead, it is well worth taking a few extra minutes to install an “under/over” curb, which places the upslope side under the roofing in a true shingled lap between the curb and the roof. This way, the upslope edge is much more protected and less likely to leak using the same shingled condition occurring on the downslope side – all creating a properly water-shedding, weathertight condition.
We all know that water seeks the path of least resistance, so the key to keep water flowing down a roof is to avoid creating pockets of resistance. This is particularly true on the upslope end of a curb as well as on the two sides parallel to the slope of the 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. Similarly, once the water reaches the two sides, at least 6 inches of free clearance is needed (i.e., without being encumbered by standing seams or other features) to allow the water to keep going and not back up to create a water head at the upslope end of the curb. Simply put, clearance means free-flowing drainage; lack of clearance can mean water buildup and leaks.
Installing curbs that rest in the flat, lower, panel area of metal roofing invites water tightness problems since the curb now has to be installed and sealed in the most vulnerable area – the surface where rainwater flows. Instead, coordinating the curb size with the rib spacing to provide a rib-to-rib curb eliminates fasteners down both sides of the curb in the pan of the roof panels. Placing the curb on and attaching it to the ribs also allows better transitioning from under the roofing on the upslope end to cover the roof on the downslope end. This type of curb has the added benefit of being able to be installed either during the roof installation of after the roof is finished.
Taking these four points into account in your next metal roofing project where roof curbs are required will help assure a well-installed, weathertight condition that should last just as long as the metal roofing system itself.