We’re having a wedding! Retrofit metal roof systems are simply a marriage of a new sloped light-gauge steel framing system to an existing roof’s support system. For most roofs, you can refer to these support systems as structural grids, which utilize primary and secondary loadbearing components to distribute loads from the roof to the building’s foundation. The new retrofit framing and cladding system is also a structural grid, but in many ways very different to that of an existing flat roof.
What’s important is you understand that this marriage requires a fundamental knowledge of how structural grids work. Doing it right ensures the structural integrity of the building is maintained and not overloaded.
The components of the primary supports can be load-bearing walls, including those at the building exterior; steel or wood girders/trusses; or steel, concrete and wood beams with their columns. The secondary supports can be open web bar joists, steel or wood joists/purlins or concrete beams. Essentially, secondary members receive the design loads and distribute them to the primary members providing a path to the building’s foundation.
The design loads are the combination of temporary loads (live load), such as workers or equipment, plus loads produced by the weight of the roof assembly itself (dead load), and of course, snow loads. There are other type loads, but for the sake of keeping this simple we will just say that Live + Dead + Snow will usually control a roof’s design unless it is located in very high wind areas along the coasts. Then wind can become the culprit.
Whether the existing roof is constructed of steel, wood or concrete, most roofs are considered “uniform load-bearing” systems. This means that the roof’s structural support will receive imposing loads uniformly throughout the overall footprint of the roof.
Picture a freshly fallen 6-inch snow on a flat roof. Now, here’s the difference. Even though the new metal roof and its support (zee purlins) are considered to be uniform loading as well, the vertical framing transfers the load from the new roof to the existing roof on what is called a “point loading” system. A retrofit framing system distributes a series of concentrated loads onto the existing roof through vertical posts into its base members. These base members will change from intermittent to a continuous type depending on the span orientation of the existing joists, slope direction of the new roof or type of existing roof construction.
(See Figure 1.) Once you know how to put all of this together, retrofitting building roofs becomes rudimentary.
Metal roofing used for retrofit is called “structural,” meaning it can support the gravity loads and itself over intermittent secondary members (purlins) without the use of solid decking beneath it. The structural effects of these gravity loads can be easily engineered on a per job basis, but satisfying wind loads is another matter.
Unlike conventional roof systems, metal roofing is subjected to the strict testing protocols of the ASTM E-1592 “Standard Test Method for Structural Performance of Sheet Metal Roof and Siding Systems by Uniform Static Air Pressure Difference.” This series of tests (usually a minimum of three per product) determines the ultimate values of negative pressure that the metal roof panel and its components can withstand. Each test is conducted all the way to the roof system’s failure in 10 pounds per square foot (PSF) intervals starting at about 30 PSF.
While most standing seam roofs will fail between 65 to 90 PSF, there are some that exceed this. Now for the bad news! No matter how good your testing values are, they are subject to a safety factor, ranging from 1.65 to 2.00, resulting in an allowed 50 to 60 percent of the test value. For example: a panel tests at 92 PSF over purlins spaced at 5 feet. With a 2.00 safety factor, you can only use 46 PSF (92 ÷ 2.00 = 46). This may appear to be good, but most standing seam metal roofs today cannot withstand 46 PSF in the corners of a building, without reducing the clip spacing to less than 5 feet. This requires additional framing needed to satisfy the design wind speed.
I recommend leaving design and testing to the design professional on your project team. Next month we will get into roof replacements and reasons to utilize the retrofit concept.
Mark James has more than 40 years experience in the retrofit and metal construction markets, working for leading manufacturers and executing great projects. Currently, he is president of Retro- Spec LLC, a consulting company offering direction and deep knowledge of the retrofit business for manufacturers and contractors. James can be reached at mark@retrospecllc.com.
