This month, we will discuss the different types of retrofit framing systems that best suit a project.
Once you have found that ideal opportunity for retrofitting an existing roof, you will probably already have a pretty good idea what the new roof’s geometry will be. While this was the first question, will you now know what the best-suited under structure will be to create the new roof geometry?
For existing flat roofs, there is basically one type of light-gauge steel framing system with many optional components that make it adaptable to the multitude of roof structures out there. For existing sloped roofs, there are several sub-framing assemblies that have been used, but only a few are code compliant, structurally correct systems, which I will explain later.
Most of the retrofit manufacturers have framing systems that are very similar. Why? Because this is not rocket science and you cannot reinvent the wheel. Their differences are primarily in the way the framing is braced both transversely (eave to eave) and longitudinally (parallel to the new metal roof’s ridgeline). Some are easier to erect, but in the end they all make this very lightweight framing system extremely stable.
Finally, once you begin to compare the new roof’s geometry to the existing roof’s framing it becomes clear when the various base system options come into play. This was discussed last month in our Fit-for-Use Applications column.
Now let’s discuss the correct framing system. Let’s say that your existing flat roof has an open-web steel bar joist support system. This project will probably be suited for “Post & Purlin” type framing (see Figure 1), understanding that the base members may or may not change from intermittent to continuous members based on the new roof geometry.
However, if your new roof slope is perpendicular to the span direction of the bar joist and is principally gabled at both ends, then you may want to look at what we call a “Wide-Bay or Span” framing system (see Figure 2). Essentially a modified Post & Purlin system, its value is in reducing the amount of framing to be erected and needs to attach to the existing roof system. Different from the Post & Purlin, this system has greater purlin spans and its vertical supports are located directly over the existing primary supports, such as beams.
This system is good as long as the purlins are not spanning more than 20 to 25 feet. The only drawback to this system is in its anchorage requirements where the required pull-out to be satisfied, can exceed four or five times that of the Post & Purlin.
As a point of interest, the two different framing systems have been combined many times for use on the same project. The Wide-Bay design is usually ideal for existing glulam roof systems due to their having a structural wood decking in lieu of joists and the laminated beams are spaced less than 20 feet on-center. It could also apply to concrete and some heavy timber roofs as well.
If your building is not a flat roof, then you have some options as well, but be cautious. For decades, the metal construction industry manufacturers and contractors have been installing new metal roofs over asphalt shingles and other sloped roof assemblies including existing metal building roofs (see Figure 3). Today, it is a little more difficult since most building code authorities have acquired jurisdiction over roof replacements. Even though some people may argue that replacing a roof falls under building maintenance, the permitting bodies don’t see it that way. The problem? You now have to have your new roof engineered to meet the local code’s regulated wind speeds and snow loads. This means a sub-framing system is engineered to accommodate the ASTM E-1592 wind uplift test results of the new metal roof, as well as supporting heavier adopted snow loads. It can get tricky, so consult with a manufacturer that knows this business and can help you choose the right system.
If you are just entering the retrofit market, there are several qualified manufacturers that furnish the engineering, framing components and new metal roofing, which provide a complete package. The ones I know of are members of the Metal Construction Association (MCA), which in my opinion establishes them as a “player” in the retrofit roofing game.
Next month, we will discuss how retrofit systems can help existing buildings achieve energy independence.
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 RetroSpec 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.
