Do I Need to Watch My Weight?

by Marcy Marro | June 1, 2023 12:00 am

Well, I have some great news for 80% of you! No, most of you do not need to worry about the extra weight of a metal over metal retrofit. Surprisingly a metal retrofit system including retrofit framing, insulation, panels and trim, will only add between 1.75 – 2.5 #/Sq. Ft. The International Existing Building Code (IEBC) excludes existing buildings from a full structural analysis in section 707.1 for the “addition of a second layer of roof covering weighing 3 pounds per square foot or less over an existing, single layer of roof covering.”

But what about the other 20%? When do you need to give an existing building a closer look? More importantly what can you do if an analysis shows an overloading condition?

Most buildings deemed to be of high importance should be checked to confirm their capacities. Additionally, buildings in areas that have significant increases in minimum snow loadings should be checked. Some areas of the country have seen their ground snow loading go from 20# psf to 30# psf or higher. And, if during a walk-thru of an existing building you see a significant amount of equipment hanging from the roof purlins it would be prudent to have an engineering firm do an analysis of the existing structure.

In most cases when a metal building is found to be overloaded it will be the purlins and not the frames that are beyond their design capacity. The only way to fix that is to add more purlins between the existing ones or to sandwich structural channels to the back of the existing purlins. This can be difficult and sometimes impossible depending on the interior buildout of the facility.

This is where a metal-over-metal retrofit can help. For many years Roof Hugger[1] has been receiving comments from installers about how much stiffer or solid feeling a building felt after being retrofitted with Roof Hugger sub-purlins. In 2010 we began running a series of simple span base tests to see if Roof Hugger sub-purlins did add strength to existing purlins. The results were impressive, but we were not satisfied because most buildings today are not simply span structures, they are continuous span structures. In 2018, our testing firm created a 50-foot, two-span test chamber to create a real-world continuous span base test. This test data along with the simple span test data allowed us to document the amount of strength various size sub-purlins will add to an existing purlin system.

In 2020, Roof Hugger hired a third-party engineering firm to create software to model existing building purlin systems to determine their capacity and then add sub-purlins in various heights and gauges to determine the exact increase in strength we can achieve for any existing building.

How does this work exactly? When you install a Roof Hugger Sub-Purlin over and existing roof panel and into the existing purlin a deeper composite purlin is created. The new deeper section of existing purlin and new Roof Hugger creates a greater capacity than just the existing purlin by itself.

Using a retrofit system provides a huge advantage for building owners because the work can be done on the exterior of the existing roof. The older building can be brought into compliance with the current building codes without disrupting ongoing production or other operations.

The installation is easy, non-disruptive and the new metal roof can have 60 years or more service life. This means photovoltaics can be added years in the future without the worry of the panels outlasting the underlying roof.

Not all roofs will enjoy an increase in capacity by retrofitting, but most will.

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Dale Nelson is the president of Roof Hugger LLC, Lutz, Fla., an LSI Group company. For more information, call (800) 771-1711 or visit www.roofhugger.com[2].

Source URL: https://www.metalarchitecture.com/articles/do-i-need-to-watch-my-weight/

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