At one time or another, every building owner will deal with the process of replacing their roof. While this is a necessity for the proper function of a building, it can often be costly, disruptive (smelly and noisy), and messy. If your building currently has a metal roof that needs replacement, one option to consider is a metal roof overlay system.

The most common metal roof overlay system includes prefabricated sub-purlins, which are z-shaped structural members that are factory cut to fit snugly over a variety of metal panel profiles. The purlins, typically 16-gauge galvanized steel, are attached through the existing metal roof panels to the building’s structural frame to provide the appropriate base to which the new metal panels are secured.

The benefits of a metal-over-metal retrofit roof system include:

  • Reduced costs and disruption associated with demolition, and provides better protection for the building’s interior finishes from unpredictable weather versus removing the existing metal roof.
  • Minimized requirements for the contractor to gain interior access, which reduces interruption of occupant day-to-day activities.
  • Increased energy efficiency. The sub-purlins can be formed to customized depths to allow installation of additional insulation.
  • Reduced labor costs from faster project completion.
  • Ability to easily upgrade an existing exposed fastener (face fastened) metal roof to a more watertight standing seam (concealed clip) roof.

There are many factors to consider before choosing a sub-purlin retrofit system. The existing building structure should be analyzed to verify that it can safely support the additional weight of an overlay. Furthermore, applicable code requirements should be researched to determine if the new overlay system will be compliant. Lastly, the overall condition of the existing metal roof system and insulation assembly should be evaluated. If the existing panels are visible from the interior and are aesthetically unpleasing, or if the need for continuous insulation is driving your roof project, an overlay may not be the right option.

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