Although metal roofs have a decades-long service life, when they do eventually reach the end of their life building owners must decide to either repair, replace, or retrofit. Retrofitting has a number of advantages, making it a popular solution. However, when building owner's chose to retrofit their existing metal roof, they have a couple of options.
The Metal Building Manufacturers Association (MBMA) published a whitepaper, Comparison of Retrofit System Over Existing Metal Roofs, by Brian Gardiner that dives deep into the options available to building owners. “The goal of this paper is to compare two commonly used methods of retrofitting existing metal roofs and illustrate important design considerations that frequently apply to candidate buildings.”
The two most common retrofit options are metal over metal and single-ply over metal. Both options allow for the original roof to remain in place, meaning that the building does not experience disruption to operations, and reuses the existing insulation. Because the original roof remains, retrofitting also cuts down on landfill waste and can reduce costs (no labor or disposal costs associated with roof removal). However, not all projects can safely accommodate a single-ply retrofit over an existing metal roof.
Ultimately the whitepaper recommends that building owners, “Engage a licensed professional engineer practicing structural engineering with knowledge of metal buildings to evaluate the unbalanced loads and potential water accumulation on a single-ply retrofit roof with respect to structural capacity and drainage provisions.” Also, “the retrofit roof designer should verify the fire classification both above and below the retrofit roof system for building code compliance and building insurer acceptance for the retrofit roof system.”
You can read the full Comparison of Retrofit System Over Existing Metal Roofs whitepaper here to learn more about each of these considerations, or you can visit the MBMA website for more info.