Comparison of Retrofit Systems Over Existing Metal Roofs
This white paper, written by Brian Gardiner of BMG Enterprises, LLC, compares two commonly used methods of retrofitting exiting metal roofs, single-ply systems and metal panels, and illustrates important design considerations that apply. The design consideration include the existing support structure, secondary framing deflection, the existing metal roof properties, drainage, roof fastening, drag load, and fire rating. The paper also contains a brief review of the building code requirement.
Retrofit roofs over existing metal roofs have several potential benefits. There are also several design considerations related to the decision to choose a metal, or single-ply roof, especially when retrofitting a metal building. The following recommendations are provided to assist with this decision:
• Engage a licensed professional engineer practicing structural engineering with knowledge of metal buildings to evaluate the following:
Existing roof and structural system to ensure it is adequate for current building codes
and to support the anticipated loads imposed by the retrofit roof system.
Retrofit roof field, perimeter, and corner zones to ensure code compliant fastening of the roof system.
Necessary resistance to anticipated drag loads on the retrofit roof system and existing structure.
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.
The following additional recommendations are specific to single-ply over metal roof retrofits:
• 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.
• Consult with the local building official, a licensed professional engineer practicing structural
engineering, and the single-ply roof manufacturer to confirm that the existing building's metal
roof is adequate to perform as a structural deck for a non-metal sheeted roof.
• Implement quality control during construction to ensure that single-ply membrane fasteners
driven through insulation board layer(s) are properly engaged into secondary structural members.
Building owners, and their design professionals and contractors, should evaluate retrofit roof
solutions for their project to determine what is the most prudent choice.