This question is best answered by first establishing what is to be accomplished. The first questions to answer are: Is the unit to remain - with only repair or replacement of components within the unit? If not and the entire unit is to be replaced, Does the replacement unit weigh more or less than the unit being replaced?

If the existing unit is to remain and you are able to simply repair it in place then quite obviously NO structural modifications or structural engineering is required.

Chapter 4: REPAIRS of Florida's Existing Building Code states:

401.3 Conformance. The work shall not make the building less conforming to the building, plumbing, mechanical, electrical, or fire codes of the jurisdiction … than it was before the repair was undertaken.

If however the existing unit requires removal and replacement then the proposed weight versus the existing weight becomes critical. If you propose to replace the existing units with new units that weigh the same or less, the code only requires that the new units are attached to the structure on curbs or rails so as to resist overturning from wind loads.

Chapter 5: Alterations - Level 1 of Florida's Existing Building Code states:

507.2.1 Replacement of roofing or equipment. Where replacement of roofing or equipment results in additional dead loads, structural components supporting such re-roofing or equipment shall comply with the vertical load requirements of the Florida Building Code, Building.

The basic issue is this: when removing and replacing existing roof top units with new units that weigh more, the Florida Existing Building Code requires an engineering evaluation. If that evaluation finds that the stress within the structural elements is increased by more than 5%, structural modifications will most likely be required.

One final thought to keep in mind is that a post-construction evaluation of existing conditions can often turn up as-built deficiencies. One such case occurred when we were tasked with addressing new units that weighed more. We performed the engineering analysis and in the process identified an original construction deficiency within the structure. Although this deficiency did not represent a threat to the safety, health and welfare of the public it did represent an item that once discovered had to be addressed.

If you have a code question, our forensic group is ready to answer your questions or assist you in developing a code compliant solution. Contact William Bracken at wbracken@brackenengineering.com.