Retrofit Roof Design Basics

When it comes to metal roofs there is no big difference between roof design for new construction and a re-roof design for an existing building, with one major exception. When we retrofit an existing building we design around the existing roof panels and the existing purlin spacing. Roof Huggers located only on the existing purlins may not meet the new building code.

ASCE-7 adopted by the "International Building Code" (IBC) divides a roof into three zones: The "Field" or center of the roof, the "Edge or Perimeter Zone" and the "Corner" zone of the roof. The loads for each "Zone" are now calculated separately.

The new roof must withstand the full forces calculated for each of these "Zones". The old design criteria assumed roof loads were evenly distributed loads (This means that even before you start, the EXISTING BUILDING most likely does not meet the new code.)

Since no two buildings are exactly alike, the size and shape of these "Zones" varies from building to building and depends on several factors. Each building must be considered individually and engineered based occupancy, existing construction, proposed use and the new panels to be used. The basic steps in the process are as follows:
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  • STEP A: Collect the basic information needed:
    • Controlling Building Code
    • Width x Length x Eave Height of Roof to be retrofitted
    • Roof Pitch
    • Note Parapets 36” or Higher
    • Building Configuration: Open, Partially Enclosed or Enclosed
    • Exposure Category: B, C, or D
    • Occupancy Category: I, II, III, or IV
    • Basic Three Second Gust, Design Wind Speed
    • Ground Snow Load if applicable.
    • Existing Purlin Spacing
    • Existing Roof Panel information
    • Special Requirements: IE: UL-90, Factory Mutual, additional Insulation, etc.
  • STEP B: Determine the Zone Pressures, (Options Below).
    • Contact a design professional; provide them the information from Step A.
    • Contact Roof Hugger for a free preliminary load estimate.
    • Visit to do a preliminary design yourself.
    • Many panel suppliers will also do a preliminary design and “Clip Analysis” for new standing seam roofs or a maximum “Span Analysis” for screw down roofs.
  • STEP C: Select the New Panel you want to use on the building.
    • Review the Zone Loads required by the code (from above)
    • Determine the maximum capacity of the new panel on the existing purlin spacing.
  • STEP D: Review the panel capacity vs. load requirements.
    • Compare the Zone Loads to the panel capacity to determine if the existing purlin spacing is adequate or needs to be reduced to provide proper attachment for the new panel.
    • If the existing purlin spacing is 5’ (typical in older buildings) you would compare the manufacturers published capacity for the new panel on 5’ purlin spacing to the values provided by an engineer or estimated.
    • If the actual/estimated loads in any Zone exceed the panel’s capacity, additional framing must be added to reduce the purlin spacing in the affected Zones.
      • Another option is to consider a narrower rib spacing or heavier gauge panel that has a higher capacity.
  • STEP E: If Additional Framing is Required:
    • Determined what that framing will consist of.
      • Add additional purlins from inside the building where needed
      • Create a grid system made from “C’s”, “Z’s” or Hats Members to meet the Zone Loads on the necessary purlin spacing.
      • Use Pre-Notched (Roof Hugger) “Z’s” with fitted integral Sub-Rafters to meet the loads.
      • Roof Hugger can assist in framing estimates upon request.
    • THE NEW PANEL WILL ALWAYS CONTROL THE FRAMING DESIGN. (Weaker panels typically require more framing than a stronger panels)
    • THE MOST ECONOMICAL DESIGN USES PANELS THAT CAN MEET THE LOADS IN ZONES 1 & 2. This design, if workable, restricts additional framing to the corner Zones only.


Roof Hugger stands ready to discuss your retrofit project and the design process, please feel free to call us at 800-771-1711.


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Physical Address: 142 Whitaker Rd. Lutz, FL 33549
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