This month, I want to address the steps to prepare for a design analysis of an existing roof, which is necessary to determine if an existing roof can support the new roof system. The analysis is a three-step process. First, determine specifically what makes up the existing roof and what it is supporting. Second, identify what the new system weights are and the new imposing design loads. And, third is the analysis, which itself is quite simple but vital to determining if the existing roof will in fact support the new retrofit system.
There are several factors affecting the design analysis:
• Building code roof loading criteria changes since the building was first constructed, which include snow and wind, as well as importance and exposure factors.
• The added weight of the new retrofit system and how it is transferred to the existing roof structure.
• The proposed new roof geometry and how it will affect snow loads due to valleys, hips or ridges and large roof penetrations
(chimneys, etc.).
• Relocation of existing rooftop equipment either repositioning on the existing roof or elevating to the new roof.
All of these changes are the reason why existing roof systems and their structural components must be analyzed to confirm their capability to support the new and existing loads.
Flat roofs are considered to be uniform loading systems; retrofit systems are point- loaded systems. The difference between these two is you have a new roof that will deliver a series of concentrated loads into the existing roof structure. As stated before, roofs utilize a structural grid, whether constructed of steel, wood or concrete. This grid combines primary and secondary load-bearing assemblies that receive positive acting (downward) loads and ultimately transfer them to the building foundation through load-bearing walls and/or structural columns. For example: A pre-engineered metal building has rigid frames (primary structural elements) that are normally spaced from 20 to 30 feet on-center, making up a bay. The rigid frames receive the design roof loads (snow, dead and collateral) from the roof panel and purlins (secondary structural elements). From these, the loads are transferred to the columns into the building foundation.
The primary assembly comprises load-bearing walls (including those at the exterior perimeter of the building, girders and trusses (steel or wood) and beams (steel, concrete or wood). The secondary assemblies can be open-web steel bar joists, purlins
(steel or wood) and concrete beams, including tees used in pre-cast roof systems. Most secondary assemblies distribute uniform loads to their outer ends where its total tributary load is distributed to the primary supporting assembly.
Tributary loads are the positive acting roof design load measured across the member to a distance equal to one half of its spacing to each adjacent secondary member multiplied by the member’s span. For example: If a bar joist in an existing roof is spaced at 5 feet on-center and spans 50 feet, then the tributary is 2 1/2 feet outward from its centerline in each direction multiplied by 50 feet, or 250 square feet. Now, if the roof’s total design load is 28 pounds per square foot, then the total load carried by the joist is 250 by 28, or 7,000 pounds. Therefore, each joist will distribute half of this load, or 3,500 pounds, to each of its ends onto its primary support beam or wall.
For a designer to perform an analysis, it is first necessary to collect information on the components that make up the existing roof assembly. These will include:
• The existing waterproofing membrane if it is to remain
• The roof insulation
• The structural decking
• Any suspended loads, such as ceilings and HVAC or electrical equipment.
• Fire sprinkler systems, which could have been added since the building was originally built
McElroy Metal Inc., Bossier City, La., has a “Retrofit Designer’s Guide” available at www. mcelroymetal.com. The guide lists common building material and assembly weights and provides a listing of open-web bar joists with their physical dimensions. This valuable tool helps identify the specific joist types in a building roof. Once identified, you need to determine the load carrying capacity of the existing. To do this, use old load tables or obtain a recent edition of “80 Years of Open Web Steel Joist Construction,” published by the Steel Joist Institute at www. steeljoist.org. For wood and concrete, most designers can determine the load capacities through calculations.
Now, you can see what information has to be collected in order to determine if the existing roof can indeed support the new retrofit roof system. The information is necessary to perform the analysis. If the existing roof cannot support the new retrofit roof, then remedial work and strengthening of the existing roof system will be required. This is why it is highly suggested to perform the analysis prior to finalizing any monetary costs on the project.
Mark James has more than 40 years experience in the retrofit and metal construction markets, working for leading manufacturers and executing great projects. Currently, he is president of RetroSpec LLC, a consulting company offering direction and deep knowledge of the retrofit business for manufacturers and contractors. James can be reached at mark@retrospecllc.com.
