A structural roof evaluation determines whether an existing roof structure can safely support new weight. This process comes up whenever a building owner wants to add HVAC equipment, solar panels, a green roof, a rooftop terrace, or additional stories. Our structural engineering team performs roof load evaluations for projects across all 50 states and delivers clear findings that owners, architects, and contractors can act on.
What Triggers a Roof Load Evaluation
The most common trigger is adding mechanical equipment. A new rooftop HVAC unit, cooling tower, or telecommunications equipment adds dead load to the roof structure. If the original design did not account for this equipment, the structural engineer must verify the existing members can carry the additional weight without overstress or excessive deflection.
Solar panel installations trigger evaluations for the same reason. A typical photovoltaic array adds 3 to 5 pounds per square foot of dead load. On a large commercial roof, that can total tens of thousands of pounds the original structure may not have been designed to support. Ballasted solar systems, which use concrete blocks instead of roof penetrations, add even more dead load per square foot.
Green roofs, rooftop terraces, and any change from a traditional low-slope roofing membrane to a heavier assembly require evaluation. Saturated growing media in a green roof can weigh 50 to 150 pounds per square foot depending on the system and depth of media. This load often exceeds what most commercial roof structures were designed to carry, and significant structural reinforcement is commonly required.
What the Engineer Reviews and Measures
The evaluation starts with existing structural drawings if they are available. The engineer reviews the original design loads, structural member sizes, and connection details to understand what the structure was designed to carry. When drawings are unavailable, which is common in older buildings, we perform field verification to measure member sizes, span lengths, and connection hardware. This is one of the specialty structure services we provide when documentation gaps exist.
With existing member sizes and spans confirmed, the engineer runs structural calculations to determine the current capacity of the roof framing, accounting for the original dead and live loads plus the proposed new load. The calculation checks for bending stress, shear stress, and deflection against code-allowable limits from ASCE 7 and the applicable building code.
The engineer also reviews the existing condition of the roof structure when possible. Corrosion, wood decay, pest damage, or prior unauthorized modifications can reduce member capacity below what the original drawings show. Any observed deficiencies are documented in the findings and addressed in the recommendations.
Understanding the Load Categories
Dead Load
Dead load is the permanent weight of the building components: the roof deck, insulation, roofing membrane, and any permanently installed equipment. The structural engineer adds the proposed new equipment weight to the existing dead load to determine the total dead load demand. Dead load acts continuously and must be supported at all times.
Live Load
Live load on a roof is the temporary load from workers, equipment, and materials during maintenance and construction activities. Code-required minimum roof live loads typically range from 12 to 20 pounds per square foot for accessible roofs. If the proposed rooftop terrace will be used by occupants, the live load requirement increases substantially to match the applicable occupancy category.
Snow and Wind Loads
In climates with significant snowfall, snow load is often the controlling load on a roof structure. New obstructions on the roof, such as rooftop equipment screens or solar racking, can cause snow drift accumulation on the downwind side. The structural engineer must account for this drift load, which can significantly exceed the ground snow load at the drift location. Our structural engineers apply the ASCE 7 drift calculation procedure to any new roof obstruction.
What the Findings Report Includes
The roof load evaluation report documents the existing structural system, the proposed loading, the calculations performed, and the conclusions. If the structure has adequate capacity, the report provides that confirmation with a PE stamp. If the structure lacks capacity for the proposed load, the report identifies which members are deficient and by how much, giving the design team the information needed to develop a reinforcement scheme. Learn more about our full range of structural services.
Reinforcement options depend on the deficiency. Steel plates welded or bolted to steel members, sister framing alongside existing wood joists, and supplemental columns installed below overloaded beams are common approaches. In some cases, the most cost-effective solution is redesigning the proposed installation to reduce the added load rather than reinforcing the existing structure.
How to Prepare for a Roof Load Evaluation
Gather any existing drawings for the building, particularly structural drawings and the original building permit set. Provide the weight of the proposed equipment, including weight per square foot for distributed loads or point loads for equipment that will be mounted on discrete supports. Our team can work from manufacturer cut sheets and equipment submittals. Submit your project at rightangleeng.com/get-started to begin.
Frequently Asked Questions
Is a structural engineer required for all rooftop equipment additions?
Most jurisdictions require a structural engineering review for any equipment addition that adds significant weight to the roof or requires penetrations into the structural deck. The specific threshold varies by jurisdiction, but the cost of a roof load evaluation is small compared to the cost of a structural failure or a failed permit inspection.
How long does a roof load evaluation take?
For a straightforward project with existing drawings available, the evaluation and report typically take three to five business days. Projects requiring field verification of unknown member sizes take longer because the field work must be coordinated with the building owner. Complex multi-load scenarios may require additional time for the calculation work.
What happens if the roof cannot support the proposed load?
The engineer provides the deficiency findings and typically discusses reinforcement options. The owner and design team then decide whether to reinforce the structure, reduce the proposed load, or relocate the equipment to an area of the roof with more capacity. In some cases, a lightweight alternative product can achieve the same function at a fraction of the weight.
Ready for Your Roof Load Evaluation?
We deliver structural roof evaluations with PE-stamped findings for projects in all 50 states. Submit your project at rightangleeng.com/get-started to get started.
References
American Society of Civil Engineers. (2022). Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-22). Reston, VA.
International Building Code. (2021). Chapter 16: Structural Design. International Code Council.
American Institute of Steel Construction. (2016). Steel Construction Manual, 15th Edition. Chicago, IL.