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In this two-part series, I delved into the various roof membranes and roofing systems typically found in commercial and industrial (C&I) buildings. Identifying the roof correctly is crucial since this determines other aspects of design and engineering in solar installations mounted on rooftops. Although these subsequent considerations come second to roof identification, they remain vital. After all, these structural, waterproofing, and balance-of-system (BOS) factors ensure that rooftop photovoltaic (PV) systems don’t get blown away or inadvertently cause roofs to collapse or leak.
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### Structural Considerations
Arguably, the most pivotal aspect of a C&I rooftop solar project is the structural assessment and engineering. Below is a concise overview of key areas of focus for structural evaluations and designs:
#### Reserve Capacity Evaluation
The structural components of existing C&I buildings are generally designed to meet the minimum load requirements set by relevant codes. Exceeding this structural capacity could lead to catastrophic outcomes. Thus, it's essential for a structural engineer to review and assess the current structure's envelope to ascertain if there’s enough reserve capacity to support a rooftop solar array.
To achieve this, the engineer will either examine the as-built drawings, if available, or conduct a site visit to inspect the actual conditions. This structural evaluation must take into account the additional gravitational and lateral loads imposed by the new solar equipment and compare these against the existing building’s capacity for gravity and lateral loads. The engineer should also factor in any modifications made during the building's lifetime that weren't accounted for in the initial design, like the addition of rooftop units or extra roofing layers.
In certain scenarios, the existing building may already have sufficient reserve capacity for the solar setup. In others, the building might require targeted structural reinforcements to handle these added loads. Hence, the structural engineer needs extensive experience in this area to devise the most efficient and cost-effective solutions.
#### Mechanical Attachments
While many low-slope C&I rooftop solar projects use ballast to counteract wind uplift, some projects or regions necessitate mechanical attachments. For instance, in a ballasted system, connecting the solar equipment directly to the roof structure can help reduce dead loads to fit within constrained load-bearing capacities.
A structural engineering plan doesn’t just specify the number and placement of attachments but also outlines hardware requirements. To maximize effectiveness, mechanical attachment details must consider the specific construction of the roof. Some roofing systems feature a thick layer of insulation between the roof deck and the membrane. Removing and replacing this insulation with wooden blocks can help minimize torsional deflection.
Various mechanical attachment techniques cater to different roofing systems. Common hardware includes U-bolts, OMG Power Grips (as shown below), and S-5! clamps. When using S-5! clamps on standing-seam metal roofs, it’s crucial to pay attention to the hardware used to attach the roof panels to the structure—a detail that isn’t always straightforward to inspect or confirm.
#### Roof Loading Plan
Once installed, the weight of a rooftop solar array is distributed fairly evenly across the roof. However, during the installation phase, the concentrated dead loads of PV modules, inverters, mounting systems, and ballast can pose significant risks.
A roof loading plan ensures that pallets of building materials are strategically placed over structural supports like columns, beams, or walls that can temporarily bear these concentrated loads. Besides identifying optimal locations for staging equipment, the plan also specifies how many pallets or what weight each location can safely support.
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### Waterproofing Considerations
While structural attachments help ensure that rooftop equipment withstands wind uplift and other environmental stresses, every roof penetration presents an opportunity for water ingress. Additionally, rooftop solar must be installed in a manner that preserves the integrity of the roof membrane and doesn’t disrupt drainage.
*Courtesy of OMG Roofing Products*
#### Roof Flashing and Drainage
Flashing involves using roof-compatible, waterproof materials to prevent water from seeping through at penetrations, joints, and intersections. Effective flashing requires collaboration between the solar EPC and a certified roofing professional. Partnering with a roofer ensures compatibility of materials and proper installation, potentially preserving any existing roof warranty.
Despite their name, low-slope roofs aren’t truly flat—they include features like shallow slopes, crickets, drains, and scuppers to keep water out. Retrofitting solar without disrupting these drainage mechanisms is essential. Ponding water not only adds unnecessary weight but also accelerates roof deterioration. It’s also linked to certain field failures in PV systems.
#### Preventing Roof Damage
Single-ply roof membranes are vulnerable to physical harm during and post-installation. Foot traffic and dropped tools can damage the membrane. Over time, environmental factors affecting the mounting system—like wind forces and temperature fluctuations—can also compromise the membrane.
Installers can mitigate these risks by using temporary protections for walkways and loading zones. Permanent solutions often involve roof-friendly slip sheets or separator pads acting as sacrificial barriers between the mounting system and the membrane. Additional permanent walkways may be necessary to accommodate increased foot traffic over the lifespan of the PV system.
If you’re seeking strategies to de-risk C&I rooftop PV systems while enhancing performance, contact Pure Power Engineering to explore our value-engineered design and construction drawing services. Our team specializes in tailoring solutions that balance safety, efficiency, and long-term reliability.
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By addressing both structural and waterproofing concerns thoughtfully, we can ensure that rooftop solar installations not only perform well but also endure the test of time without compromising the integrity of the roof.