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One of the most difficult constraints that project developers, system designers and integrators in the solar sector face is whether a roof’s dead-load weight limits can withstand the additional weight of a solar system.

Roofs that have very low dead-load capacities of 4 pounds per square foot (psf) or less can make it difficult for many ballasted rack and module installations to be built. With roofs that have capacities below 3 psf, many installers simply have to walk away from a deal altogether for want of a suitable solution.

This article will examine the root causes of this problem, how it affects the market and deal size, and the impacts of system weight on a building.

The amount of allowable dead load - i.e., the force that is relatively constant over time - on a roof is entirely a function of the building’s design and roof structure. Almost all existing roofs in the U.S. are designed and built to meet existing code at the lowest costs possible, and without any consideration that a solar system might later be installed.

Unfortunately, many of the best roofs for distributed solar generation - large, flat roofs with few obstructions, such as those on distribution centers and big-box retail stores - have very low dead-weight bearing capacities.

Developers working on these projects are often faced with selecting the least unattractive of four options: install a smaller system than the roof would otherwise allow, retrofit the roof’s substructure to accommodate the weight of a system - a very expensive solution that often kills a deal - replace ballast with a dramatically increased quantity of roof penetrations and thus increase the liability and cost of putting holes in the roof, or walk away from the opportunity.

A developer or integrator often will not know the dead-load capacity of a roof until well into the construction phase of a project. It is often not possible to know the capacity until a full structural analysis is prepared by a structural engineer - a costly and time-consuming process that is not typically included in the bid and proposal phase of a project.

Consequently, projects are won based on assumed load limits to be verified when the project is awarded. The developer and integrator face the risk that, after the structural study is conducted, the project may be affected by dead-load roof constraints that either increase costs or kill the project altogether. In either case, weight constraints burden the project with undesirable risks.

In addition to limiting the total addressable market of commercial roofs that are viable for solar, weight constraints can limit the size of the installation, as dead-load capacity can be unevenly distributed within the same roof.

The result is a much smaller system than would otherwise be deployed if weight were not a constraint. Another effect is higher overall costs because the same costs for selling, general expense and administrative and installation mobilization costs would be amortized against a smaller system. Downstream impacts include difficulty establishing financial backing, tighter profit margins for all project participants and reduced energy production for the end customer.

In addition to immediate weight constraints, another weight-related set of concerns involves the long-term impact of the system on the roof over the 25-year-plus life of the PV system. Remember, building owners - as the system’s host - want low-cost electricity generated by solar with no negative impact on their facility. A heavy system (more than 5 psf) can cause problems, even if it is within the allowable dead-load capacity of a given roof.

Heavier systems can lead to trouble over the long term, particularly if their designs cause high points (feet) or edge loads. In other words, even if an array’s average distributed weight across the roof is below 4 psf, the psf applied by each racking foot or edge where the racking system comes into contact with the roof can be many times heavier.

Even if a recommended slip sheet is used, the weight of these edges digging into fragile roof membranes can cause abrasions over time, compression of insulation, and premature roof wear on the roofing membranes most commonly used on large, flat roofs.

The result is increased roof maintenance and, in the worst cases, leaks and severe damage. None of these issues are desirable outcomes for the end customer, and they are often not captured in the operation and maintenance expenditures of the typical power purchase agreement financial analysis.

In addition, maxing out the available dead-load capacity of a building limits the ability of the current or future tenants - or owners - to place additional equipment (e.g., AC equipment, chillers, ventilation equipment) on the roof, should the building need to be retrofitted for another use.

This limitation represents a significant opportunity cost, particularly for end customers that own a number of properties and are renting to tenants, such as real-estate investment trusts.

Given that dozens of racking companies are selling solutions into today’s solar market, there are a few product characteristics that a developer or integrator can use to quickly identify which systems are likely to meet weight needs.

The first and most obvious characteristic is to analyze the weight of the racking system itself. What materials are being used: steel, aluminum or plastic? If it is the latter, is that material proven to be durable over 25 years? Does the system require many accessory pieces that will drive up the overall weight of the system? If the solution requires modules that have a frame, it can add more than 5 lbs. per module.

Although the material weight of the rack is important, what is more important is the overall system weight as determined by how much ballast (or weight-offsetting penetrations) is required before the system meets all applicable codes and standards.

A number of physical characteristics affect these calculations. Systems that have a low profile relative to the roof deck and have wind deflectors that prevent uplift have better aerodynamics than products that are several inches off the deck. In addition, products with higher tilt angles (above 10 degrees) require more ballast to reduce both the lift and drag of their higher angle.

All of these factors drive the ballast calculator that determines the weight required for a given system. This calculator is proprietary to each racking product and should use unique coefficients of friction and lift derived from a wind tunnel study performed by a third-party wind tunnel testing house. The calculator should be developed in partnership with a structural engineer who can verify that the coefficients are properly applied.

Although there are many wind tunnel testing facilities in North America, developers and integrators should also be sure that the products they are using were tested by a reputable firm with a deep background in solar rooftop mounting systems.

Using an ultra-light racking and module solution is an ideal way to reduce the risks and avoid difficulties posed by weight-constrained roofs and excess weight on roofs. Opting for a lightweight system that is specially engineered to reduce the amount of ballast and/or number of penetrations needed reduces the potential need for costly rework after the system is installed, as well as the risks discussed earlier with excess weight on roofs.

There are also now commercially available solar systems that are designed to reduce point and edge loads to the lowest possible levels by distributing system weight over the entire area of the rack, rather than single points (or legs).

In addition to a physically light product with aerodynamic characteristics and a properly developed ballast calculator tool, the final piece of the puzzle for selecting a truly lightweight system is working with a supplier that is able to skillfully apply the product and its engineering calculations to a given roof.

For instance, roof wind analysis typically calls for heavier weight in the north corners and southern rows, and along the edges of a given array. The larger the array blocks, the lighter the system due to load sharing between modules/racks.

Therefore, for a truly lightweight install, the racking manufacturer should provide layouts that minimize ballast load by maximizing array size, reducing north and south edges (no saw teeth), avoiding north corners, and eliminating isolated modules and unnecessary peninsulas. S

 

John Rethans is the director of product management at SOLON Corp., located in Tucson, Ariz. Rethans led the team that launched SOLON’s SOLquick product for commercial rooftops in late 2011. SOLON can be contacted via Patricia Browne at (520) 647-8754 or info@solon.com.