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It is no secret that industrial roofs have become one of the most widely used sites for large photovoltaic solar systems, and for obvious reasons: The availability of space coupled with practicality make it an ideal habitat for solar energy production.

Many of the nation’s largest corporations have found solar to be a worthwhile investment for their facilities. On-site energy production offsets operating costs and enables companies to become less dependent on local utilities. And it certainly does not hurt the company’s image. Then why are more buildings not being outfitted with PV solar arrays? There is no shortage of space or use for the energy.

Most manufacturing, distribution and retail facilities are designed as “big-box” steel-frame buildings. These structural systems are optimized and have little if any extra capacity, not even to squeeze in a few pounds per square foot (PSF) for an ultra-light array. Oftentimes, especially with older buildings, the roof is not up to code and would need updates even without a solar installation. It is easy to give up on a solar roof project when the building cannot support it.

Though not uncommon, it is risky for an installer and property owner to simply assume a roof is strong enough for a project. These expansive roofs are designed for maximum efficiency, and the 3-8 PSF associated with a solar system can easily overstress the roof structure. For installers and owners brave enough to build without consulting an engineer, there are some serious legal implications if anything were to go wrong.

Fortunately, nothing catastrophic has occurred so far as a result of a solar installation, but the industry should continue to make improvements. The due diligence of any solar roof project should include a structural roof assessment. Otherwise, the installer and owner are taking on substantial unknown risk.

 

Weight concerns

The weight of a solar system may be more significant than it seems. For instance, the total design load for a roof structure can be as low as 25 PSF, depending on how much it snows in the region. Add 5 PSF for a solar array, and the design load increases by 20%, which is significant. In fact, it is very unlikely that the roof structure was designed with that kind of excess capacity in mind.

This is not to say that a project should come to a halt if the roof is not up to snuff. Structural upgrades are common when it comes to big-box industrial buildings. Anytime a new mechanical unit, piping, conveyors or any other piece of equipment is placed on the roof, the structure needs to be assessed for the proposed loads and strengthened as necessary. The same is true for a solar system. Bringing the right people together to find a well-rounded solution results in a better design, better investment and safer building.

The good news is that these buildings can be reinforced to support just about anything, especially the light loads associated with solar systems. The truth is that building modifications can open the door for a solar project and allow greater flexibility. The building is modified to suit the system, rather than the system being selected for the building. Structural engineers and mounting-system designers can work together to produce the most productive, easiest-to-construct, cost-effective solar system possible, even with upgrades. Building modifications should not be the hurdle that stops a project. When compared to the cost of a large solar project, building upgrade costs can be a drop in the bucket.

The first step in deciding whether or not a building is suitable for solar is to consult a structural engineer, preferably one that can determine the actual as-built roof capacity along with the cost of upgrades, if necessary. If the solar system has already been selected, then the project will either get the go-ahead or need upgrades. A better approach is to have the structural assessment done on the front end so more possibilities can be explored. An approach where the solar panels and panel infrastructure are designed together maximizes the system’s effectiveness. A balance among the panel loads, building upgrades and mounting system result in getting the most use out of the space at the lowest cost.

 

Roof systems

To understand structural reinforcement as it pertains to industrial roofs, it is important to know a little about their structural system. For the typical big-box industrial building, the roof deck is supported by roof joists, which rest on open web girders or beams that transfer the load to the columns and foundation. In the case of adding, for instance, 5 PSF, the roof joists are the most likely member to need upgrades, followed by girders. Columns and foundations are less likely to need any upgrades because the added load is often a smaller percentage of their total capacity.

Roof joists are designed on a tight margin with little room for increased load, and they often need upgrades. Reinforcement is designed to strengthen the joists for the precise loads. Steel is welded to the existing members to beef them up so that the trusses can support the extra weight. Sometimes the addition of as few as two steel bars, strategically placed, can add the desired strength to a joist.

The roof joists - and, if necessary, the girders - are accessed from inside the building from lifts, and steel is welded in place to strengthen the members. Engineers and erectors who specialize in retrofitting and reinforcing these buildings can determine not only the building’s capacity, but the magnitude of the upgrades.

In terms of solar mounting systems, the International Code Council’s “Acceptance Criteria for Modular Framing Systems Used to Support Photovoltaic Panels” is the main publication outlining requirements. But nowhere in this publication is building structure itself mentioned. More attention needs to be devoted to the structure that ultimately supports the panels.

There is a disconnect between the mounting manufacturers, who are connected to the panel manufacturers, and the building engineers. The mounting systems are designed to support a specific panel, and the building itself is often not considered. The main goal for mounting-system manufacturers is to design a system that is quick and easy to install that cuts down on material and labor. In the case of industrial roofs, the system is normally designed to distribute the load on all the joists so that they all share the load.

There are a number of ways to accomplish this goal, and the best way to find a solution is consulting and connecting the right people and knowledge. Installations are often being carried out without exploring all the options. As the industry grows and makes use of available roof space, there will need to be greater interaction between building engineers and installers to make a safe, sound investment.

For instance, atop the Toys-R-Us distribution center in Flanders, N.J., sits one of nation’s largest photovoltaic solar arrays. The 5.38 MW system provides roughly 70% of the energy needs for the toy giant’s largest distribution center. The 37,000 ultra-lightweight solar panels produce 6.36 million kWh of electricity annually.

In order to carry out this massive undertaking, Toys-R-Us had to address some structural inadequacies before the project took off. The existing roof structure was inadequate to safely support the load from the proposed system, so a structural engineer was consulted to solve the problem. The building’s roof structure - specifically the roof joists - needed to be reinforced.

The process took place in the factory around conveyors, racks and all the existing functions of a distribution center. By coordinating with in-house personnel, the roof was ready to accommodate the solar installation in less than a week. In terms of cost, it was approximately 4 cents per square foot to analyze and upgrade the 1.5 million square-foot building.

The solar energy sector stands to gain a great deal from the optimization of solar panel structural systems as they pertain to big-box industrial buildings. The amount of space and energy production associated with these buildings makes them a prime spot for solar installations.

But we have some more work to do. Installers and owners need to be aware of these buildings’ structural systems and how they can be modified and used to support solar arrays. A balanced approach conceived by building engineers and mounting-system designers will help open the door for many solar projects that were previously scrapped because of assumed rooftop structural inadequacies. S

 

Philip Hodge is president of Tennessee-based Habco Erectors Inc., and Bryan Rainey is an engineer-in-training with the company. They can be reached at (423) 365-4684.