To Attach or Ballast?
The pros and cons of roof-mounted PV installation options
Factor in tax breaks, energy savings, and other local or state-sponsored incentives, and it’s no surprise that companies and buildings with 5,000 square feet or more of low-slope roof space are eager to incorporate solar arrays.
However, there is still some debate as to whether or not a ballasted system or attached system is the best choice. The short answer is that the best choice depends on the specific roof being used. If the roof’s fundamental construction can accommodate easy-to-install attachments, there can be noticeable cost savings for an attached system as opposed to a fully ballasted system. It is important to realize price should never be the only determining factor in an industrial-size solar installation. There are quite a few other factors that should play a role in the decision-making process as well.
CONSIDERATION #1: The gap between roof function and roof construction
For decades, the primary purpose of a roof has been to protect interior structures from exterior elements such as water, weather, and more. Construction methodology has followed this idea and sought to make commercial roofs more reliable and affordable through the use of lightweight materials and minimal structures supporting large spans. The decision to change the main purpose of a roof to also become a support platform for solar equipment means that some new technical aspects must be considered in order to ensure long-term performance of the roof and any existing warranties.
In many cases, only large lightweight roofs are well suited for, or can successfully accept, a fully penetrated (attached) or hybrid (ballasted and attached) system. Depending upon the characteristics of the racking system being used, these options are typically faster to install, and with a lower risk of damage to existing roof structures or membranes. In some states, building regulations require these more robust, traditional roof designs to cope with weather conditions such as hurricanes, seismic activity, or safety concerns – such as for schools or civic buildings.
The National Roofing Contractor Association (NRCA) has been supporting attached mounting systems because it requires a roof and a PV system be designed to work with one another rather than a one-size-fits-all solution. The organization has been offering various solar technical programs, certifications (such as RISE), and various installation recommendations to assist in wise decision making, which is the single-most important factor for successful installations.
CONSIDERATION #2: The age of roofs vs. the age of solar systems
Unless the commercial property is a new build or has completely redone its roof within the past 2-5 years, it is likely the roof will need service or replacement well in advance of any solar array. Most solar systems today have life expectancies between 25-30 years, while roof systems are typically built to satisfy 20-year warranties. The additional cost of 25- to 30-year roof warranties are often unattractive to property owners who first installed roofs for “standard” purposes only. If a roof system cannot be expected to last the life of a solar array, it will require disassembly and re-installation of solar equipment in order to service or replace the roof – thus doubling the expense of the solar installation.
Those companies and property owners wishing to experience an immediate short-term benefit from solar may be better suited for an attached system over a ballasted system for 4 to 5 years, or until their current roof needs replacing. This is due to the expense of removing or uninstalling the roof attachment connections compared to moving thousands of pounds of ballast block only to replace them afterwards. When installed correctly, the attached mount can be re-installed on a new, more appropriate roof base, and should require less maintenance over time as there is no concern about shifting, inadequate ballast, or heavy weather conditions.
CONSIDERATION #3: Optimized total system weight
Because of accepted building codes and commercial clients’ desire for cost efficiencies, the vast majority of commercial roofing systems are built to accept maximum weight loads, creating a structural loading design problem, and requiring total PV system loads to come in less than 4 psf. This fact often designates a fully attached system is more appropriate for a particular commercial property. However, since roof warranties are often voided when surface membranes are deliberately penetrated, attached systems are only applied when approved attachments are applied. At the same time, higher-level wind zones require additional ballast requirements in order to properly anchor the system, and those requirements will eliminate ballasted systems because they will exceed the roof’s total weight threshold. Therefore, attached systems are more effective at lowering the overall structural building load.
Generally speaking, if the roof membrane can accommodate easy-to-install attachments then it truly is the preferred method because it is a secured system, an overall lighter structure, easier to install, and requires less maintenance. Otherwise, a ballasted system is the ideal second choice.
From CEO, Bill Taylor.
North America Clean Energy Magazine: View Article