Flat Roof Mounting Systems
Inside this Article
An increasing number of PV systems are being installed in commercial applications across the US. While many of the available roof areas at the commercial facilities are considered flat, even flat roofs have a minimal slope to them. Often a rise of 0.25 inch over a 12-inch run - referred to as a 1/4:12 pitch - is typical. So these are more properly thought of as low slope applications. While low pitch roofs are very often a dream installation site for many solar integrators, they present a number of design and installation challenges: valleys and ridges for drainage, the drains themselves, vents, exhaust fans, rooftop units, curbs and parapets. All of these have to be taken into account during the design process. In this article I examine the design constraints and considerations for flat roof mounting techniques. I also outline commercially available racking solutions for a variety of roof structures.
Three main options when racking a PV array on a flat roof exist: attached, ballasted and a hybrid option that uses both ballast and structural attachments. The latter is often referred to as a minimally attached system. Each has its own advantages and disadvantages that need to be weighed against each other.
Attached. A structurally attached type of system relies on penetrations in the roof surface and connections to the framing. Several options for fastening the racking system to the building are available. Acceptable methods will be determined by the building construction method and an engineering review. Attachment details may include standoffs welded or screwed in place, curbs integrated into the roofing or steel grids suspended above the roof surface. In some cases direct attachment to steel or concrete deck material is structurally adequate; in other cases attachment to trusses or rafters is required. The advantages of attached racking systems include reduced dead loading to the structure, the ability to engineer specific requirements for live loads, increased options for array tilt and the ability to have an array that is level regardless of roof drainage features.
Ballasted. Ballast mounts rely solely on the weight of the array, racking system and additional material, like concrete pavers, to hold the array to the roof. Their biggest advantage is the lack of roof penetrations. This does not eliminate the need for working with a roofing contractor, but it can significantly reduce the coordination required between the roofer and the PV installer. These arrays can generally be installed while maintaining the roof warranty. But in order to do so, the installation must be coordinated with the original roofer or an approved representative for the roofing manufacturer. Ballasted systems need to be carefully analyzed due to the increased roof loading imposed by the array. Also, many ballasted systems will be limited to a pitch of 20° or less to minimize wind uplift forces.
Hybrid. A minimally attached, or hybrid, system takes advantage of both attached and ballasted features. A hybrid racking system will require a minimum number of penetrations and some level of ballasting. The concept for the hybrid system is rather simple: the fewer penetrations used, the more ballasting required and vice versa. An example of the trade-off between attachment and ballast is published in the UniRac RapidFoot Installation Manual. The RapidFoot attachment foot has a maximum uplift rating of 1,200 pounds. Installing one RapidFoot attachment per 15 modules results in a ballast reduction of 3.84 pounds per square foot, whereas one attachment per every six modules reduces the ballast requirement by 8.70 pounds per square foot. This allows the system designer to optimize the racking system design based on known building factors, such as its load bearing limit and the spacing of its structural support members.
ATTACHED RACKING SYSTEMS
In some cases, attached systems may be the only option available to designers. Compared to ballasted systems, for example, structurally attached mounts result in the least dead load to the building. For projects where minimizing dead loading is a design driver, an attached racking system is ideal. High wind areas may also require an attached mounting solution. Even in moderate wind zones, positive attachment to the building is often required in order to have array tilts in excess of 20°.
Structurally attached racks offer many advantages. On flat roofs in North America, for example, achieving the optimal energy output per array capacity will generally require an attached solution. These racking systems have the additional flexibility of making the array slope and orientation independent of the roof deck. This means the array can be leveled, which is visually appealing. It also means that the racking system is less likely to interfere with the roof ’s ability to shed water or debris. Not only can the array span a ridge or valley along the roof, but also roof drains may be more easily avoided. Roof maintenance, replacement and repair may also be simpler with a structurally attached system. In general, the array angle can be dictated in the initial design. In some cases, tilt angle can be adjusted after installation. Modules can often be arranged in either landscape or portrait orientation with a varying number of modules on a given rack.
Whatever the selection criteria, designers can choose from several structurally attached racking systems.