Flat Roof Mounting Systems Issue Solutions for the Wide Open Commercial Landscape
Level Roof Mounting Systems Answers for the Wide Open Commercial Landscape
An expanding number of PV frameworks are being introduced in business applications over the US. While a significant number of the accessible rooftop territories at the business offices are viewed as level, even level rooftops have an insignificant slant to them. Regularly an ascent of 0.25 inch over a 12-inch run – alluded to as a 1/4:12 pitch – is run of the mill. So these are all the more appropriately thought of as low slant applications. While low pitch rooftops are frequently a fantasy establishment site for some sun based integrators, they present various plan and establishment challenges: valleys and edges for waste, the channels themselves, vents, exhaust fans, housetop units, checks and parapets. These must be considered amid the plan procedure. In this article I look at the plan limitations and contemplations for level rooftop mounting methods. I additionally plot financially accessible racking answers for an assortment of rooftop structures.
Three primary choices while racking a PV exhibit on a level rooftop exist: connected, ballasted and a cross breed choice that utilizes both counterweight and basic connections. The last is frequently alluded to as an insignificantly connected framework. Every ha its very own preferences and disservices that should be weighed against one another.
Appended. A fundamentally appended sort of framework depends on entrances in the rooftop surface and associations with the surrounding. A few alternatives for attaching the racking framework to the building are accessible. Worthy strategies will be controlled by the building development strategy and a designing survey. Connection subtleties may incorporate standoffs welded or tightened put, controls coordinated into the material or steel matrices suspended over the rooftop surface. Now and again guide connection to steel or solid deck material is basically satisfactory; in different cases connection to supports or rafters is required. The benefits of joined racking frameworks incorporate decreased dead stacking to the structure, the capacity to build explicit necessities for live loads, expanded alternatives for cluster tilt and the capacity to have an exhibit that is level paying little mind to rooftop waste highlights.
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 solar 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.
DPW Solar. POWER-FAB Roof/Ground Mounts are available in a variety of configurations, manufactured by Direct Power and Water. As the name suggests, this is a mounting structure appropriate for use on the ground or on the roof. In the latter case, the roof itself serves as the foundation. The rack utilizes structural aluminum angle and stainless steel hardware. Attachment options are L-feet, ideally used in conjunction with flashed stanchions. The product line offers one-piece or telescoping legs. A 20° to 65° tilt angle is standard.
Professional Solar Products (ProSolar). ProSolar manufactures SolarWedge and Solar Wedge XD racking systems. Both systems are designed for low tilt angle applications on flat roofs. The Solar Wedge XD utilizes a deeper rail and allows for attachment spans up to 8 feet. The footings are designed to work in conjunction with the ProSolar support rail and offer integrated leveling hardware. Array angle options are 5°, 10° or 15°. Components are stainless steel and aluminum.
RoofScreen. The Solar Racking System from RoofScreen is a modular, commercial racking system designed to minimize roof penetrations. Each system is custom designed and engineered per the specific job requirements and comes with a certified plan set. Allied Tube and Conduit, a division of Tyco, supplies specially galvanized steel tubes per the system design. Footings consist of a base support in varying lengths for different insulation, a flashing and base assembly.
Schletter. The Flat-Roof System from Schletter accommodates a wide variety of applications, including structural attachment. Connected options from Schletter include the FR-CompactVario and the FR-Connect-DT. Attachment options vary by roof type. Schletter offers a variety of tilt support legs depending on the module used, as well as different support racks for varying lengths between roof penetrations. Design support includes the AutoCalculator configuration tool and the ShadeCalculator for determining inter-row spacing.
UniRac. UniRac offers two structurally attached mounting solutions. The first is a tilt-up version of its SolarMount rail system, with high or low profile tilt angle options. Adjustable legs are also available. Manufacturer supplied attachments include standoffs and the FastFoot, which attaches to concrete, metal or wooden decks. The second structurally attached racking option is the UniRac Large Array (U-LA) system. The U-LA mounting structure is intended for use in a ground mount application or on rooftops. U-LA components, in combination with steel pipe supplied by the contractor, form a truss-like support structure. UniRac SolarMount rails are used to mount the PV modules. Designed to accommodate a wide range of applications, this is a highly customizable mounting solution, one that can be engineered to span long distances or to withstand strong winds.
BALLASTED AND HYBRID RACKING SYSTEMS
The main advantage to ballasted racking systems is the reduction or elimination of roof penetrations. But there are other advantages as well. These systems often result in decreased labor costs, as they are generally faster to install. The total labor savings may even exceed the increased material cost for the racking system. These are low profile, low tilt angle mounting solutions. For this reason they are seldom visible from the ground, especially where a parapet wall exists. Where a 100% ballasted solution is not possible or desirable, many ballasted racking manufacturers provide an attachment detail designed for use with their product. As the number of anchors is increased, the amount of ballast required for these systems is reduced.
While these systems are customizable and convenient, they are not suitable for every application. The roof structure may not support the additional dead load. Ballasted racking systems also have their own inherent limitations as specified by the manufacturer. There are basic wind speed restrictions, for example, seismic rating limits and limits on the site’s exposure category. In addition to these engineering limitations, there are also certain site conditions that the ballasted racking manufacturer may not support. Common site restrictions may include edge and corner of roof setback requirements, maximum allowable roof slope and a maximum building height of 60 feet.
In some cases there are array shape restrictions specified by the manufacturer. Unauthorized layouts and even unapproved layout changes may void the racking manufacturer’s warranty. Fortunately, many manufacturers have configuration tools or provide design support to the installer. In many cases, the location of ballast materials varies from the edges to the middle of the racking system, for example, so it is very important to follow any plans provided by the manufacturer and all design guidelines and installation instructions.
A brief survey of the manufacturers of ballasted or minimally attached racking solutions follows. These are aluminum products with stainless steel hardware that are suitable for commercial flat roof applications of varying scale. Most warranties are for 10 years, but finish warranties may be briefer.
DPW Solar. Direct Power and Water manufactures two fully ballasted in place racking products. The POWER-FAB Ballasted Roof Mount is a nonpenetrating family of products most appropriate for small array configurations. This product line can accommodate tilt angles up to 45° and is designed for 90 mph maximum wind speeds and exposure category C. The bottom of the rack is covered with EPDM rubber to increase friction and protect the roof. The POWERFAB Power Tube CRS is a low profile racking system specifically designed for commercial installations. The Power Tube CRS will accommodate a design wind speed of 125 mph at a 5° tilt and a wind speed of 90 mph at a 10° tilt. At either tilt angle, it is designed for exposure category C and seismic zone 4. Each north-south Power Tube rail is installed with an EPDM strip on its bottom. These UV resistant pads protect the roof and increase the system’s coefficient of friction. EPDM pads are also adhered to the bottom surface of the ballast trays.
Krannich Solar. The fully ballasted K2 mounting system for flat roofs available from Krannich Solar is typically configured for array tilt angles of 10°, 15° or 20°. Higher tilt angles are an option if the roof will support the load. The basic design is a mounting triangle that sits atop flat rails that contain the required ballast materials. Few tools are required for assembly. The hardware provided, such as T-head bolts, simplifies installation.
PanelClaw. The Polar Bear is the first commercial flat roof mounting system available from PanelClaw. The Panel- Claw hardware, from which the company takes its name, grabs the inside of the module frame at four corners. Modules tilt up in the support base for easy installation and maintenance. This system utilizes a nonrail based architecture with a low component count. A rear wind deflector is utilized. The Polar Bear racking system is acceptable for wind exposure categories B, C, and, upon request, D. It is compatible with EPDM, TPM, PVC and tar and gravel roofs. A roof connector option is available for minimally attached designs. The array tilt varies depending upon module width, but a 10° to 15° range is typical.
Schuco. The SolarEZ Flatroof Mounting System is a ballasted commercial racking system from Schuco. It will accommodate five array tilt angles ranging from 7.5° to 37.5°. The system is designed to accommodate snow loads up to 30 pounds per square foot. It can also accommodate wind speeds up to 120 mph. The system can be designed for exposure categories B, C or D. The installation manual explains how to calculate ballast requirements. Structural attachment to the roof can also be accommodated.
Schletter. The FR-Windsafe is the ballasted or minimally attached commercial flat roof racking system available from Schletter. The system includes a rear deflector to minimize wind uplift loads. A variety of attachment options are available, depending upon roof type. The configuration tools previously mentioned help with array layout and schematic drawings.
SunLink Corporation. The SunLink commercial flat roof mounting system is a non- or minimal-penetrating system that utilizes a unique panelized architecture. Once installed, these panels of modules tilt up, making array wiring or roof access and maintenance easier. Flashable posts with a stainless steel clamp arm are used for making mechanical attachments. Wind deflectors are installed on the north end of the array. Array tilt angles of 5°, 10°, 15° or 20° are possible. Design support and as-built documents are provided by the manufacturer, as is a 15-year standard product warranty.
UniRac. The RapidRac G10 from UniRac is a low parts count, ballasted mounting system for commercial flat roofs. The product uses a fixed array tilt angle of 10°. It offers integrated WEEB grounding and supports the use of PV laminates. Support includes an online estimator and configuration tool. Code compliance documentation is included with the installation manual. With many RapidFoot attachments, the RapidRac G10 can be installed as light as 2 pounds per square foot; it can also be installed as a fully ballasted, zero penetration system with a higher roof loading.
Proprietary products. Not all ballasted, commercial flat roof racking systems are available through direct sales or distribution. In some cases integrators are supported directly by module manufacturers with proprietary racking solutions. The Schott Solar FS racking system is a notable example. Schott’s FS racking system is primarily used with the company’s ASE-300 series of modules and is available through direct sales. Commercial racking products developed by PowerLight, like PowerGuard roof tiles, are likewise available to SunPower commercial dealers. These products are notable in part because they have been deployed in the field for a relatively long period of time.
When beginning the design process, the goal of the PV system should be clearly identified. Does the client need a minimum number of kilowatt- hours generated, as is the case in many LEED projects? Has a certain system size in kW been established, as is common in RFP situations? Or is the client looking for the biggest system possible? Aesthetic issues, such as street visibility, may influence product selection. Structural issues may also dictate certain approaches. This would be the case if the roof could not structurally support a ballasted mounting system. Very often a combination of design considerations will need to be taken into account.
As a PV designer or integrator, your input into these design drivers will help guide the client to an optimal system. In a situation where the energy yield needs to be maximized in a limited space, for example, thin film products may be eliminated. There may be applications where using a product integrated into the roofing membrane is the most advantageous solution (see sidebar). Also, knowledge of the local building and planning department requirements will help establish some basic system guidelines.
Familiarity with the local planning department and its requirements can save a number of hassles and redesigns up front. Some municipalities require that all rooftop equipment not be visible from street level. While traditional rooftop units (RTUs), such as HVAC systems, can be placed behind roof screens, generally PV arrays cannot be hidden in this manner. Therefore, the PV array may be required to be lower than any parapet wall in order to properly hide it. The inverter and associated equipment may also be of concern to the planning department. Careful consideration, proper documentation and clear communication will be required if any of this equipment is to be placed outside and in public view.
Beyond customer requirements and those of the local AHJ, the following are important design considerations for any PV installation on a flat roof.
Roof type. There are a number of common flat roof assemblies that installers may encounter on commercial facilities, such as built up roofing (BUR), modified bitumen (MB) roof covering and multiple membrane roofing systems.
BUR is one of the most common types of low slope roofing methods. BUR consists of two or more layers of felt reinforcing piles topped with a cap sheet or aggregate, very often asphalt. Another method is MB, which consists of one or more layers of polymer-modified asphalt sheets. The sheets are either adhered or mechanically attached to the substrate or held in place with a ballast layer.
Another popular roofing method is using a membrane such as synthetic rubber like EDPM or a thermoplastic like thermoplastic polyolefin (TPO) or polyvinyl chloride (PVC). Each type can be used in low slope applications and has similar qualities. When installing PV on these roof surfaces, it is imperative to work in conjunction with a properly licensed roofing contractor to maintain the roof ’s warranty.
Before specifying a PV system on any of these roof types, be sure that you have selected a racking solution that is compatible with the roof itself. With ballasted systems, a slip sheet is often used between the roof and the racking system. It is important to make sure that the slip sheet material is compatible with the roofing. Verifying compatibility may mean consulting the roof manufacturer. This is especially true with membrane roofs.
PV Integrated into Roof Membrane
With the increased use of membrane roofs such as vinyl and TPO, PV integrators, building owners and architects are eager to incorporate solar electric systems into these roofs. Through the use of BIPV modules, these roofs can have minimal penetrations or roof loading considerations. Typically, thin film technologies – amorphous silicon (aSi) in particular – are used in these applications, although there are crystalline modules for use in BIPV arrays on flat roofs.
Some of the immediate benefits of such systems include integrating the PV array into the roof and thereby eliminating the potential damage associated with adding an array onto a membrane roof. The structural element is nearly eliminated due to the low additional weight added to the roof surface. Depending on the shading characteristics of the roof, a greater portion of the roof may be available for installing PV as the wind loading issues of the roof edge will be minimized.
The first trade-off that comes to most installers’ minds is the reduced energy output due to the flat array mounting. Depending on geographic location, this can have a significant impact when compared to an array mounted at even a slight angle, like 5° or 10°. There is also the concern about high cell temperatures due to a lack of airflow around the cells. Another trade-off is the reduced output due to dirt and debris collecting on the array. The fact that aSi products tend to tolerate heat and shading better than crystalline products may alleviate some of these concerns.
Coordination between roofing contractor and electrical contractor will be even more important when the PV array is integrated into the roof. While BIPV arrays are becoming more popular, they are not as common as traditional, framed module installations. With a framed module installation, the footings and potentially racking systems can be handled directly by the roofing or construction contractor with limited input from the electrician. With BIPV products, both parties will need to be in constant communication— possibly working together directly—to verify proper installation techniques.
Manufacturers of BIPV products for flat roof applications include the following.
Open Energy. Located in Grass Valley, California, Open Energy manufactures a BIPV product that uses crystalline cells encapsulated into a PV membrane. The modules utilize Multi Contact connectors located on the top of the module. This allows for the wiring connections to be made on the roof surface, minimizing roof penetrations. Modules listed at 450 W with a relatively large footprint (48 inches x 96 inches) still maintain a distributed weight less than 1.7 pounds per square foot.
Roof condition. Naturally, PV systems, especially ones ballasted in place, should be installed only on roofs that are in good condition. This means verifying the age of the roof and the remaining length of any warranties. It also means surveying the roof for stress, damage or other existing problems. Note if and where water is ponding, for example, taking notes and pictures. Also note the condition of edge and flashing details. These notes and photographs may prove useful later.
Another important consideration is the roof wear and tear resulting from the actual PV installation. Cardboard or slip sheet material can be used to minimize wear in high traffic areas. Yellow Spaghetti Roof Walkway Rolls are commonly used to protect a flat roof during the installation of a PV system. Whenever drilling or cutting, be sure to collect the metal shavings. Finally, never drag components into place on any roof. Always lift and place components down in an effort to preserve the roof ’s integrity.
Keep in mind that even if the roof manufacturer gives an application a thumbs-up, not all racking manufacturers support their products on every roofing system. Be sure to read the manufacturer’s installation instructions and understand any application-specific restrictions relevant to your project. This is especially true when specifying or deploying a ballasted racking system, which has a greater impact on the roofing membrane or the roofing system than an attached system.
PV integrators will also want to contract with either the original roof installer or another manufacturer-approved roofing contractor. Roofing contractors will verify that the roof is in good condition before the project is started; after the PV installation is finished, they will complete a follow-up inspection of the roof. Request a written report for both visits. This subcontractor will also seal requisite penetrations for attached racking systems or patch any accidental membrane penetrations. To best prepare for the unexpected, always have the roofing contractor provide PV installation technicians with a suitable sealant product and instructions for temporarily patching the roofing material. Workers should mark any temporary patches and track these locations so that they are easily located later. This will allow the roofing contractor to make all repairs to the roof in a single site visit at the project’s completion.
Low angle vs. high angle. If the decision was made to install framed modules at an angle off of the roof surface, one of the first design considerations is to determine the right angle to mount the array. To maximize the PV array’s energy output, the optimal angle – generally close to the site’s latitude – may require significant distances between rows to eliminate inter-row shading. (See “Calculating Inter-Row Spacing,” December/January 2009, SolarPro magazine.) In areas with substantial snowfall, an increased tilt angle may be required in order to effectively shed the snow off the array.
Structurally, the biggest trade-off for an increased tilt angle is the increased wind loading that the array will experience. There can be significant uplift forces imposed on the array depending on the array location and wind exposure category. If the PV racking is ballasted, this results in a greater dead load on the building; if it is structurally attached, more attachments may be required. The rack manufacturer or an engineer needs to determine if heavier gauge materials spanning the footings should be used.
A lower tilt angle is often best when a ballasted rack system is desired. The live loads imposed on the array will be minimized in this situation, resulting in reduced ballasting weights or fewer roof penetrations to keep the array in place. Mounting the array at a lower tilt angle reduces its wind exposure and structural requirements. It also reduces the potential energy yield per installed array capacity.
While this sounds unfortunate, there is an interesting trade-off at work. Reducing array tilt minimizes inter-row shading. This allows rows of modules to be mounted closer together, effectively maximizing the available array capacity. Rooftop power density increases as array tilt is lowered. Increasing the amount of power that a roof can facilitate will generally increase its annual energy harvest as well.
A quick check on a flat roof scenario in Portland, Oregon, illustrates this. According to PVWATTS, an array mounted at 30° – near optimum for energy yield – will produce 6% more annual energy compared to an array of the same capacity mounted at 10°. But given a limited roof area, the lower PV tilt angle will allow for as much as 50% more array capacity. This is a location where 1,080 kWh is produced annually per installed peak array kW. So in this scenario, the additional modules mounted at a lower angle will result in both a larger overall system as well as higher energy production values.
At 30° tilt:
8 rows of 12 modules = 96 x 175 Wp = 16.8 kWp
16.8 kW x 1,080 kWh/yr/kWp = 18,144 kWh/yr
At 10° tilt:
12 rows of 12 modules = 144 x 175 Wp = 25.2 kWp
25.2 kW x 0.94 x 1,080 kWh/yr/kWp = 25,583 kWh/yr
Maintenance. Another early consideration for designers is the level of system maintenance that the client is willing to perform. As more and more systems are being specified and sold by energy production rather than initial system size, maintenance – particularly cleaning modules – becomes an important consideration. Letting the rain deal with washing the modules may not be sufficient for some commercial systems. The location of the array will play an important role in this decision. At installations in agricultural settings, for example, where there is a high level of dust and particulate matter settling on the array, modules will need to be cleaned frequently to keep the array performing at its peak. Studies have been performed on losses associated with soiling with varied results. Each site will require an assessment of the soiling potential and best methods to mitigate the problem. In some cases, increasing the tilt will reduce soiling, especially at the bottom edge of the array.
Roof loading considerations. The structural element of rooftop PV systems can often be one of the most difficult for designers and integrators to overcome. Nearly every commercial roof structure will need to be analyzed by a structural engineer in order to verify the roof ’s ability to withstand any additional loading. The American Society of Civil Engineers (ASCE) publishes the Minimum Design Loads for Buildings and Other Structures (ASCE 7-05) to establish the design loads used in the US. Many jurisdictions use the International Building Code (IBC) as a guideline, which in turn references ASCE 7-05. It is also very common for jurisdictions to have additional requirements based on the IBC above and beyond the general requirements. The current IBC was released in 2006 and, like the NEC, is released on a 3-year cycle.
Loads to be considered include dead loads from the PV array and associated equipment, as well as live loads such as wind, snow, rain and seismic events. The IBC has specific definitions for loads—live loads, dead loads and nominal loads—that will have an impact on the array’s design. In order to properly evaluate the PV system, the engineer needs to have an understanding of the PV array to be installed, approximate locations for all rooftop equipment and the structural as-built plan set for the building.
The first load to be considered is the dead load of the PV array. Typically, a framed module and associated penetrating racking system will add less than 3 pounds per square foot. While this may seem like a minimal load, the combination of dead loading from the PV array plus live loads such as wind, seismic events and snow loading needs to meet the minimum code-prescribed levels in order to meet ASCE and IBC requirements. Generally, ballasted racking systems will have dead loads greater than 5 pounds per square foot; depending on site variables, a ballasted system may require excessively large dead loads.
Wind loading is another significant consideration. The IBC publishes a map of the US, IBC Figure 1609, that indicates the basic wind speeds to be used in designs. The values listed are nominal design 3-second gust wind speeds at an elevation of 33 feet in an area classified as exposure C (open terrain with scattered obstructions). Wind loading can have a major impact on the PV array characteristics – tilt angle, array location and mounting method in particular. The design wind loads in portions of the southeastern coast of the US are based on a 150 mph wind speed, for example, while the design wind speed on the West Coast is as low as 85 mph. In special wind regions, the local authority determines the exact requirements.
PV array size will partially be dictated by the wind loading at the edge of the building. Edges of roof structures have greater wind loads than the center of the roof, requiring additional structural considerations closer to the roof edge. Wind tunnel tests can help define the wind drag and uplift forces imposed on the array. Racking manufacturers often have this data available for engineering calculations. China Solaracks is top solar racking manufacturers provide detailed design and installation instructions that include edge or corner setback requirements. Often additional ballast material is required in these regions with a ballasted racking system.
Snow loads are another variable load consideration. PV arrays mounted at an angle to the roof surface distribute snow in a variety of ways. Snow will tend to drift along the roof and collect along the backside of the modules, increasing the load on the roof. It is also possible for the array to collect snow during a storm and then shed that snow once the sun comes out. This will increase the point loading on the front side of the array. The IBC has minimum snow load requirements listed on a map of the US. Many geographic areas have multiple snow load values listed, based on elevations. There are other locations on the map listed as CS, an abbreviation for “case study.” In these areas, the local building department publishes the required snow loading values.
Seismic loads must also be factored into the structural calculations. ASCE 7-05 and Section 1613 of the IBC cover earthquake loads and seismic design categories. In order to meet the seismic requirements with a ballasted system, ASCE 07-05, section 13.4 will need to be addressed. Local jurisdictions often have prescribed requirements, ranging from using ballast weight as the sole means of placement to requiring a combination of ballasting and minimal roof penetrations.
How much roof can you cover? One common desire PV designers encounter is to fill the roof with as much solar as possible. While it is tempting to do a shading analysis of the roof and then find a way to fill in every nonshadowed corner, this is generally not considered good practice and will not win any friends on the construction or maintenance crews.
Roofs are not a barren wasteland where no one will ever venture again. Very often existing HVAC and other RTUs will require maintenance. It is important to remember that there will be people on the roof, and you do not want the PV array to be compromised in any way due to clearances, or lack thereof, around the rooftop equipment. It is common to see tie-in points built into the roof for window washers. If these are present, it is best not to place a PV array between these and the edge of the roof. If there are existing walk pads on the roof, it is also good design practice to avoid impeding those paths.
There should always be walkways designed into the PV array. This is helpful for both the installation crew as well as future maintenance technicians. These personnel will likely have tool belts and tool boxes in tow, so there should be some walkways wide enough for this level of activity. People should be able to pass one another without damaging the array. Regardless of the space between rows, it is good practice to provide paths so that people are not tempted to make shortcuts through the array. If the distance between the rows is sufficient for walkways, give careful consideration to conduit and racking runs between the rows. Elevated conduit should be considered trip hazards and labeled where appropriate, especially at intersections with walkways.
Another best practice is to set a perimeter around RTUs to allow easy access on all sides of the equipment. A general minimum setback of 4 feet is a good target value. There will also be minimum NEC requirements for working clearances around the live components; Article 110 defines these requirements based on nominal voltage to ground. The NEC dictates a minimum 3 feet of working depth. If a minimum perimeter around these units is established, regardless of shading, the PV array will not impede on the other mechanical units located on the roof.
Last but not least, the Occupational Safety and Health Administration (OSHA) imposes requirements for the safety of installation and maintenance crews. The distance that the workers are from the roof edge dictates whether or not they have to be roped in. Depending on the array layout and future foot traffic on the roof, this requirement may not only apply during the installation but also during future service calls. Setbacks and fall protection requirements are often determined by the parapet wall height. As a general rule, any workers within 6 feet of the roof edge will need to be harnessed, but the height of the surrounding parapet wall may dictate different setback requirements. The installing contractor should verify compliance before construction begins.
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