7. Select and size your racking and mounts.
As explained in the Balance of System section, solar racking is used to hold modules in place. In a typical flush mount system on a residential rooftop, each row of modules sits astride two rails. If the rows are long, multiple rails are spliced together to cover the distance. For most home arrays, you'll be using rails that are either 136" or 162". These are bolted securely to the roof with mounts, which may or may not be sold by the same company that makes the rails. You'll also need splice kits, fasteners and some other hardware and supplies to secure the modules to the rails, and the mounts to the roof.
Modules must be clamped to rails (or otherwise fastened) so they're flat and level with each other. Tilted panels require specialized attention that takes into account the potential wind load and sail effect. The array installed in this photo uses a portrait orientation for the modules. Thus the module's width dimension is used when calculating the row size.
Besides knowing the module brand you plan to install, and the number of modules, a few other key pieces of data must be gathered, such as:
As you shop for rails and mounts, beware of the weight of the material and type of metals used. Lightweight aluminum rails and stainless steel hardware is the preferred combination. The aluminum keeps the structure lightweight, while stainless steel fasteners are strong, durable, and do a better job of resisting galvanic corrosion than galvanized steel.
To calculate the exact length of each row, start by retrieving the module's width from its spec sheet. Then multiply that dimension by the number of modules in each row. The equation looks like this:
Initial Row Length = #Modules X Module Width
For a landscape orientation (sideways), use the module length spec instead.
The Sharp module referred to in Steps 5 and 6 of this guide, for example, is 39.1" wide by 64.6" long. For a portrait configuration of 10 modules in each of 2 rows, that math is 10 X 39.1"= 391".
You'll have to factor in extra space for fasteners and any other requirements specified by the racking product instructions. For example, the rail installation might call for:
The final equation looks like this:
Array Rail Length = ( #Modules X Module Width ) + ( Mid-Clamp Size X
( #Modules - 1 ) ) + (End Clamp Size X 2) + (Margin on End X 2)
For a landscape orientation (sideways), use module length instead of module width.
For our sample 20-module array, the math is thus: (10 X 39.1") + ( 1" X (10-1) ) + (1/2" X 2) + (2" X 2) which equal extra spacing adds up to 405". The next easy calculation determines how many row lengths the rails will have to cover.
#Rail Rows = Module Rows X 2
Since two rows of 10 modules will accommodate the 20-module array, the math is 2 X 2, which equals four total array rail lengths. Next you'll have to decide which quantity of the available standard rail sizes will do the job for the least amount of money. This equation is divided into three parts:
Step 1: #of Rails Per Row = (Array Rail Length / Standard Size)
Let's look at the larger size rail of 162" first. When that's divided into the rail length of 405", the result is 2.5 rails. The second size 136" divided into 405" yields a result of 2.97. This number rounds up to 3. Now on first glance, you might conclude that 2.97 rails isn't be long enough to create one array rail length out of three 136" rails. However, the opposite is true, and three rails will exceed 405" by three inches. (Here's the math: 3 X 136"= 408".)
At this point, both sizes appear to fill the order efficiently, leaving little or no waste material. Let's move on...
Step 2: #of Standard Size Rails to Order = ROUNDUP ( #Rail Rows / #of Rails Per Row )
The math for the 162" rail looks like this: 4 X 2.5 = 10 rails. "Round Up" means a decimal number must be raised to the next highest whole number or integer. Since 10 is a already a whole number, it doesn't need to be rounded up. Meanwhile, the math for the 136" size is this: 4 X 2.97 = 11.88. Since 11.88 isn't a whole number, we must round it up to the next integer, 12.
So now we know that we can order either 10 rails sized 162" or 12 rails at 136" for our array. Before checking their prices, we must also factor in the number of splice kits we'll have to order in conjunction with each standard size of rail under consideration. Thus:
Step 3: #Splice Kits = #Rail Rows X ROUNDUP (#of Rails Per Row)
For the 162" rail, the math is thus: 4 X (RoundUp 2.5) = 4 X 3 = 12. For the 136" rail, the solution is 4 X (RoundUp 2.97) = 4 X 3 = 12. The number of splice kits is the same for both option. Consequently, this will have no impact on either the price or the labor time spent installing them.
Pricing is the next step in our journey to choose a rail size. Keep in mind that rails are long lengths of non-foldable metal requiring shipment in a truck. If you or your installer are not picking up the material from the supplier, there may be an additional charge for the longer rail, since 162" translates to 13.5 feet, while 136" is a somewhat less wieldy 11.3 feet. With all these details in mind, here's the the final calculation concerning the rails and splices.
Step 4: Cost for Rails = (#of Standard Size Rails to Order X Price per Rail )
+ (#Splice Kits X Cost per Splice Kit) + freight charge
When the time comes, you can perform this math on your own to see which standard rail size is most cost-effective. Next, you'll have to consult the product literature to determine all the fastening hardware and quantities required to get the job done (plus a few extras of each part). Most of the time, you'll use the same products regardless of the rail size you choose. Here's a list of common parts and the mimimum number of them required:
Panel racking, like this line of products from Iron Ridge, includes rails, roof mounts, fasteners and optional hardware. Notice the "tilt leg kits (5 to 45 degrees)" on the right. These attachments allow you to adjust the array tilt manually. Photo: Iron Ridge XRS Solar Panel Racking
Mounts connect solar rails the roofing structure below the roof tiles and plywood decking. In the photo above, L-feet brackets are used in conjunction with what appear to be roof or tile hooks and flashing to fasten the rails to the rafters. Flashing is sheet metal that cradles a mount and slides nimbly beneath the roofing surface. Some types have a rubber boot to provide extra protection against water seapage. Mounts may also include one or two 5/16" lag screws are bolted down through roofing material and into the rafters. The length of these screws varies, so you'll need to know how thick the roofing material is.
This product is commonly known as an Oatey flashing. A lag bolt is first fastened through the hole and into the roof, securing the mount. Then a cylinder-shaped hunk of metal called a standoff is inserted. The rails attach to the standoff. Standoffs allow for higher placement of modules off the roof, which is better for air circulation and lowering heat.
For most racking systems, a mount is placed every four feet along a row. However, there shouldn't be more than 20 inches of a rail at either end unsupported by a mount (called cantilevering), which means your first mount should be placed no farther than 20 inches down the row. The specs and requirements vary, however, depending on the product. A manufacturer's warranty may be voided if you don't follow the instructions laid out in the installation manual.
The installation shown in the photo includes rails for two rows of modules. Notice the closeness of the two inner rails. Although it's not the case here, you may be able to connect both of them to a single mount in the center using an optional braket extension like the one sold by ProSolar (shown at right). Another good practice is to alternate between studs when locating your mounts. The photo here indicates that a few studs (or rafters) are having to shoulder the entire load of the array. Photo: Homeco Energy
The product literature should also tell you how to determine the number of mounts needed for your solar array. One common sizing calculation you should perform involves the weight dispersal of the racks and modules across the mounts. This tells you how much of a dead load each mount is supporting. (A dead load refers to the material that will be permanently added to the roof.) To find that number, you divide the weight of the racking and modules by the number of mounts you're using. Mounts are generally limited to a load of around 45 pounds, but the actual weight is stated in the specs.
For example, if you divide 900 pounds worth of material across 24 mounts, you'd get a weight dispersal of 37.5 pounds per mount.
Depending on where you live, local building codes may also specify a maximum dead load distribution on a residential roof. This requirement is in addition to the product specifications of your racking. Building codes usually allow dead loads of no more than 5 pounds per square foot of roof space Fortunately, most racking products and modules add only about 3 pounds per square foot. (If the roof is covered with heavy concrete or ceramic tiles, the allowable dead load may be smaller.) To compute the dead load of the array, divide the total weight of the racking, mounts and modules by the square footage the array will occupy. For example, 1,125 pounds of material divided by 374 square feet of array space (34' X 11') equals 3 pounds per suare foot.
Solal ABC's provides a worksheet for making a dead load measurement as part of Expedited Permit Process form. See Step 1 for instructions.
The calculation to determine the number of mounts you'll need will depend on the product specifications. However, the quantity most often equals the number of L-feet you'll be attaching, since an L-foot always attaches to a mount. It's sometimes possible, however to use fewer mounts, by incorporating mounting bracket extensions, like the one featured in the photo above. The extension allows you to connect the L-feet of two adjacent rails to a single mount located between them. This saves you time and money spent on installing mounts, and reduces the number of penetrations into your roof. But don't forget to do your dead load math first, since you'll be using fewer mounts to hold up your array.
When you buy standard solar racking, the product engineering data should not only address dead loads, but also wind, snow and seismic loads. If you live in place where any of these factors are a concern, your local building inspector may have additional requirements. Be sure to research this subject before making your racking purchase. Engineering data concerning loads (as wells as the other producs specs) are referred to as cut sheets in the world of building inspectors. Copies should be submitted with your construction permit application and made available on site whenever an inspector visits.
Occasionally, DIY solar installers decide to build their own racking from common steel or aluminum channel. Thinking this will save money, they eventually discover they must hire an engineer to compute the required data on dead, wind and other types of loads. Otherwise, their installation won't be approved. Today, a slough of companies are competing for you business in the solar racking market, so you can rest assured affordable solutions are available. To browse products from Unirac, Ironridge, ProSolar, Quickmount., and other manufacturers, visit our Links page.
Additional supplies required to complete the installation include polyurethane caulking (to apply in and around roof penetrations), wire clips, grounding clips for the modules, grounding jumpers (electrical cables that connect across rail splices), and grounding lugs for fastening a bare copper ground wire to the rails.
Needless to say, before pursuing a solar project, don't forget to inspect the roof carefully. Here are some scenarios that must be addressed before installing a PV array:
Finding a dip, warpage or weak spot in a roof can sometimes be fixed by shoring up the rafters with wood blocking. The video below explains how this is done:
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