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what's the consideration when mounting on solar roof

July 27 , 2017

Mounts (also referred to as L-feet, standoffs or hangerbolt ) connect solar rails to the roofing structure below the roof tiles and plywood decking. For example , 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. Shingle 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 leakage. 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.

For most roof solar 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.

How to determine the number of mounts

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.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. Today, many companies are competing for you business in the solar racking market, so you can rest assured affordable solutions are available.


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