Racking: Roof Mount Systems

Roof mounted solar arrays need to be designed to withstand the highest winds expected for the location, as determined by the American Society of Civil Engineers. The Applied Technology Council publishes the design requirements for all U.S. locations at http://www.atcouncil.org/windspeed. Solar arrays mounted on single family homes and most other occupied structures (Risk Category II) in Salt Lake City and surrounding areas are to be designed to withstand 90 to 100 mph winds, depending on location.

Snow loads must also be considered in a photovoltaic solar energy system. Though snow loads appear to be evenly distributed across a roof – and they are if the roof has no solar panels on it – the load on a solar array is actually borne by a limited number of base mounts. This can create large point loads on the rafters that support the racks. This is typically only a problem in Salt Lake City on older structures, especially when the roof is not steep enough for snow to slide off of the panels. In those cases the base mounts can be located two feet apart (on every rafter) instead of every four or even six feet.

Rack mount systems are highly engineered and, as with solar modules, certified to meet load requirements if installed as directed. Color (black or aluminum) can be a consideration in some installations, but because the racking is largely hidden by the panels that is a lesser factor for most people.

Roofing materials dictate the “standoff” design – the method of attachment to the roof scructure. Excellent designs exist for nearly every roofing design and material.

Flat roofs require “ballasted” racking if they rely on a membrane to seal out moisure; penetrating that membrane with fasteners would introduce an opportunity to leak. Ballasted systems rely on weight to prevent wind for affecting the array.

Utah Applied Solar Energy, LLC  2015