The installation of solar water heating (SWH) collectors is an investment in one’s energy security. Proper installation protects that investment.
A roof-mounted array has the economic benefit of utilizing an existing structure—your home or garage’s roof—rather than having to build a separate structure to support the collector array, which can be costly (see “Ground-Mounted SWH Systems” sidebar).
Even so, a roof-mounted SWH system should not affect the building’s structural integrity. Examine the roof structure and be certain that the rack can be attached to the roof framing—not the sheathing! If the roof already looks like it is having a difficult time doing its job—i.e., it is sagging or bounces when you walk on it—you may need to reinforce it. If you plan to use a tilt rack (see “Tilt or Parallel?” sidebar), be sure to understand collector loads on the roof. It may be necessary to consult a professional for advice or have the system engineered.
This article provides a glimpse into the mind of a structural engineer and discusses the impact of a collector array on a structure. This insight should help you determine the various forces that can affect a project, and to appreciate the expertise that an engineer can lend to your project.
The forces upon a collector and roof are the major consideration when designing a rack or selecting mounting hardware. These forces are categorized as dead loads, live loads, and environmental loads.
Dead loads result from the weight of the collector, the mounting hardware, and the collector fluid—and remain constant. For SWH systems that use glazed flat-plate or evacuated-tube collectors, the collector dead load is approximately 3 to 5 pounds per square foot (psf). For comparison, a layer of shingles has a dead load of 2 to 3 psf. The exact empty and filled weights of a particular collector can be obtained from the manufacturer’s specification sheets. Integral collector storage (ICS) units and thermosyphon systems contain tanks and may have dead loads that range from 25 to 70 psf.
Live loads are intermittent, resulting from movable objects. The weights of the staged equipment and of solar installers working on a roof would be considered live loads. These loads typically don’t affect rack design, but the roof itself must be able to support live loads.
Environmental loads come from rain, snow, earthquakes, and wind. Rain is rarely considered when designing the mounting for a solar array unless the design causes pooling on the roof. It is best practice to avoid such a situation by orienting the attachments to the roof in a manner that permits normal drainage of the roof surface.
Typical snow loads in the United States range from zero in areas such as Florida and Southern California to 100 psf in northern Maine to up to 400 psf in some locations in Alaska. Drifting and sliding snow can increase these loads significantly. Snow loads for collector racks are affected by local siting—whether they are in an exposed area where wind will readily blow snow off, or they are in a sheltered area where the snow is unlikely to be blown away.