Structural (foundation)

The support of the rooftop solar collector becomes part of the building’s structural system. For small collectors serving a single wood frame building the collector system can typically be secured directly into the roof rafters. For larger installations more typical of that on US Army barracks, dining facilities, and other buildings, an engineered substructure is generally required to meet local codes and to support the collector arrays. The design object would be to use less material and fewer roof penetrations. Considerations of the supporting structure design are:

• Penetrations though the roof for connection to the existing structure must be water tight using
  properly sealing methods. Apply insulation where needed.
• Dynamic loads of wind and snowfall (see Figure 5.2) must be included in the structural analysis
• Expansion and contraction of system components must be considered.
• Where geographically required, seismic loads shall also be included in the structural design.
• The system design must provide access for maintenance and safe movement near roof edges.

A major concern with the installation of solar collectors is the wind impact on the structure. Increased wind loading on the support substructure can be offset by increasing the securing mounting points and reducing the tilt of the solar collector. A taller solar collector has a greater wind resistance and thus a higher wind load.

A typical engineered substructure supporting the solar collectors consists of beams or open web joists mounted on supports attached to the building structure below the roof. These reasonable spaced attachments are optimized with the cost of the rest of the supporting structure. The cost on a flat roof is ~$10/sq ft (~75.4€/m2). If the use of reasonably spaced support points is not desired, the support structure must span longer distances up to the width of the roof. The cost of using the longer span can approach 2.5 times the structure having intermediate supports. The frame that supports the solar collectors should be made using a non-corrosive metal such as aluminum.

Collectors mounted on sloped roofs typically take the roof slope as their tilt angle. This should be between 20 and 50 degrees. Slopes within this range on a sun facing roof will have only a slight reduction in performance when compared with the optimum tilt angle. These collectors may be attached a few inches above the roof to allow for rain water to flow underneath. Another style of placing solar collectors on a sloped roof is to integrate them into the roof surface. Figure 5.3 shows how these collectors form part of the roof replacing the roof materials being used. Pipe connects directly to the collectors from the ceiling space below. In-roof placement typically requires roofs with a slope of not less than 20 degrees to avoid standing water on the glazing, which may void the manufacturers water tightness guarantee.

Collectors can also be placed in a vertical wall (Figure 5.4). Flat plate collectors attached to a full surface of the building façade can eliminate the need for insulation and a weatherproof covering for the affected wall area, thus providing avoided cost savings that will offset the reduction in solar energy collection performance.

Collectors placed on flat roofs or the ground need to be arranged so that the optimum performance can be achieved. The materials used for constructing the structural supports need to be protected from corrosion. The use of steel hardware and fasteners in contact with aluminum collector frames and copper piping create a high likelihood of corrosion. Separating dissimilar materials with fluorocarbon polymer, phenolic, or neoprene rubber materials is recommended,

Placement distance between rows must be such that shading of one row by the next row behind is minimized. A slight amount of shading in the winter when the sun is lowest in the horizon can be permitted if the space for collectors is limited or piping costs are a concern since the solar radiation lost is a small percentage of the total annual amount. In areas where snowfall is normal a space of at least 12 in (30.5 cm). between the roof and the lowest collector part should be provided to allow for the collection of snow that has slid off the collectors. There also should be space for safe human movement between the collectors and around the end of collector rows. The space between the roof edge and the collector row must satisfy local building codes. Figure 5.5 shows a diagram of the space between collector rows (located on the ground or flat roofs) when the collectors are placed one behind the other on a horizontal surface.

When placing the collector on a roof, care must be taken not to damage the roof surface or interfere with other roof functions. Placement should allow for proper operation roof drains, HVAC and exhaust systems, plumbing vents, flues, or chimneys and antennas. Space for maintenance of the roof and those system placed on the roof shall also be provided. If the roof can handle the addition load of the solar collectors, then the collector supports can be attached to concrete slabs placed on the roof. To avoid harming the roofing below them these pads need to be placed on protective mats. Roof penetrations for structural connections and piping need to be made water tight. This is typically done using sleeves that surround the connecting structural or piping components, and that pass through the roof. A sealant is placed in the sleeve to form a watertight barrier and the top of the sleeve is ~3 in (77 mm). above the probable roof water level. The sleeve is appropriately flashed around for a good weather tight roof penetration. Figures 5.6 to 5.8 show several methods of supporting solar collectors on the ground or on a flat roof. It is advised that the roofing company that installed the roof be used to perform the roof work required to install the solar collector to preserve the remaining roof warranty.