Eight factors to consider when designing solar-ready roofs

Bullitt Center roof

The roof of the Bullitt Center in Seattle, designed by Miller Hull and featuring 575 PV panels on the roof at 425 watts per panel.

When you're ready to invest in solar, keep all of these steps in mind to design the best roof possible

Energy use and production is one of the largest sources of carbon emissions in the US, as well as one of the costliest expenses for businesses and households. To improve efficiency of resources, many building owners have developed an interest in solar photovoltaic (PV) panels. Rooftop solar installations can provide long-term economic and environmental benefits for building owners, and future tax breaks, technological advances in solar panels, and caps on carbon production may further increase the return on investment (ROI) for renewable energy sources.

At the same time, certain variables make it difficult to generalize on immediate returns. The viability of systems varies greatly, depending on the cost of electricity at the installation site, as does the price that state utility regulators set for the re-purchase of excess renewable power generated by the installation. The cost of installation itself can also vary, depending on the capacity of the roof to support panels.

Due to this uncertainty about the present-day economics behind panels, clients may be more interested in reserving space for a later installation. Whichever they prefer, here are eight factors to consider when designing a solar-ready roof.

Zoning/historic property restrictions

The first step in determining whether a building is solar ready is to make sure there are no zoning, historic property, or district restrictions that would prevent the installation of solar panels by the municipality. Another consideration point is that some municipalities have solar-shading ordinances that protect solar installations from shade cast by adjacent properties.

Building ownership

In most areas of the United States, solar panels require utilization of investment tax credits to have an attractive rate of return. To take advantage of these tax credits, a private entity needs to have taxable income to offset with the solar tax credits. Therefore, who owns the panels and the building is an important fundamental question for project viability. If the building is publicly owned, the panels can be installed and owned by a for-profit entity that can take advantage of the tax credit and sell the power generated to the public entity.

Proximity to transmission infrastructure

Buildings that are closer to substations make it more economical to sell excess power back to the grid, where the interconnection between the utility and the customer takes place. Since solar energy is generated during the day and the building owner will need to buy power when the PV array is not producing enough to meet needs, such a connection is required unless the energy can be stored in another way. The further the distance of the substation from the building site, the greater the loss of energy due to distribution losses. Hence, substations within a mile away are preferable.

Energy-generation goals

Ideally, to achieve a net zero building the solar panels would create more energy than the building uses. This goal, however, is an ambitious one that requires a large solar array. Following USGBC’s LEED v4 credit for New Construction, when incorporating on-site, nonpolluting renewable energy generation—such as solar—the panel’s production capacity must be at least:

  • 5 percent of the project’s annual electrical and thermal energy cost for one credit (exclusive of existing buildings)
  • 12.5 percent for two credits
  • 20 percent for three credits

To determine the size of the array the total electrical load of the building needs to be known, along with the generating capacity of the panels. For example, the LEED Platinum Bullitt Center in Seattle (a city not known for its sunshine) utilizes 575 PV panels on the roof at 425 watts per panel. There is 13,400 square feet of solar collection area with a DC (direct current) rating of 44.38kW.

In 2014, the Center's rooftop photovoltaic panels produced roughly 244,000 kilowatt hours of energy while the building used only 153,000.

Shading and orientation

To work properly, solar panels cannot be shaded by higher roofs, adjacent structures, tree canopies, or adjacent solar panels. Therefore, a shading analysis will need to be performed to make sure the solar panels are free from these potential obstructions. To prevent shading of the adjacent row of solar panels from the panel in front of it, roof collectors usually have 5 to 10 degrees of tilt to allow panels to be closer together and maximize the number of the panels on the roof; ground panels typically have 20 to 30 degrees of tilt. Ground panels have a greater power density due to the sun heating the panel in a more perpendicular manner (power density is greater for each panel, not per square foot of ground area).

This illustration shows panels shading potential at 5, 10 and 20 degree tilts on December 21 (the day with the longest shadows). For the same roof area, 630 panels are possible at 5 degrees, 533 at 10 degrees, and 437 at 20 degrees. The effect of panel shading on roof surfaces should be studied using a WUFI analysis to make sure that cooler shaded surfaces do not result in a condensation buildup in the roofing system that might require installation of a vapor barrier.

For unshaded locations, the maximum solar energy is from a panel that faces due south; the ideal tilt angle is slightly less than the local latitude. Maximum demand for power is generally in the afternoon; therefore, a southwest orientation of the panel can be advantageous to match demand and production.

Roof structure

Generally, it is more economical to install solar panels on roof heights below 60 feet. Wind velocity increases with building height, thus the cost to reinforce the armature supporting the panels increases accordingly. To prevent vandalism and damage from animals, solar installers prefer installations to be at least 12 feet off of the ground.

The weight of a solar system is generally 2 to 6 pounds per square foot, depending on if the system is ballasted. For new buildings, this relatively small load increase may be accomplished by the structural engineer selecting the next size up in the manufacturer’s roof joist table. For existing roofs, especially roofs in northern climates that were designed prior to more stringent requirements for snow drift loads, a more detailed analysis of existing roof capacity will be required.

For a system that is more than a demonstration, a viable square footage minimum area of array should be set to cover the design and infrastructure costs that decrease as the size of the project increases (for instance, setting a minimum 30,000 square feet of array area to support a .25MW system).

Roofing issues

PV panels do not need to be attached to the roof structure or penetrate the roof membrane to keep them from shifting or flying off due to wind. A variety of manufacturers make ballasted support systems that are held down by concrete block ballasts or similar weights. Such systems allow the panels to be easily removed when the roofing membrane needs replacement. When negotiating agreements with third-party suppliers, owners should include costs for removing and reinstalling panels when the roof membrane needs repair or replacement.

Since solar panels require minimal maintenance, foot traffic around the panels should be limited compared to maintenance of rooftop HVAC equipment. In areas where higher traffic volumes are anticipated, walk pads should be incorporated into the roof design. A white roof can help keep the roof cooler and reflect additional sunlight onto the panels, while a lower roof temperature will increase panel efficiency. Whatever system is chosen, consult with the roof manufacturer to insure that the installation will not affect your warranty.

As with any installation of rooftop equipment, most building codes specify that items located within 10 feet of the roof edge be protected with a guardrail or tie-off system unless there is a parapet that is a minimum of 42 inches high. These considerations should be taken into account when designing the array area.

Electrical requirements

Designers should take care to provide a pathway from the area where future inverters will be located (e.g. the main electrical room or adjacent space outdoors, preferably on the north side of the building).

Overall, there are many factors to consider when making a building design solar-ready, including ordinances, utility infrastructure and pricing, energy generation goals, and roofing factors. While a lot to weigh, the decision to install or allow for future installation of solar panels can be of great benefit to the building owner once all considerations are thoroughly examined.

This story also appeared in the AIA TDBP Technique newsletter for August 2016.

Brad Gellert, AIA, is a senior architect with ThenDesign Architecture and a member of the AIA TDBP Advisory Group.

Image credits

Bullitt Center roof

Nic Lehoux for the Bullitt Center

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