by Harry J. Lubitz, CSI, CDT
Last year, the California Energy Commission introduced new energy-efficient standards for all newly built residential and commercial structures. Essentially, ‘solar-ready roofs’ will be required on all new construction. While home and building owners are not required to install solar panels at the time of construction, their roofs must be prepared if they choose to install them in the future.
Following California’s lead, more states and municipalities are expected to require ‘solar-ready’ assemblies going forward. Whether mandated or not, it is a good idea to understand what can be done to minimize costs and keep the option to add photovoltaics (PVs) down the road without too many headaches.
The best time to think about a solar roof plan is during the design stage; pre-planning is a must for an installation during the build or sometime in the future. This entails developing a review that focuses on optimizing the design and power production while minimizing the initial and longer-term costs. To this end, this article looks at the economics of opting for a membrane or metal roof in terms of outlasting the PV array above it.
Orientation and roof pitch
Regardless of roofing material, there are a few general rules that can help maximize the power production of a PV system. For example, the general orientation of the building can be a very important factor. When steeper slopes are involved, south-facing roof surfaces are ideal (Figure 1). When this is impossible, southwest or southeast can also be good options.
Orientation is not as important for very low slope roofs (i.e. five degrees or less). It is important to note the solar community uses angle in degrees while the roofing community uses pitch (Figure 2).
Modules are normally installed planar to the roof surface on steep roofs, and planar or very slightly tilted on low slope applications. Aggressive tilting of modules is seldom done anymore primarily due to economic considerations adversely affecting payback periods, and also due to adverse wind effects. Tilted systems are still used in very northern geographies, or on some roofs not oriented to the south. It is a delicate balance of increased cost versus increased power production.
In many cases, the roof pitch is pre-determined and cannot be modified. In theory, the best pitch is the latitude of the jobsite, but in practice this is seldom the case. A lower-than-optimal pitch is not as critical as orientation—the difference in power production is nominal. The increase in power production is not usually worth the premium cost of unusually steep pitches, unless the steeper slope is also an intentional design element. Even a 10 percent increase in power production does not usually warrant the added cost of achieving it (Figure 3).
The best design for the solar system is an unobstructed roof area (or areas) with no shading issues. Building components such as plumbing stacks, skylights, chimneys, and side walls can create shadows on the solar system. To eliminate shadows, the solar system design would be modified to accommodate any of these conditions. Consideration should also be given to any future buildings that could cast a shadow on the system or plants, like trees near the building.