The roof of a Federal building is a common placement for a number of renewable energy technologies, so they should be addressed anytime a roof renovation is undertaken, including roof-mounted photovoltaics (PV) and solar hot water (SHW) systems that consider structural loads, accessible wiring/plumbing, and available roof space; daylighting, including skylights, clerestories, and solar tubes; and energy-efficient roofing technologies such as vegetative roofs.
Renewable Energy Options for Building
In a Federal building renovation, a variety of equipment may vie for roof space. Decisions about using roof space should involve a range of stakeholders and ensure that all competing uses are fairly evaluated. If all technologies and equipment vying for roof space are assessed at the same time, the most economic and effective use of the available space can be found rather than trying to piece the roof together after the design of various systems has progressed. Additional factors must be considered, such as the provision in ASHRAE Standard 189.1, which mandates that building design allocates space for onsite renewable energy systems of at least 6.0 kBtu/ft² (20 kWh/m²) for single-story buildings and at least 10.0 kBtu/ft² (32 kWh/m²) multiplied by the total roof area in ft² (m²) for all other buildings. The exception to this rule is buildings without adequate solar resource.1
The spacing of technologies also needs to be considered in roof design, since a variety of factors—fire safety, structural loading, and solar access—will dictate the ability to utilize the space. Some technologies, such as skylights, may only be of use on the roof, but others, such as PV; heating, ventilation, and air conditioning equipment; etc. may be located to other areas. Consider placing non-solar equipment in shaded areas of the roof or building exterior. In additional to local jurisdictional issues related to roof access and fire safety, the U.S. General Services Administration also has guidelines for roof placement and walkways, such as its Fire Safety Guideline for Photovoltaics.
Solar access needs to be assessed and maintained for any solar equipment placed on the roof. Solar water heating must have unshaded solar access and needs to be located on the roof or adjacent to a building to minimize heating losses from piping. PV is the most rigorous in needing unshaded solar access throughout the year, since shading on one module can reduce the output of the entire string of modules. PV can be very flexible as to location since it is commonly used on south-facing building awnings, shade structures in the parking lot or landscaping, or on a nearby parking structure. This should not discount the need to assess PV design in conjunction with the roof design, it just provides some flexibility if designed properly.
When assessing the structural integrity of a roof design, the full complement of potential technologies should be assessed to ensure the design load can accommodate both the live and dead loads associated with the various technologies. PV systems mounted on flat roofs are often ballasted—in other words, they are not physically connected to the roof, but rather held in place by weights. According to NREL's Solar Ready Buildings Planning Guide the dead (static) load of PV system can vary from 2–6 lbs/ft² or more depending on the mounting and ballasting of the PV system. Roof mounted solar thermal systems are typically at the high end of that same range. Additionally, most roof-mounted systems will be placed at an angle, and can experience significantly higher live (dynamic) loads than a flat roof, especially in windy conditions.
Solar Water Heating
Both SWH and PV systems need to be designed by experts in the technology, but design should be closely coordinated with roof design in any renovation. This involves not only managing the available roof space and structural loading, but also ensuring design elements do not conflict. For example, if the roof anchors for a SWH system penetrate the roof surface, will those penetrations affect the thermal properties designed into a high-efficiency roof? In addition, the "boundaries" of the solar system versus the roof need to be clearly defined, and responsibility for system components need to be delineated in contracts and warranties.
When renewable technologies are not included as part of a roof renovation, the design should consider the future use of these technologies and be designed to accommodate phased implementation of renewable energy technologies.
This involves fully considering the needs and impacts of renewable energy systems on all aspects of the roof design from an adequate structural rating to accommodate the additional load to bringing wiring and plumbing connections to the roof.
This also entails carefully placing existing equipment to maintain unshaded space for future solar installations.
Detailed information about how to design a solar-ready building is provided by the National Renewable Energy Laboratory (NREL) in the Solar Ready Buildings Planning Guide. NREL has also developed an overview that includes cost information in Solar Ready: An Overview of Implementation Practices.
In addition to roof-mounted renewable systems, there are various energy-efficient roofing technologies and practices, including cool metal roofing materials and ENERGY STAR®-rated. Additionally, vegetative roofs can mitigate heat gain within a building and reduce heat island effect.
Utilization of cool roofs not only reduces building energy consumption by lowering air conditioning demand and extends the life of cooling systems, they add to occupant comfort by keeping the building cooler during warmer seasons. The Federal Energy Management Program has produced guidelines for selecting cool roofs.
1American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Standard 189.1: Standard for the Design of High-Performance Green Buildings Except Low-Rise Residential Buildings, Section 7.3.2 On-site Renewable Energy Systems.