In recent years, green building advocates have proposed alternative roof treatments for the achievement of greater energy efficiency. Green building certifications – like LEED and others – place particular emphasis on green roofs. In 2009, United States Secretary of Energy Steven Chu advocated cool roofs for greater energy efficiency – a method most recently adopted by New York City in their Cool Roof Program.
Much has been published about green roofs and cool roofs with new construction, but what about existing buildings? This issue of Reduce, Reuse, Rehab will explore green roofs, cool roofs, and how they can be compatible with the Secretary of the Interior’s Standards for the Treatment of Historic Properties (the Standards). In addition, the issue will show how historic buildings can also realize greater environmental efficiency through the use of traditional roof materials and treatments.
A green roof features a system of vegetation, drainage, insulation, and structural support installed on flat or low-sloped roofs. The vegetation grows at the top layer, with the growing medium, filter membrane, and drainage layer directly beneath it. Beneath those layers are the waterproof root repellant layer, support panel, thermal insulation, vapor control layer, and the structural support.
When installed and maintained correctly, green roofs provide a number of environmental benefits, including: heat and cooling reduction, roof life span increase, water runoff reduction, pollutant and carbon dioxide filtering, and urban wilderness or natural habitat creation.
While a green roof system can be installed on a new building with fewer restrictions than it can on a historic building, compatibility between a green roof system and the Standards can be accomplished in certain situations. Of course, every historic building has its own situational and individual quirks and concerns that require specific research and planning before undertaking a project such as green roof installation.
As discussed in Interpreting the Standards Bulletin 54: Installing Green Roofs on Historic Buildings, the project must pay special attention to the prevention of water infiltration, retention of historic character, compatibility of new additions/alterations, and the reversibility of additions.
Meeting the Standards on a sloped roof of a historic building would be quite difficult, but since green roofs are most often installed on flat roofs, there exists a greater chance for compatibility. With historic buildings, a green roof should be installed only when the vegetation is not visible from the street, thus retaining the building’s historic appearance and character. The invisibility of vegetation can generally be achieved because of the parapets often featured on flat-roofed historic buildings.
Provided that it does not negatively affect the building’s historic character, a green roof can meet the compatible alteration standard, and since it can be removed at a later date, the green roof can also meet the reversibility standard as well.
Cool roofs reflect solar energy while also emitting absorbed heat and infrared radiation. A cool roof is usually white or light-colored and reflective, as lighter colors reflect solar energy, thus reducing heat transfer to the building. The coating used to create a cool roof is not simply white or light-colored paint; it is a specially designed material that must achieve both solar energy reflection and heat and infrared radiation emission.
According to the Cool Roof Rating Council, a cool roof will reduce the need for air conditioning, often resulting in 10 to 30 percent reduction in energy use, thus also reducing the output of greenhouse gas emissions. Cool roofs contribute to the reduction of the heat island effect, where temperatures increase due to a lack of vegetation coupled with a concentration of heat-absorbing surfaces, such as parking lots, pavement, and dark rooftops. Further, cool roofs increase the life span of the roofing membranes through less exposure to high temperatures and mechanical cooling equipment through less frequent use.
Cool roofs can meet the Standards relatively more easily than green roofs, as long as the historic building’s roof is flat. With a flat roof, dramatically changing the roof’s finish color – from dark to light in this case – would not adversely affect the historic character of the building as it would not be visible and would be reversible.
On a low- or high-sloped, dark-colored historic roof, however, a cool roof installation would not be compatible with the Standards, as the color change would significantly change the historic appearance of the building. In addition, it would be impossible to reverse the creation of a cool roof if the roofing was original material, such as slate or wood shake.
Traditional roofing materials
While attaining compatibility between modern, energy efficient roof treatments and the Standards can be difficult, historic building roofs – particularly most sloped historic roofs – can offer energy efficient alternatives.
If the historic building retains its original slate, clay tile, wood shake, or metal roofing, then it is already more energy efficient than a roof with non-natural, non-recyclable materials, such as asphalt shingles. This is because many types of modern roofing require the use of potentially hazardous chemicals and plastics in their creation. Plus, most modern roofing material does not last nearly as long. When properly maintained, a slate roof can last up to 100 years in conservative estimates, whereas a new, asphalt-shingle roof’s lifespan is generally estimated to last only 30 years.
Traditional, naturally-derived roofing should be repaired and preserved whenever possible, for maximum energy efficiency. By retaining and maintaining roofing that currently functions, a historic home owner avoids the purchase of potentially wasteful roofing materials.
If the historic roofing has reached its natural life expectancy, replace it in-kind whenever possible. When a slate roof that has not yet reached its life expectancy is replaced with asphalt shingles, the slate can be recycled for use on a building whose slate requires replacement.
Sustainable rehabilitation of historic buildings requires thoughtful planning to reach compatibility between green standards and preservation standards. Green roofs and cool roofs can be installed on historic, flat-roofed buildings in certain circumstances. On sloped-roof buildings, original, historic, and/or natural roofing materials should be used and saved whenever possible, so that the building both achieves environmental efficiency and retains its historic character for future generations to enjoy.
For more information:
“Green Roof Retrofit Case Study,” Architecture Week. 9 April 2003.
Connor, Steven, “Obama’s Climate Guru: Paint Your Roof White!” The Independent. 27 May 2009.
Jenkins, Joseph, The Slate Roof Bible. Grove City, PA: Joseph Jenkins, Inc., 2003.
Skorbach, Kristina, “Keeping Things Cool with White Rooftops,” The Epoch Times. 11 May 2010.