Is your community facing resurfacing its flat roof(s) in the near future? This is a generational question as roof surfaces tend to last 25 to 30 years. By ‘flat roof(s)’ I mean a low-slope roof surface having a slope between ¼ and 3-inches per foot. In the Northeast, most existing flat roof surface membranes are a black rubber-like material called EPDM (Ethylene Propylene Drene Monomer). Most roof resurfacing projects choose using the same roofing material again but there are other choices.
The typical alternatives to EPDM include PVC (Polyvinyl Chloride); TPO (Thermoplastic Polyolefin); Flat Seam Metal; BUR (Built-up Roofing); and Modified Bitumen. Unless there are strong issues such as cost or design needs, only EPDM; PVC; and TPO will be the likely candidates for selection. In today’s energy conservation consciousness, the selection often comes down to a black (EPDM) and white (TPO & PVC) issue.
Modern design and regional building codes recommend ‘cool’ roofs designed to reduce energy consumption and what is commonly called the ‘urban heat island effect’. Cool roofs are categorized into three basis types: white, reflective coated, or green (vegetated) roofs. White roofs are the most common with TPO (thermoplastic polyolefin) and PVC (polyvinyl chloride) being the typical choices. Though PVC and TPO roofs have the same wear and cost factors, PVC materials have negative characteristics, such as high toxicity and un-recyclability, so for ease of comparing black to white we will only consider EPDM vs. TPO in our comparison discussions.
To start with TPO roof membranes have longer lives lasting on average 25 to 30 years compared to EPDM lasting 20 to 25 years. This is due to TPO’s resistance to UV and thermal expansion damage. Some TPO products developed bad reputations in the past due to their inability to manage severe cold which in some cases caused the membrane to shatter. These problems have been eliminated with today’s TPO roofing materials.
Secondly, the initial cost favors EPDM roofs. However, when life cycle and energy cost issues are considered the black and white roofs become competitive. Installation methods differ in that EPDM seams are taped or adhesively sealed while TPO seams are welded by a thermal process.
There are strong forces pushing the general acceptance of white roofs in the future. States such as California have passed laws in 2005 requiring the use of reflective roofing materials as well as individual cities such as Chicago establishing building codes to favor its use. There is evidence showing the heat island effect of black surfaces, which include not just roofs but also parking lots, paved roads, and building facades, can have an impact on local weather characteristics.
In addition, the nation’s largest green building advocates are influencing architects and building owners by favorably rating buildings with cool roofs. Under the joint program of the US Environmental Protection Agency (EPA) and Department of Energy (DOE) for a roofing product to receive an Energy Star label under its Roof Product Program it must have a solar reflectivity of at least 0.65 and weathered reflectance of at least 0.50 in accordance with EPA testing procedures.
The Cool Roof Rating Council (CRRC) has created a rating system for measuring and reporting the solar reflectance and thermal emittance of 850 roofing products and provides this to energy service providers, building code bodies, architects and specifiers, property owners, and community planners. The Green Building Initiative has instituted its Green Globe system in the US and Canada to develop benchmark energy consumption criteria because of roofing material’s solar reflectance and thermal emittance.
The US Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system is used for most publicly funded building projects and high-profile non-government buildings because of legislation, executive orders, resolutions, ordinances, policies, and tax incentives. Architects seeking a LEED certification for their project will receive credit for white, cool roofing meeting LEED solar reflective index guidelines.
So, with all this horsepower pushing for cool roofs, it seems like an easy black or white decision for the condo board facing a roof replacement project. Maybe not. For northern condos, the problem is a little complicated. CRRC admits to a “winter penalty” for northern climate cool roofs. DOE building modeling data reveal that in the north heating is a much more significant factor in energy use than cooling. In fact, heating accounts for 29 % of energy used compared to only 6% for cooling.
It turns out that insulation is a more crucial element for energy efficiency than cool roofs here in New England. It has to do with the amount of Heating Degree Days (HDD) and Cooling Degree Days (CDD). As an example, Boston has 5,841 HDD and 646 CDD as compared to Albuquerque’s 4,361 HDD and 1,211 CDD. Therefore, using DOE’s cool roof calculator, Boston’s high number of HDD’s and positive winter heat gain results in lower energy usage and fewer carbon emissions with an EPDM roof.
In my experience, the best way to ensure a quality roof surface is to hire a competent roofing contractor with experience in installing your selected roof surface material. I recommend seeking the advice of your community’s professional engineer or architect to ensure the right choice.
Written by Jack Carr, P.E., R.S., LEED-AP, Senior Consultant Criterium Engineers
Published in Condo Media