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A New look at old roofs: replace or re-cover?

As homes and commercial buildings age, their roofs often begin to leak. When a roof is 15 to 20 years old, a typical building owner will have it replaced. But is a costly roof replacement needed so soon? Not always, say some researchers at the Department of Energy's Buildings Technology Center at Oak Ridge National Laboratory (ORNL).

The researchers are finding ways to dry a wet roof and significantly extend its life at one-third the cost of replacing it. They also are identifying methods of building new low-sloped roofs that dry easily and can be re-covered later to make them last longer. These techniques, they say, will save energy and money.

ORNL researchers David Kyle and Andre Desjarlais have written an Assessment of Technologies for Constructing Self-Drying Low-Slope Roofs, which evaluates currently available techniques for removing water from flat roofs. Tom Smith, technical director of the National Roofing Contractors' Association has called their report "the most important U.S. report regarding roofs in at least the past five years."

Traditionally, after 15 years of use, an old roof assembly is torn from the metal deck and replaced at a cost of about $9 per square foot. Adding to the expense is the cost of landfill disposal of the large volume of old roof parts, including asphalt, foam, and possibly asbestos. If asbestos is contained in the roof waste, it must be removed and disposed of in special ways, boosting the cost.

To reduce the cost of roof repair to one-third that of the traditional tear-off approach, more and more building owners are asking roofers to dry out their roof insulation and cover it with a new top layer. ORNL researchers say this approach offers benefits to consumers and the country.

"An increase in roof service life of five years in the United States should reduce the cost of roofing by 21 percent, saving the country $2.5 billion a year and cutting in half landfill waste from roofing, which currently represents almost four percent of the total volume of solid wastes in the United States," said Jeff Christian, director of ORNL's Building Envelope Research Center. "This saving is possible if roofers could dry out and re-cover old roofs and build new self-drying roofs that can be safely re-covered later."

Starting from the bottom up, a typical roof comprises a corrugated metal deck, insulation, and a membrane made of a single-ply sheet or plies of felts mopped together with asphalt. Water eventually enters gaps in the roof assembly, leading to its deterioration.

As moisture accumulates, problems develop. Metal fasteners may corrode. The ability of roof insulation to keep out heat may decrease by as much as 40 percent. Dripping water may enter building interiors, causing damage and motivating owners to consider roof repair.

How can an old roof be dried most rapidly before it is covered with a new membrane? ORNL researchers and their industry collaborators at the Buildings Technology Center have identified effective ways to reduce and prevent moisture accumulation in roofs. They have found some special materials that block the flow of water into a roof assembly. Other materials absorb water as liquid and release it downward as vapor. When this vapor reaches the building interior, it can be removed by air conditioning.

ORNL researchers Christian, Desjarlais, and Phil Childs, in collaboration with Dow Corning researchers, are demonstrating "downward drying" in a building at ORNL. Using infrared cameras and neutron sources, they found that 40 percent of the insulation in the 27-year-old roof was saturated with water.

They repaired the leaks and increased the roof's R-value (insulating ability) from two to 13 by spraying the roof with polyurethane foam. Before reroofing, holes were drilled in the metal deck and vapor retarder to let vapor pass through the roof deck below. In this way, the wet insulation dried out.

In summer, the large difference in the pressure of vapor in the roof and in the air-conditioned space inside forces the vapor down into the building, where it is removed by air conditioning.

The roof's outer membrane was covered with white granules (donated by 3M Corporation) that reflect sunlight more effectively than typical black roofs. This change reduced the air-conditioning load on the building.

The researchers removed wet roof samples from the building and thermally tested them in ORNL's Large-Scale Climate Simulator while weighing them on "load cells." The Large-Scale Climate Simulator was programmed to reproduce summer outdoor weather conditions. The researchers measured a loss in the amount of water over time, confirming downward drying. Based on this and other information, they modeled the roof system on the computer so they could predict how fast the roof will dry.

By not replacing the ORNL roof using the typical procurement process and by having Dow Corning re-cover the roof as part of its contribution under a cooperative research and development agreement, the Buildings Technology Center effort saved ORNL $250,000 in roof replacement and disposal costs.

Based on computer modeling of energy use before and after the roof work, the researchers found that the installed insulation and reflective granules reduced heating costs by 42.5 percent and cooling costs by 18.5 percent, saving the Laboratory $2,300 a year in energy costs.

"Lessons learned from this work," Christian said, "are being incorporated into computer software that will help roofing contractors decide whether re-covering individual old roofs is a viable alternative. The software may also guide the design of future roofing systems."

ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Martin Marietta Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.