The Architectural Challenge of Green-Roof Rats


July 11, 2014

Jim FredericksWhen most pest management professionals (PMPs) discover rodents are active on a roof, they focus on roof rats, which have a propensity for nesting in trees and attics. Norway rats, on the other hand, tend to excavate subterranean burrows in the soil. But what happens when soil is on a roof? More frequently, PMPs encounter rodent problems associated with green roofs (buildings designed with rooftop lawns and gardens), which can make rooftop rodent management challenging.

Green roofs have their roots (pardon the pun) in Scandinavia, where they’ve been around for centuries. The modern green roof movement, however, dates to the 1960s and is becoming increasingly popular in urban architecture. Green roofs are attractive to builders for their water and energy conservation benefits. Vegetated roofs can reduce the amount of runoff directed from rooftops and insulate buildings from extreme temperatures during the summer and winter. Green roofs also can qualify for U.S. Green Building Council LEED (Leadership in Energy and Environmental Design) credits in the Sustainable Sites, Water Efficiency, Energy Optimization, and Materials and Resources categories.

PMPs might encounter two significant types of green roofs defined by the maintenance level and soil depth required to maintain vegetation. Extensive roofs are designed to require minimal inputs from humans and are engineered to support less vegetation per square foot of roof compared to intensive roofs, which can require additional inputs such as water and fertilizer to support larger amounts of soil and plant matter. Intensive roofs require at least six inches of soil, though some can have as much as 12 inches, especially in rooftop vegetable gardens with raised beds. Extensive green roofs typically require less than six inches of soil to support low maintenance plants.

In addition to the potential for secondary insect pest infestations resulting from water problems caused by faulty moisture barriers or improper drainage, intensive roofs featuring vegetable gardens provide excellent resources to support rodent infestations. Norway and roof rats will find fresh produce as excellent food in addition to water on hand for irrigation. Norway rats also might burrow into the deep soil substrates on intensive roofs and gnaw through waterproofing layers, resulting in leaks and secondary damage to structures. Rats also can gain entry into upper floors of buildings from the roof, where they nest.

In May, PMPs and researchers at the National Conference on Urban Entomology discussed the challenges of pest management in buildings with green roofs. It’s evident there’s great need for more related research and training. PMPs servicing buildings with green roofs should incorporate the roof areas into their inspections and provide integrated pest management advice about how to make these areas less hospitable to rats by limiting access to food and water, and keeping a watchful eye out for immigrant rats.

You can reach Dr. Fredericks, chief entomologist and VP of technical and regulatory affairs for the National Pest Management Association (NPMA), at



About the Author

Dr. Jim Fredericks, PHOTO: National Pest Management Association

You can reach Dr. Fredericks, BCE, executive director of the Professional Pest Management Alliance (PPMA), at

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