The Washington National Cathedral

North Lawn Intensive Green Roof

In 2000, the Cathedral starting meeting to determine how to construct a parking garage and bus depot on the Close and once it was determined that these structures would be placed underground, the question then became how do you conceal such as large concrete structure. The answer was to build the largest intensive green roof in Washington D.C. covering more than 75,000 SF. The Cathedral’s mandate was to have the North Lawn appear the same post construction, as it did before work commenced. This was accomplished with the exception of the very northeast corner of the garage where the final grade was 6” higher to cover the corner of the structure. As the Cathedral’s Project and Construction Manager, APM was called on to lead the efforts in designing and constructing the green roof. First, a hot fluid applied membrane system with protection board was installed on top of the concrete structure. This was followed by a drainage board with filter fabric, which was then covered with planting medium ranging from 10” inches in lawn areas to 4’ in depth where trees and bushes were to be planted. Due to the immense quantities of growing medium required (over 7,500 CY), a custom mixed plant medium was developed by a prescribed formula in order to provide percolation at a steady rate while sustaining adequate moisture for effective growth and adequate enrichments for plants and lawns. In order to achieve the proper depths of the planting medium, APM developed a detailed quality control plan to not only protect the waterproofing membrane and drainage board, but to avoid over compaction of the medium as well as overworking the material, which can have negative consequences on the materials structure and render high quality material ineffective. To this day, visitors to the Cathedral are surprised that there are such large structures under the lush lawns and extensive trees and foliage.

"In my 37 years of leadership and management experience, the APM team serves as a shining example of "how to do it."
– Major General (Ret.) Stephen T. Rippe, USA, Executive Vice President and Chief Operating Officer Protestant Episcopal Cathedral Foundation

Awards: 2nd Largest Green Roof in DC Metropolitan Area Managed by APM's Accredited Green Roof Professionals