Roof gardens are at the forefront of the green building movement as a beneficial building technique

Although they are commonly considered cutting-edge by today's standards, roof gardens have actually existed for thousands of years. The Hanging Gardens of Babylon were built approximately 2,600 years ago for Amytis, wife of King Nebuchadnezzar II, who desired a roof garden plaza at her home. Construction included a waterproof root barrier made of pure lead topped with several feet of soil-nourished trees and greenery sourced throughout Persia. Unfortunately, limited history does not allow us to know whether or not the King received any LEED points for the water-resistant lead, which was likely sourced within 500 miles of the building site. While the fundamentals remain the same, roof garden technology has drastically improved during the past two millennia. In 2009, roof gardens have been thrust into the forefront of the green building movement as a universally beneficial building technique that ranks among or beyond other energy-conscious building methods. Where there were once roofing materials that would make the EPA wince, there are now recycled materials, specially designed drainage mats, and stringently tested growth media. A roof garden is only as good as its waterproofing, and a true "belt and suspenders" approach is taken to roof garden waterproofing design. The heaviest of single-ply membranes, ranging from 80 to 145 mils in thickness, usually lie beneath the surface. Another reliable method is to use rubberized asphalt as the primary waterproofing membrane, which, by nature, needs to be covered with some sort of overburden. These systems typically utilize in excess of 200-mils of reinforced liquid-applied membrane. Once the building is waterproofed, a non-woven felt is applied to protect the primary waterproofing membrane from abrasion and puncture damage. Above the protection fabric, there is a drainage layer, usually consisting of a one-inch thick network of cups and drainage holes that provides a degree of segregation between the waterproofing membrane and the veneer of growth media and plants. Shallow roof gardens tend to use a moisture retention mat above the drainage layer. This holds additional stormwater and provides a place for roots to congregate away from the waterproofing membrane. Recent manufacturing developments take all of the above layers and combine them into a single, flexible drainage composite. Doing this makes installation far less complicated, reduces labor costs to the building owner and simplifies application for the contractor, benefiting everyone involved. As supplied from most manufacturers, the growth media used on a roof garden is a highly engineered blend of minerals and organic matter. This isn't your father's, or Nebuchadnezzar's, dirt. Before being carefully blended, a gamut of tests is performed to establish specific and proven characteristics. The objective is to provide a sterile, soil-less medium that is lightweight, yet able to hold large quantities of rainwater and nutrients while allowing the plants' roots access to sufficient oxygen, even when in a saturated state. The most common species of plant used on roof gardens is sedum. These hearty plants must live in an extreme environment that is rarely shaded, drought-like in many locations and often exposed to higher winds than nature intended for their ground-dwelling existence. Roughly 80% of all roof gardens in the United States are populated solely by sedum, which exists in over 500 species worldwide and comes in every color of the rainbow. This succulent groundcover can live for many weeks without water and requires little to no maintenance. For the building owner, there are few products on the market that perform as many functions as a roof garden. Common reasons for a roof garden installation include improved stormwater management, reduced cooling costs, reduced heat island effect in urban areas, sound reduction for the building's occupants, additional outdoor space and a roof life expectancy twice as long as an exposed waterproofing system. Advances in technology have made these green gems in the sky more accessible and affordable to all who want to reduce their environmental impact and reap the many benefits that roof gardens offer. In an age where building owners are more commonly utilizing green building practices, it is refreshing to see a revival of one of the oldest techniques known. History and modernity collide with today's advanced roofing techniques - make mine green. Zach Williams is roof garden manager for Carlisle SynTec, Carlisle, Penn.