School districts, universities and health care facilities are beginning to appreciate the benefits of green roofs, or roof gardens as they are sometimes called. A green roof manages storm water run-off, reduces cooling costs, mitigates the heat-island effect and softens loud noises.

Though considered cutting edge by today’s standards, roof gardens have existed for thousands of years. Built approximately 2,600 years ago for Amytis, wife of King Nebuchadnezzar II, the Hanging Gardens of Babylon was a roof garden constructed of viscous bitumen with a reed grass “scrim” over stone, overlaid with brick pavers in a gypsum bed. Wrapping the pavers was a waterproof root barrier made of pure lead. Directly above, several feet of soil nourished trees and greenery.

Roof gardens can reduce sound infiltration through the roof
by more than 40 dB (the difference between front seats at
a rock concert and calm day fishing on a lake).

While the fundamentals remain the same, roof garden technology has improved in the past two millennia. Roof gardens have been thrust into the forefront of the green-building movement as a universally beneficial building technique. 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 mediums.

Modern roof garden assembly design often starts with the thickest and most durable membranes recommended for the job. In the case of single-ply roofing membrane systems, additional safety measures, such as armoring field seams with cover strips, are implemented. These single-ply membranes are often fully adhered to gypsum cover boards such as half-inch DensDeck ®. This allows for uniform weight distribution over insulation, especially during the overburden installation. Many membrane systems can last 30 years or more when exposed to ultraviolet light, wide temperature fluctuations and flying debris. Such factors, which shorten the life of any standard roof, are significantly minimized with the use of a roof garden.

A roof garden is only as good as it’s waterproofing, so 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 lies beneath the surface. Another reliable method is to use rubberized asphalt as the primary waterproofing membrane, which needs to be covered with some sort of overburden. These systems typically use more than 200 mils of reinforced, liquid-applied membrane. Once the building is waterproofed, a heavy, 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 that provides a degree of segregation between the waterproofing membrane and the roof garden itself.

In Europe, where roof gardens are more commonly used, this drainage layer can consist of a one- to four-inch layer of smooth stone covered with a filter fabric that prevents migration of fine, growth-medium particles into the drainage system.

On this side of the Atlantic, plastic drainage mats are manufactured with upward-facing cups to hold additional hydration. When the cups are full of rainwater, they overflow and quickly drain excess water to the membrane level. The drainage mats offer tremendous compressive strengths that can equal or exceed 20,000 pounds per square foot and provide a high degree of internal system segregation while reducing assembly weight.

For roof gardens with less than six inches of growth medium, a moisture-retention mat above the drainage layer will hold further storm water. Because of their absorption capacity, moisture-retention mats provide a place in which the roots of the roof-garden plants naturally congregate in their quest for water. This acts as a built-in safety feature; roots that are kept away from the primary waterproofing membrane are much less likely to cause a roof leak.

Recently, manufacturers have taken all of these layers and combined them into a single, flexible drainage composite. This makes installation far less complicated, reduces labor costs for the building owner and simplifies application for the contractor.

It’s Not Dirt
As supplied from most manufacturers, the growth medium used on a roof garden is a highly engineered blend of minerals and organic matter. This is not your father’s dirt (or Nebuchadnezzar’s, for that matter). Before the components are blended, testing is customarily performed to achieve Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FLL) standards. Germany, a pioneer in the roof-garden industry, has set rigid standards to ensure excellent multi-decade performance. Tests establish dry and saturated densities, particle-size distribution by mass, saturated-air porosity and hydraulic conductivity. The objective is to provide a sterile, soil-less medium that is lightweight, yet able to hold large quantities of rainwater while allowing the plants’ roots access to sufficient oxygen, even when saturated. The FLL standards also help to prevent compaction of the growth medium.

Roof garden plant species must be hardy in order to grow in an environment that is rarely shaded, drought-like in many locations and often exposed to higher winds than found at ground level. Roughly 80% of all roof gardens in the United States are populated solely by sedum, which exists in more than 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.

Planting is done in a variety of ways. Many roof gardens are propagated with standard plugs. This involves inserting small plants with roots grown in a plug of dirt into the surface of the growth medium. Another method is to use pre-vegetated sedum mats or tiles, which are best described as “sedum sod.” Lay the sod on the surface, apply water and the building now has full vegetative coverage, which eliminates much of the maintenance required in the first year of a roof-garden installation. An increasingly common method involves spreading sedum cuttings over the surface of the growth medium. The cuttings will root quite readily if kept somewhat moist and given access to light.

Options for Plant Material
For the aesthetically inclined, there are options other than sedum. Dozens of plant varieties, ranging from Armeria maritima (seathrift) to Thymus vulgaris (common thyme) to Achillea millefolium (yarrow) provide deep textures and dazzling colors throughout the growing season.

There are not many products on the market today that perform as many functions as a roof garden. The most common functional reasons for a roof garden installation include:
• Storm water management
• Reduced cooling costs
• Reduced heat-island effect (in urban areas)
• Longer roof life expectancy (twice as long as an exposed waterproofing system)

What makes roof gardens so effective at keeping things cool during the summer are the plants. Basically, they are water pumps that operate at relatively high pressure and low volume. The plants’ roots sip water and release it as water vapor, mostly on the undersides of the leaves through their stomata (plants’ equivalent of human pores). Whenever a liquid changes phase to a vapor, heat energy in the surrounding environment is absorbed. Water is particularly good at this; for every pound of water moved through the roof-garden plants, 8,000 BTUs of heat energy is absorbed. As a result, during hot summer days, the roof membrane temperature is typically 5 to 10 degrees cooler than the ambient air temperature. This relieves the work required by air conditioners and holds everything on the rooftop at a reasonable temperature.

Sound reduction is a strong consideration for schools, hospitals and other institutions. Roof gardens can reduce sound infiltration through the roof by more than 40 dB (the difference between front seats at a rock concert and calm day fishing on a lake). Roof gardens are particularly good at reducing those hard-to-attenuate, low frequency noises. Building occupants near an airport flight path or train tracks will enjoy this benefit.

Roof gardens help the environment as well. Plants take in carbon dioxide and release oxygen. Rain with impurities (acids generated from oxides of sulfur and nitrogen) becomes cleaner on its way to the local sewer or watershed as it trickles down through the layers of mineral-based growth medium. Some structures even grow food on their roof gardens. Other benefits include additional usable space on top of the building in the form of landscaped plazas and courtyards.

One potential disadvantage to roof gardens is the weight they add to the building structure. Thankfully, systems have become much lighter over the years. Where 50 pounds per square foot, fully saturated, was once common, it is now not uncommon to see systems weighing 12 to 22 pounds per square foot, fully saturated.

Gardens on Sloped Roofs
The building shape is proving less of a hindrance to the emergence of roof gardens throughout the world. Where once the roof had to be relatively flat, steep-slope roofs are now more commonly greened. Forty-five-degree angles have been successfully turned into beautiful rolling landscapes and even fully vertical green walls have begun to emerge.

Roof gardens are becoming far more common within this country and their numerous benefits more widely appreciated. Advances in technology have made these green gems in the sky more accessible and affordable to those who want to reduce their environmental impact and reap the many benefits that roof gardens offer.

With a new conventional roof, the owner is, at most, thankful that the building will stay dry for a good long while. In contrast, the smile on the face of a building owner viewing a new environmentally friendly ecosystem covering the building is a pure and wonderful moment. History and modernity collide with today’s advanced roof-building techniques.

Zach Williams is the roof garden product manager for Carlisle SynTec.