Design Questions

What are the most important factors in designing a green roof?

Charlie says: We consider many interactive factors, balancing considerations for optimal performance, including:

  • Climate, especially temperature and rainfall patterns
  • Microclimates on the roof top
  • Strength of the supporting structure
  • Size, slope, height and directional orientation of the roofing structure
  • Type of underlying waterproofing
  • Drainage elements, such as drains, scuppers and drainage conduits
  • Accessibility and intended use
  • Visibility, fit with architecture, and owner’s aesthetic preferences
  • Fit with other ‘green’ systems, such as solar panels
  • Materials and labor costs

What is the difference between ‘extensive’ and ‘intensive’ green roof?

Charlie says: There is no strict demarcation. Generally speaking, ‘extensive’ green roofs are less than 6 inches deep, and, depending on depth, may support a range of plants. Think sedums (low-growing succulents), herbs, meadow grasses and perennials. Deeper ‘intensive’ systems can sustain complex landscapes, including small trees and even small ornamental ponds and fountains. Very dramatic.

How much does a green roof weigh?

Charlie says: Green roofs vary greatly in weight, depending on depth and the material components. The important measurement is ‘wet’ weight – because that’s when fully saturated fabrics and plants are at their heaviest.

Let’s crunch some numbers. For example, Roofmeadow® green roofs, engineered to be both lightweight and efficient, generally weigh about 6.75 pounds per square foot for each inch of media depth. Thus, the saturated weight of a 3-inch deep system is about 20 pounds per square foot, including a mature plant cover. Mid-range, 5-inch, systems weigh approximately 34 pounds per square foot and are compatible with wood or steel decks. Heavier intensive green roofs generally require a concrete supporting deck. For more in-depth information, the ASTM (Standards E2397 and E2399) has published specific protocols for determining the weight and dead load of green roofs.

Be cautious of systems marketed as light weight; do your homework! Very lightweight media often contains high amounts of organic content, like pine bark, peat moss or coir, resulting in media that is inconsistent with the industry standard – media that could cause structural and compression problems later. While these types of media may appear to support flush aggressive plant growth for the first couple of years, after the organic content biodegrades, the plants begin to struggle and the amount of media is seriously diminished.

How does a green roof affect the conventional roof below it?

Charlie says: Based on German experience, a green roof can be expected to double or triple the life of the underlying conventional roof in several ways: protecting it from human activity and wind-blown debris; shielding it from UV radiation; and buffering temperature extremes, minimizing damage from daily expansion and contraction.

Are modules the lowest cost, tried-and-true method for green roof installations?

Charlie says: Always keeping an eye on the young and dynamic U.S. green roof market, we’ve asked ourselves lately, “What’s with the modular-green-roof craze?” How do modules compare to the traditional German-style green roofs planted in situ? (In situ installation involves assembling the green roof, layer-by layer, directly on the roof.)

Modules may be an effective approach for do-it-yourself enthusiasts and roof gardeners with small projects to create a green roof space, but are modules really the cost-effective green roof solution for large-scale projects? Can the type and quality of the underlying waterproofing be discounted when modules are used?

There are good reasons that modular systems virtually disappeared from the German green roof marketplace 15 – 20 years ago. These reasons include (a) erratic and generally poor long-term vigor of plants, (b) insufficient protection of the underlying waterproofing system, and (c) poor stormwater management performance. The German breakthrough in green roof design was to recognize that plants installed in thin exposed soils can draw strength and durability from the laterally contiguous environment of soil and plants. The classic extensive (or thin) green roof can provide a lush planted surface that is simply not attainable with a collection of independent containers. Let’s address the arguments against modules one by one:

Those who market tray or module systems suggest that, in the case of a leak, the modules can simply be removed until a leak is found. Then the leak can be repaired, and the modules can be replaced. We have never understood this “selling point.” In the case of a leak on a in situ (planted in place) green roof, an electronic survey, like EFVM®, would identify the leak without removal of the green roof or disruption of the cover. While the procedure for electric leak detection is identical for both modular and in situ installations, it’s a lot easier to do in a properly designed in situ green roof. Once the area of the leak is identified, a one to two square-foot area of vegetative cover is rolled back like a carpet and the membrane is repaired. Then, the green roof is rolled back into place. The whole repair takes about 20 minutes, and nothing in the entire endeavor requires any heavy lifting. Compare that to the difficulty of removing a 2’x2’ tray with four inches of lightweight mineral media and plants; a minimum dry weight of 80 pounds. Add a little water and your typical 2’x4’ tray with slightly moist soil would weigh 190 pounds! Removing and shuffling these modules around the roof is not a simple exercise.

Green roofs are universally prized for their capacity to help manage rainfall runoff. These benefits pertain to reduction in runoff volume, runoff rate, and the delay in runoff from roofs. However, recent research has shown that, while modules can be effective in reducing runoff volume (by absorbing rainfall), they are relatively ineffective in reducing runoff rate or lengthening runoff times. Modular green roofs respond like a cluster of tiny adjacent plant containers: Once the water-absorbing potential of a module is exceeded, the runoff drains rapidly to the drains or downspouts. In in situ green roof assemblies, however, water percolating down through the upper layers must then travel long distances horizontally through the root-zone of the vegetation before it reaches an internal drain or downspout.
Under the best circumstances modules can only approach the protectiveness of in situ green roofs.

Unlike modular green roofs, in situ installations completely cover all waterproofing surfaces, especially the critical areas at parapets, drains and penetrations areas, where problems tend to first appear. In fact, modularized green roofs often leave these critical areas exposed to the sun’s UV radiation and ‘wear and tear.’ In Germany, roofs with this type of layout were actually found to need replacement sooner because wear from maintenance crews was concentrated at vulnerable roof locations.

Modules are often proposed as a method of providing an instantaneous cover; however, most modular green roofs systems installed in North America do not arrive at the job site fully pre-grown but rather planted with plugs (either at the nursery, just before installation, or on the roof). Due to the more stressful growing environment in trays, plugs tend to establish a full ground cover more slowly than do plugs in a comparable in situ assembly with the same initial density of plants. When modules are installed in a fully pre-grown state, the cost will exceed that of using pre-grown mats, a sod-like approach to rapidly vegetating green roofs (which avoids the disadvantages of modules). Furthermore, creating a uniform cover with rectangular modular units can prove challenging and labor intensive: Aligning the trays accurately and addressing curves, penetrations, and corners inevitably leads to aesthetic compromises.

Additionally, like other types of planting containers, modules are not immune from edge effects that can retard plant growth. In particular, temperature variations at the tray edges and interruptions in moisture dispersion will tend to stunt plants near these boundaries causing the grid-like pattern of the modules to re-emerge over time.

Due to the need for free drainage of each tray, roots are as much a problem as with trays as with any other green roof. A green roof design rule states, ‘Wherever water can go, the roots will certainly follow.” Consequently, the underlying waterproofing system must be of high quality and invulnerable to roots (or include a root-barrier protective layer). Additionally, over time, the roots can clog the drainage holes, causing the module to flood and die.

Irrigation, when required, is more challenging with modules. Unless spray irrigation is used, each module must be separately irrigated. In situ systems, on the other hand, lend themselves to inexpensive base irrigation methods that minimize water use and water wastage in a very reliable, efficient and easy to maintain way.

Recyling-wise, modules offer little benefit over in situ installations as both Roofmeadow® in situ assemblies and modularized green roofs can be salvaged and the mature vegetated layers resold. Which leads us to the (big) little tray problem . . .

The modules or trays, themselves, introduce an irreducible additional cost. Trays also involve supplemental labor costs associated with double-handling of the materials. As green roof installations in the U.S. become increasingly efficient, and high-rate material handling equipment is brought to bear, the cost burden associated with modularized green roofs will make them increasingly expensive relative to comparable in situ installations.

Modern innovations have resulted in in situ green roofs that overcome the inherent disadvantages associated with the potted-plant roof garden. Modularized installations are essentially aggregations of isolated pots or planters. In situ green roofs, by contrast 1) allow for lateral migration of water and roots and 2) reduce the exposed surface area, thereby minimizing the potential for thermal shock. That’s why we prefer to think outside the box and plant outside the module.

Is attaching all elements of the green roof the underlying structure important?

Charlie says: Based on 35 years of German experience, usually not. Plant roots bind the layers of media and fabrics to create a unified cover, and the plants themselves create enough surface wind turbulence to foil potential uplift. (It’s the converse of what happens on an airplane wing.) If your green roof will be located in an unusually high wind area, such as a high rise building or coastal area, discuss your concerns with your green roof engineer, who will specify appropriate wind stabilization measures. For example, with our concealed ballast design, we’ve guaranteed our green roof installation on Boston harbor’s World Trade Center for up to 94 miles per hour. Not to mention, three of our Florida green roofs have successfully weathered hurricanes.

How steeply pitched can the roof be?

Charlie says: The maximum slope recommended for conventional green roofs is 30 degrees, or a 7:12 pitch. A variety of methods utilizing meshes, slope stabilization panels, cribbing or battens can be used to secure the green roof and prevent media shifting and erosion. We have used all methods successfully on various sloped roof projects. Roofs with pitches greater than 7:12 can be greened; however, special techniques are required. We’re particularly proud of the advanced technology 54-dgree pitch green roof we designed for a demonstration at the Ariel Rios courtyard at the EPA in Washington, DC.

In what climates do green roofs work?

Charlie says: Green roofs have been built most widely in temperate climates, but special techniques allow them to thrive in semi-arid, tropical and even windy coastal areas. We have designed and built green roofs across the continent, from hot, moist Florida to the cold, windy Boston harbor. Here’s a selection of great examples.

Is irrigation essential for green roof success?

Charlie says: Well, no and yes. With thoughtful engineering and appropriate horticulture selection, permanent irrigation is rarely necessary. Only about 15% of currently installed Roofmeadow® green roofs are irrigated. When it is required, due to climate, media or plant selection, the water should be delivered deep under the surface – where the roots will seek it and where it will not be wasted to evaporation. Surface irrigation systems (drip or spray systems) are wasteful and require more maintenance than subsurface irrigation methods.

Is the most common reason for green roof failure inadequate moisture?

Charlie says: Without a doubt, when green roofs fail to thrive, the common culprit is actually too much moisture! It is not uncommon for green roof designers to focus on moisture capture and retention to the detriment of maintaining good drainage and aerobic conditions in the growing media. Unless you have a wetland green roof landscape, that chronic high-moisture content – not the lack of water – will lead to the rapid decline of most green roofs.

Why is drainage important?

Charlie says: Proper drainage ensures that the growing medium will be maintained in an aerated condition suited to healthy plant growth. Basal drainage must also be designed with large rainfall events in mind. The goal is for all rainfall to percolate to the base of the system. The portion that is not absorbed should move ‘underground’ toward roof drains or scuppers. During very large storms, brief episodes of surface runoff may occur, but standing surface water should not. In a Roofmeadow® system, such surface runoff is designed to enter the roof drains at gravel surfaced areas that surround the drain access chambers, channeling excess water out of the system. Surface ponding, even during large storms, is evidence of a poorly designed green roof and can lead to long-term problems.

Can fabrics make good moisture storage layers?

Charlie says: Fabrics offer an effective method for intercepting and distributing water, but distributing and storing water are two very different things. By their very nature, fabrics never can be very effective as water storage components. Don’t be misled by claims of moisture retention fabrics holding many times their weight in water. Fabrics are very light and water is heavy. You can never fill the volume of a fabric by more than 100%! Fabrics also have low capillary potentials relative to most components of green roof media. As a result, water captured in fabrics 1) will be rapidly consumed by the plants and 2) will drain out of the fabrics over time under influence of gravity. What then are fabrics good for? We use them to distribute moisture evenly across the bottom of the vegetative cover, to protect the underlying waterproofing and to prevent the migration of soil materials – all most decidedly NOT moisture storage.