MYTH 4: Modules are the lowest cost, ‘tried and true’ method for green roof installations
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 commercial projects? Can the type and quality of the underlying waterproofing be discounted when modules are used?
Module or tray systems have been commercially available for only 4 years. During this time, they have not gained a foothold in the bustling European green roof market for several reasons:
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.
Removal of modules for whatever reason (e.g., building remodeling, leak detection/repair) is just as difficult and costly as removal of many in-situ installations. In both modular and in situ installations, leak detection and isolation procedures are identical. Moreover, a 2’x 2’ module filled with 4” of mineral media could weigh about 80 lbs when saturated. Selectively moving modules of such weight can be challenging. Both Roofmeadow® in situ assemblies and modularized green roofs can be salvaged and the mature vegetated layers resold.
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 sunlight and ‘wear and tear.’
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).
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 are very reliable, efficient and easy to maintain.
Finally, 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. Therefore, we say: Think (and Plant) Outside the Box: Let the Plants Grow Free!
