Each roof is unique. The success of a greenroof depends on the thorough understanding of all components and also the specific needs and conditions of each particular site. All greenroofs contain basic components, and generally have a layering system that begins with the roof deck, followed by a waterproofing membrane, a drain layer, the growing medium and, finally, the plants.
Integration of all layers and selection of appropriate components are critical to the greenroof’s sustainability and performance.
Before the construction begins, it is imperative a licensed structural engineer or architect conduct a structural analysis. Most importantly, they will determine the amount of weight the roof can support.
Waterproofing Membrane:
Correct and meticulous application of the waterproofing membrane is essential to the viability of the greenroof. The ultimate priority is to keep the building watertight. The waterproofing system may consist of a liquid-applied membrane, a specially designed single-ply sheet membrane, or a built-up roof system consisting of multiple layers. To prevent root damage to the waterproofing membrane, some manufacturers incorporate a root-repellant agent in a roof membrane's formulation and others offer a physical root barrier.
Drain Layer:
The drainage layer is installed over the waterproofing membrane to remove excess water and prevent saturation of the growing medium. In some cases, the drainage layer can be designed to retain water and serve as a reservoir for irrigating plants between rainfalls.
The drainage layer is applied over the entire roof area. It forms an extremely stable and pressure resistant sub-base, and sometimes serves as an additional thermal air layer.
A function of the drain layer is the application of a root resistant filter fabric. It is placed directly atop the drain layer for maximum, long-term success of the drainage layer.
The root resistant filter fabric prevents roots from affecting the efficiency of the drainage/aeration layer. It also acts as a filter fabric, preventing fine particles from being washed out of the substrate soil, ensuring efficiency of the drain layer.
Growing Medium:
The growing medium supports plant growth. Good soil composition is vital to the success of plant growth. Soil cannot contain silt that would clog the filter fabric; it must provide a permanent means of supplying internal aeration to prevent compaction of the mix, drain adequately, be able to supply or absorb water and nutrients for the plants to use over time, and it must retain much of its original volume. Unless cost is a great factor, single-source engineered soil pre-mixes are highly recommended.
Vegetation/Plants:
Vegetation (green) is the most visible layer of the greenroof. Plants add aesthetics and ultimately determine the success or failure of the greenroof.
Plants should be selected that have a low growth height, yet grows, and spreads rapidly. Ideally, plants should have a high drought tolerance, without special needs for irrigation, nutrition, or high maintenance such as pruning. Also, avoid plants with taproots, but instead choose fibrous root systems that will not damage the roof membranes. Finally, avoid plants that generate airborne seeds, as these could invade other landscape plantings.
An experienced horticulturist should be involved in plant selection.
