Green Roof Materials From the Bottom Up

The dimples on the drainage mat collect some water for plants, while allowing excess to overflow into the drainage system (Image courtesy of www.j-drain.com)

Perhaps surprisingly, green roofs are made with a wide mixture of natural and man-made materials. When you are selecting components for your green roof you may face the challenge of balancing your desire to use completely sustainable materials with your desire for a leakproof and long-lasting job. Some manufacturers sell all components necessary for a green roof, while others specialize in only one or two of the components. The materials described here are examples of the wide variety you will find when you shop for green roof components.

Waterproof Membrane

After structural soundness the next critical element in your green roof is the waterproofing membrane. Some membranes come in fluid form and are applied with brushes. One formulation includes refined asphalts and synthetic rubbers combined to form a thermoplastic material. This membrane adheres to roof imperfections and when applied thick enough it is self-healing to mitigate changes in the roof surface below it, like when cracks develop in a concrete roof. Another type is a reinforced rubber sheet that is rolled out over an elastomeric waterproofing adhesive. Seams are double sealed with more adhesive, rubber sheeting and a reinforced mesh. A third type is a self-adhering, elastomeric sheet that is made of coal tar pitch combined with copolymers and reinforced with polyester fibers. This is rolled out and overlapped at the seams. It does not require a root barrier or protection fabric above it.

Root Barrier

When your waterproof membrane requires a root barrier one option is high density polyethylene (HDPE) sheeting. The seams and edge laps are sealed with electric heat welding equipment to create a long lasting seal. Low density polyethylene (LDPE) is another option that comes in various thicknesses. The material is sold in widths to 20 feet with typical thicknesses of 20, 30 and 40 mil. The higher the mil the higher the tensile strength and the greater its resistance to hydrostatic pressure, punctures and tears.

Protection Mat or Fabric

Depending on your waterproof membrane and your root barrier, and the system you are installing, you may need a protection mat. These mats go on top of the waterproof membrane, or the root barrier if one is used, and they protect the roof from sharp objects. They also may retain water for plant use during dry spells.

Drainage Layer or Mat

This layer of your green roof regulates how water is used and not used. It retains water during dry periods and releases water during rains to help lower the weight on the roof and keep plants from getting too saturated. One option is a mat with a dimpled polystyrene core covered top and bottom with a geotextile material. The top covering resists root growth while still being permeable to water and air, while the bottom covering protects the roof membrane, adds stability and filters fine particles from the water. Other products for this layer use polyamide filaments in a looped pattern that are bonded to non-woven filter fabric.

Growing Medium

Here’s where the plants’ roots are and where all the critical aspects of moisture and plant nutrition come together. There is a wide variety of growing layers, also called planting medium, available. One company uses recycled materials, mineral aggregates and composted materials. Others use composted organic matter, lightweight aggregate and mineral products. In many cases the seeds for the plant variety that suits your location and installation are included in the growing layer when you buy it.

The materials used for green roof components continue to evolve and improve, and depending on your location and the type of system you are installing there may be other components needed.

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