More green roof assemblies are being installed every year. However, a green roof assembly (GRA) is not simply a roof assembly covered with plants or landscaping. It's actually a complex waterproofing assembly that is landscaped to some extent.
Properly designed and installed, a GRA is a reasonable and effective way to enhance the “greening” of a building. It can be highly visible and display the building owner's desire to be part of the sustainable building design movement. But no matter how sustainable or beautiful a GRA may be, if the green roof leaks, the building owner and occupants will not enjoy the benefits of the GRA.
A GRA generally will have standing water within it and upon the membrane layer. To protect the concrete deck from moisture attack, the membrane within a GRA should be a waterproofing membrane.
Because a building owner is likely to hire an architect or consultant to specify his GRA, it is incumbent upon the specifier and installer to be knowledgeable about proper materials and components for an effective project.
A roof assembly consists of a roof deck, membrane or primary roof covering, and roof insulation designed to weather-proof and sometimes improve a building's thermal resistance.
A waterproofing assembly consists of a structural substrate or deck; membrane; and protection, drainage, and insulation course designed to waterproof a habitable space.
A green roof assembly consists of a structural deck, waterproofing system (membrane and associated components), and overburden with plantings. It also has a waterproofing membrane to prevent moisture from entering a building or space.
A GRA should incorporate a robust waterproofing membrane, membrane protection, stability in a wet environment and full adhesion to the substrate. It does not include plaza deck systems, individual planters, or a water protection system that can be identified as a roof assembly or waterproofing assembly. Planters and moveable, planted containers are considered green coverings.
The components used on a GRAare: structural deck, primer, waterproofing membrane, protection course, root barrier, drainage layer, moisture-resistant thermal insulation, aeration layer, moisture retainage layer, reservoir layer, filter fabric (or geo-textile—a combination drainage layer and filter fabric), and an engineered soil-based growth medium with plantings.
A GRA should incorporate a fully adhered waterproofing membrane. This reduces the potential for leaks, and if leaks do occur, reduces the ability for moisture to travel horizontally, making it easier to locate the leak. The waterproofing membrane must withstand a moist environment for a prolonged period.
The substrate for the waterproofing membrane should be sloped to provide positive drainage. Flat stock or tapered insulation should be installed under the membrane to provide thermal resistance or positive drainage for green roof assemblies. When an insulation layer is included, locate it above the waterproofing membrane. This insulation should be moisture-resistant. If insulation is not included, an aeration layer is not needed.
The protection course and root barrier must be capable of protecting the waterproofing membrane during subsequent construction and resisting root penetration, respectively. The drainage and moisture retaining layers must provide adequate flow characteristics and retaining ability to support the growth of the plantings in the growth medium.
The specifications for the deck, waterproofing membrane, protection course, root barrier, drainage layer, insulation, aeration layer, moisture retaining layer, reservoir layer, and filter fabric or geotextile are dictated by the GRA type (intensive versus extensive) and plantings chosen. The design of these components is based on dead and live loads, such as expected water retainage (ice and snow loads), expected water drainage, and root structure.
From a waterproofing standpoint, one of the most important elements of a successful GRA is a structurally sound deck.
For a concrete deck, form release agents and concrete-curing compounds must be compatible with the waterproofing materials being used or must be removed from the concrete's surface. Honeycombs and other voids in the concrete substrate must be filled with a non-shrinking concrete patching compound. Concrete fins or projections should be removed to provide a smooth surface.
Concrete substrates should be properly cured, and the surface should be dry before installing waterproofing materials. Hydration must occur in concrete substrates to the degree that the surfaces become dry enough to accommodate applying waterproofing membranes. If a suitable surface cannot be obtained within a reasonable time, direct membrane adhesion should be postponed. An adhesion test is recommended to determine appropriate membrane adhesion.
Several waterproofing membranes are appropriate for green roof assemblies. The following GRA design and application guidelines are sound and time-proven and apply throughout the United States.
Coal-tar Built-up Membrane
Coal-tar built-up waterproofing membranes are composed of alternating layers of coal tar and reinforcing ply sheets. Four-ply minimum coal-tar membranes should be used for green roof assemblies. Coal-tar built-up waterproofing membranes should not be installed when ambient temperatures are below freezing. The completed membrane should not be exposed to prolonged periods of sunlight before installing overburden to prevent slipping and softening of the bitumen.
Coal tar used in waterproofing applications (ASTM D 450, Type II) differs from coal tar used in roofing applications (ASTM D 450, Types I), in that waterproofing coal tar has lower softening-point and flash-point temperature. Therefore, it has somewhat different physical properties.
Prime the surface with coal-tar primer. Reinforce all inside and outside corners with two 12-inch-wide plies of reinforcing material centered on the corner. Apply a minimum of four plies of reinforcing felt. Apply the coal tar in a continuous, firmly bonding film and with pressure to assure good adhesion. For vertical flashing applications, orient the reinforcing material vertically in workable height lifts, fastening the reinforcing at the top of each course. Flash all penetrations with two additional plies of reinforcing material.
Hot-fluid-applied Polymer-modified Asphalt Membrane
Hot-fluid-applied polymer-modified asphalt waterproofing consists of refined asphalt, synthetic rubbers, and extenders. Fabric-reinforced membranes with nominal 215 mils (5.4 mm) minimum thickness should be used. Hot-fluid-applied polymer-modified asphalt membrane waterproofing should not be installed when ambient temperatures are below freezing. Hot-fluid-applied, polymer-modified asphalt membrane waterproofing cannot be left exposed to prolonged periods of sunlight and must be covered to protect it from ultraviolet light.
Prime the surface with asphalt primer. Reinforce inside and outside corners, cracks, and construction joints with a 6-inch-wide piece of reinforcing sheet embedded in hot-liquid-applied, polymer-modified asphalt. Flash larger cracks, expansion joints, and similar details according to the manufacturer's recommendations. Apply the hot polymer-modified asphalt as a continuous coating to the required thickness. Embed the reinforcing fabric and apply additional hot polymer-modified asphalt. Exposed vertical surfaces need additional protection when other finish materials are not used.
Polymer-modified Bitumen Sheet Membrane
Polymer-modified bitumen waterproofing sheet membranes are composed of polymer-modified asphalt and one or several layers of reinforcing material. The polymer modifier extends the low-temperature flexibility and improves the high-temperature properties of the membrane sheet. Two-ply minimum polymer-modified bitumen sheet membranes should be used. Most polymer-modified bitumen sheet waterproofing membranes should not be installed when the ambient temperature is below 40° F. Heat-fused membranes may be applied at lower temperatures.
Polymer-modified bitumen waterproofing sheet membranes may be installed by one of three techniques depending on the composition of the modifier used. APP-modified sheets are either heat-fused or installed in cold adhesive.
Heat-fused means to install the sheet by heating the underside with a propane torch or other heating device, melting the polymer-modified bitumen on the bottom side, and adhering the sheet in the molten material. SBS-modified sheets are either heat-fused or installed in hot asphalt or cold adhesive. Prime the surface with primer. Reinforce inside and outside corners, cracks, and construction joints with a 6-inch-wide piece of reinforcing sheet.
Butyl Rubber Membrane
Butyl rubber waterproofing membranes consist of factory-fabricated sheets of reinforced butyl rubber. Sheets normally are 60 mils, 90 mils or 120 mils (1.5 mm, 2.3 mm or 3.0 mm) thick. It is recommended that minimum 90 mil (2.3 mm) thick, reinforced butyl rubber membranes be used for green roof assemblies. Do not install butyl rubber waterproofing membranes when ambient temperatures are below 40° F. Butyl rubber membranes only should be used in conjunction with adhesives and tapes recommended by the membrane manufacturer.
Lay the membrane on the substrate or a flat surface and allow it to relax for at least 30 minutes before use. Clean any membrane surfaces to be bonded of loose dust, dirt, and release agents as the manufacturer recommends. Position the membrane on the substrate without stretching it in a manner that minimizes voids, wrinkles, and entrapped air. Clean overlapping areas between sheets, and join the sheets with recommended adhesives and tapes.
EPDM waterproofing membranes consist of factory-fabricated sheets of ethylene propylene diene monomer. Sheets normally are 45 mils to 60 mils (1.1 mm to 1.5 mm) thick, with special thicknesses of 90 mils to 120 mils (2.3 mm to 3 mm) available. Minimum 60 mil (1.5 mm) thick reinforced EPDM membranes should be used. EPDM waterproofing membranes should be adhered to the substrate, not mechanically attached or loose-laid. EPDM waterproofing membranes generally should not be installed when temperatures are below 40° F. EPDM membranes only should be used with adhesives and tapes recommended by the membrane manufacturer. EPDM waterproofing membranes should not be used in contact with bituminous cements or mastics.
Lay the membrane on the substrate or a flat surface and allow it to relax for at least 30 minutes before use. Clean membrane surfaces to be bonded of any dust, dirt, and release agents.
Polyvinyl Chloride (PVC) Membranes
Polyvinyl Chloride (PVC) waterproofing membranes consist of factory-fabricated sheets of reinforced polyvinyl chloride. Sheets are at least 80 mils (2.0 mm) thick. Reinforced, 80 mils (2.0 mm) thick minimum PVC membranes with stripped-in laps should be used. PVC membrane waterproofing should be adhered to the substrate, not mechanically attached or loose-laid. Do not install PVC waterproofing membranes when temperatures are below 40° F.
The completed membrane should not be exposed to prolonged periods of sunlight before covering or backfilling to prevent slipping. PVC materials are not compatible with polystyrene insulation products and certain bitumen-based products such as coal tar.
Lay the membrane on the substrate or a flat surface and allow it to relax for a minimum of one-half hour before use. Clean membrane surfaces of any dust, dirt, and release agents.
One- and Two-component, Fluid-applied Elastomeric Membranes
Fluid-applied elastomeric waterproofing membranes consist of high-solids content polyurethane elastomers in liquid form. Some products are made solely of polyurethane, while others are modified with coal tar or asphalt. There are different grades of product for horizontal, vertical, and special applications. Fabric-reinforced, one- or two-component, fluid-applied elastomeric membranes should be used for green roof assemblies. Fluid-applied elastomeric waterproofing membranes should not be installed when temperatures are below 40° F.
The material cures to form a monolithic waterproof membrane. Single-component elastomers eliminate the need for mixing products at the jobsite. Two-component materials require jobsite mixing but cure faster in cooler weather and in higher humidity than one-component materials.
James R. Kirby is senior technical director of the National Roofing Contractors Association. He presented this paper at the Greening Rooftops for Sustainable Communities conference in May 2006. E-mail him firstname.lastname@example.org, or visitwww.nrca.net.
To Learn More…
Green Roofs for Healthy Cities (GRHC) is developing a course on Waterproofing and Drainage as it relates to green roof design and implementation. The course will be launched at the Fifth Annual International Greening Rooftops for Sustainable Communities Conference, April 29–May 1, 2007, in Minneapolis. Visit http://greenroofs.org/minneapolis for more information.