By William Kooncea and Mark Anaterb, Dow Chemical Co.
Abstract
Green roofs are becoming increasingly popular in urban areas. At the simplest, a green roof is usually a flat or slightly inclined roof on which plants are grown in a thin layer of soil.
As well as the basic roof function — shielding the building from cold and heat, rain and snow — green roofs have additional benefits. These include conservation of energy, extension of the service life of the roof, reduction in storm water run-off and flow rates, an overall cost reduction over the life of the roof and a restoration of ecological and aesthetic value to open spaces in urban areas.
To capture all these benefits, it is vital that green roof systems have a high-performance waterproofing membrane which protects the underlying substrate from ingress of water and any subsequent short- and long-term damage.
Among the materials being used, the Verdiseal polyurethane-based monolithic membrane offers a high level of performance. It is easy and fast to apply and has good performance characteristics, such as dimensional stability over a wide temperature range, low-temperature flexibility, seamless sealing, crack-bridging ability, rupture resistance, and low VOC (volatile organic compound) and low noxious odour installation.
Introduction
Green roofs, also referred to as eco-roofs, landscaped roofs, living roofs or roof gardens, are defined as either partially or completely vegetated. The concept dates back as far as the Hanging Gardens of Babylon, constructed around 600 BC, but modern designs developed in the 1960s in Germany.
Today green roofs account for 10 percent of all roofs in Germany, and a number of European countries and cities strongly encourage new buildings to include them. They are relatively new to North America, but growing quickly in popularity as property owners and municipalities see the advantages over alternatives such as asphalt and single membrane rubber sheets.1 A distinction must be drawn between two types of roof construction labeled as green.
Traditional roof gardens, which use relatively thick layers of soil (12 inches, 30 cm, or more) as a growing medium, are called “intensive†roofs because they require regular maintenance, irrigation and fertilising, just like a conventional garden. They can sustain grass, flowers, shrubs and even trees, but also require strong supporting structure for the added weight of the soil and plantings. This makes them expensive to build and an ongoing expense to maintain.
Pitched roofs made from sod have been fashioned for centuries in Scandinavia and the British Isles, and do not require as much maintenance since the pitch reduces the risk of pooling water and fewer drainage layers are needed. But they still require thick soil underlay for the grass to survive.
Most modern designs for green roofs are another type, labelled “extensive,†which use relatively thin growing media and require far less maintenance than the traditional approach.
They produce lower roof loading, which also reduces the structural requirements and makes them less expensive to build or retrofit onto existing buildings.
They may be decorative, park-like in utility, or strictly function as a roof. Most customers prefer to use their systems as any other roofing material, with nothing more than annual inspections for damage, pruning potentially troublesome plants, and checking drainage.
Extensive roofs are generally flat or gently sloped.2 Several factors are driving a move toward using green roofs in North America. Roof service life can be significantly extended over conventional roofing material since the waterproofing layer is not exposed to sunlight — the main limiting factor in service. The added insulation provided by plants and soil, the ability of the vegetation to reflect sunlight rather than absorb it and create heat, and the tendency of plants to respire moisture keeps radiant heat away from the building and lowers heating and cooling costs.
The same factors reduce the “urban heat island†effect, keeping ambient temperatures down in summer months and improving the microclimate around the building. Storm water is more easily managed when plant media absorb the majority of precipitation, rather than allowing most of it to drain into storm sewers as an ordinary roof does.
Another factor is that national and local governments are increasingly considering and passing regulations (energy efficiency standards, storm water taxes) and incentives (cash payments, property tax breaks) for building owners to install green roofs.
The US Green Building Council’s Leadership in Energy and Environmental Design (LEED) certification has become a major driving force.
This points-based system rates buildings on energy and water efficiency, emissions reduction, environmental quality and renewable material use. Under this system green roofs contribute significantly to a positive score.
More and more, environmentally aware customers and communities are demanding better living environments, more sustainable building practices, and more ecologically responsible development. And they want all this without sacrificing comfort or massively inflating costs.
Green roofs can also be good for a firm’s public relations. A particularly large or welldesigned example can bring favourable media attention and demonstrate goodwill toward the local community.3,4,5,6 Trends in the industry include development of lighter-growing media, allowing retrofit of green roofs onto existing buildings to become more feasible. Vegetation can be customdesigned for aesthetics. Native and threatened plant species are being incorporated to aid in local biodiversity. Photovoltaic arrays are often incorporated to decrease the external energy supply required for the building. The industry was formerly focused on commercial buildings but is now moving more toward the residential market, as builders of houses, apartments, condominiums and even whole developments become interested in the technology.7
Green roof design
The concrete or insulated steel roof deck is first covered with a waterproofing membrane to keep out rain and irrigation water. Next a root barrier goes down to prevent plant roots from penetrating the membrane and compromising the waterproofing (this layer is not needed with the Verdiseal system). A drainage mat allows excess water not taken up by the growing media to flow away from the plantings, and stops fine particles from washing into drains and blocking them. The water-retention layer soaks up precipitation and serves as the main water source for the plantings.
A fabric mat impregnated with seeds or plants is laid over the water retention layer, and growing media, usually specially formulated compost, goes on top.
Because extensive roofs have relatively thin growing media, two to four inches typically, and depend on rain for most if not all of their water, the types of plants used on them are limited.
Suitable plants must have shallow root systems, high drought resistance, and grow low to the surface.
The most common plants are sedums, also called stonecrops. Botanically these are succulents, meaning they store water in their leaves, helping them to survive in the dry, harsh conditions on a roof. There are about 400 varieties of sedums, and most species have five-leafed flowers, each of which has a characteristic colour and appearance, allowing green roofs planted with them to have a pleasing variety rather than a constant colour and texture.8 Sedums are established in a mat made of natural or man-made fibre with enough growing media to allow the plants to develop to maturity. The growing media is usually a mixture of mineral aggregate and organic material designed to foster the initial establishment of the plants and encourage the development of a root mat that knits the top layer together. The water retention layer can be as simple as a layer of old towels or as high-tech as a geotextile membrane.
The drainage mat is an engineered plastic and fabric part; the fabric filters water runoff to keep the growing media in place, and the plastic mesh allows the runoff to fall away from the top layers and toward drains. A root barrier, usually a geotextile sheet, goes directly over the main membrane and prevents plant roots from penetrating the waterproofing and causing leaks.9
Choice of waterproofing is critical
The choice of waterproofing material is critical to the success of a green roof. Although the same materials used in these applications – bituminous coatings (polymer-modified asphalt), rubberised asphalt sheets, single membrane EPDM (ethylene-propylene diene monomer) rubber, PVC (polyvinyl chloride) membrane and spray-applied coatings – may also be used as roofing, the requirements for water resistance are significantly more demanding on a green roof. Water will usually, if not always, be present on top of the waterproofing, unlike with ordinary roofs where water runs off or evaporates quickly. Factors to consider when choosing a material include cost, level of performance, compatibility with other components, and the capabilities of the contractor.
Bituminous coatings are hot-applied with a mop, roller or brush, and set up within a few hours. They are fairly easy to use, inexpensive and have a long track record as roofing, but they require open flames to keep them liquid during application, are prone to cracking and delaminating over time, and can be easily damaged by walking over them.
Thermoplastic and thermoset rubber membranes are supplied as rolls which are cut to fit the roof deck and around any obstructions or penetrations, and either adhered or welded together to form a uniform layer of protection across the roof. They are highly effective as waterproofing and require no special equipment to install, but must be installed with great care since poorly joined seams, terminations and fasteners can compromise the seal.
Bituminous and roll roofs require flashing, detailing and sealing around joints and roof penetrations, must be protected against puncture during and after construction with cement cover boards, and may have restrictions on the load per unit area they can safely support without losing integrity.
Spray-applied coatings can be one-part or two-part materials. They are more durable than bituminous coatings, and form a seamless membrane with considerably less time and effort than roll materials. To function adequately they must be applied to a clean, dry substrate and the material thickness must meet the manufacturer’s minimum recommendation.
Rigid coatings such as epoxy are sometimes used. They are easy to apply, have good chemical resistance and require only thin films, but because they cannot move with the deck as temperature changes, they are prone to cracking.
Elastomeric coatings offer the same seamless, fast and fully bonded application, while accommodating the movements of the roof deck and bridging cracks that typically form with temperature cycles.
They require thicker film builds than rigid coatings, special application equipment and skilled contractors, and do not have the long case histories of other technologies, but the list of advantages shown in the boxes on p29 and below right more than compensates.
The Verdiseal approach
When using Verdiseal, first a primer is laid to seal the substrate, preventing moisture from creating hydrostatic pressure and delamination, and also preventing any air pockets within the substrate from causing blisters or pinholes in the subsequent coating. The primer also penetrates and reinforces the concrete, and creates a chemically friendly surface on which the subsequent coating can bond.
Spray elastomer is applied next to provide the main waterproofing layer. A wear coat, in conjunction with aggregate, provides an antiskid surface and eliminates the need for protection boards for the rest of the installation.
The top coat seals the aggregate from the wear coat, provides the final colour if it is to be left exposed, and can be made UV stable for areas exposed to sunlight.
Summary
Green roof technology is well-established in Europe, and its inherent advantages for energy efficiency, longevity, storm water management, ecological responsibility and improved quality of life are making it more popular in North America. Although the final result appears “natural,†they are highly engineered systems designed to deliver good weather protection as well as the other expectations of a green roof.
The key to making the technology work is the waterproofing layer, which must function flawlessly from day one to avoid structural damage to the building.
Dow’s Verdiseal system gives multilayer protection as a green roof underlay against water infiltration, chemical attack, root damage and thermal cracking. Compared to alternatives such as hot-applied asphalt coating and single-ply membrane it offers the advantages listed in the box below.
Application technology
Preparing the roof
Before any application takes place, the roof area must be inspected. The age, strength and moisture content of the concrete substrate must be known.
Defects such as large cracks must be repaired. The schedule of trades working on the site needs to be coordinated to avoid conflicts and potential contamination of the surface. Weather is always a factor in a roof installation, especially so when using the Verdiseal coating system since the temperature must be between 40-104°F (4-40°C) and at least 6°F (above the dew point at the start of the process).
Precipitation will interfere with the cure or adhesion, and wind will carry overspray away from the job site onto neighbouring structures and automobiles.
Access to the site, storage and safety risks need to be assessed.
The substrate must be cleaned and roughened as a first step. Any old coatings, salts, growths of mould, efflorescence and laitance must be removed. To increase surface area and give the concrete a “tooth†for the coatings to anchor, it should be blasted with abrasive media or water jet, or abraded with a scabbler, scarifier or grinder in accordance with ASTM D4259 standards for concrete, to give a 1/8 inch peak-to-valley surface profile.
Primer application
The concrete primer for Verdiseal is sold under the trade name Traffideck E4. The Part A epoxy resin and Part B hardener are supplied as proprietary kits of 10 kg total, and mixed in batches of about three gallons at a volume ratio of two parts epoxy resin to one part hardener. It is applied usually by roller at 100-150 microns (4-6 mils) wet/dry film thickness. Before the primer dries, quartz aggregate is broadcast onto the surface using a shovel and stiff broom to provide mechanical interlock for the spray elastomer to come.
Primer coverage is about 350 ft2 (32.5 m2) per kit, but may be lower depending on the roughness and porosity of the substrate, and recommended aggregate coverage is ¼ lb/ft2.
At temperatures above 6°C (43°F) the primer should gel in about 30 minutes, set after 4 hours cure time, and can go 24 hours without requiring reapplication. For applications where cure speed is critical or low temperatures are encountered (40- 60°F), Traffideck E4 Fast Cure is offered with about half the gel and set times. For uncoated, ungalvanised steel surfaces Traffideck P2 polyurethane primer can be used, and for other metal substrates (copper, aluminium, galvanised or stainless steel) Cilbond 41 (product of CIL) solventbased polyurethane is recommended.
Waterproofing layer
The main waterproofing layer is Flex 2000SG, a twopart polyurethane/urea hybrid. The naming convention used is that of coatings, with the polyol mixture labeled as part A and the isocyanate component labeled as part B. The materials are mixed by direct impingement under high pressure and are applied as atomised particles which flow together and form the coating.
The resulting material has both urea and urethane functionality. The material can gel very quickly with no additional energy required, allowing it to be applied even at temperatures down to freezing. Flex 2000SG offers high flexibility, low VOC and a good balance of physical properties. For application on concrete the most notable advantage is its crack-bridging ability. With 320 percent elongation at break in tensile testing, it can accommodate movement along a concrete joint of an inch or more, although stripe coating is usually recommended for joints between separate slabs and around penetrations, and control joints still require filler strips.
Dynamic cracks which form with time and temperature cycles in the concrete will not cause surface cracks in the elastomer. Overburden like soil and plantings on a green roof will not compromise the integrity of the membrane, and chemical agents like acid rain, pollutants and fertilisers used on intensive roofs will not damage it, so there are no restrictions in the types of overburden that can be used. Flex 2000SG will resist root penetration even from aggressive plants, eliminating the need for a root barrier.
The components for Flex 2000SG are supplied in drums and applied via high-pressure impingement mixing onto the primed substrate at a 1:1 volume ratio of iso to polyol. Flex 2000SG gels within 5-10 seconds, gives a tack-free cure within one minute, and sets for light duty after 2 hours.
For ease of inspection, the polyol is coloured yellow and the isocyanate is coloured blue. When properly mixed and cured the material should have a characteristic green colour. The target film thickness for Flex 2000SG is 2 mm (80 mils) with a theoretical coverage of 2050 ft2 (190 m2) per 420 kg (924 lb) kit.
The recoat window for Flex 2000SG is 2-24 h with no additional surface preparation. For 24-48 h recoat, sanding and priming with Traffideck P2 polyurethane primer is recommended.
Wearcoat is next
The third step in the coating process is Grip 1000, a two-part polyurethane which is supplied in 15 kg kits, mixed at 2:1 polyol resin to isocyanate hardener by weight, and applied by squeegee over the Flex 2000SG, then back rolled to give an even coating. The top coat ensures no pinholes remain in the waterproof membrane, and allows the acceptance of another broadcast of quartz aggregate.
Aggregate here serves two functions, providing an anti-skid surface for any subsequent work on the roof, and toughening the surface so that protection boards are not necessary to prevent the main waterproofing layer from being punctured by workers walking on it, equipment set on it, or tools dropped on it.
Recommended film thickness is 2 mm (80 mils) with aggregate included, with a theoretical 265 ft2 of coverage per kit. Grip 1000 gels in 15 min at 20°C and sets within 2 h of mixing. Optionally the aggregate can be sealed in place with another application of Grip 1000, or for added UV stability the aliphatic urethane ASP Topcoat can be used.
Before overburden is installed the completed system should be allowed at least 8 hours to finish curing. Pull-off adhesion can be checked using ASTM D4541, with a final value of at least 150 psi desired. A water test according to ASTM D5957 should be performed as well by ponding water to a depth of 2 inches and allowing it to stand on the roof for 24-48 hours. Any leaks must be repaired immediately using Verdiseal HA repair material.
| Material properties of Flex 2000SG | ||
|---|---|---|
| Component Properties | ||
| Part A | Part B | |
| Appearance | Yellow Liquid | Blue Liquid |
| Viscosity at 77°F | 350-650 cps | 1500-2500 cps |
| Specific gravity at 77°F | 1.01-1.03 | 1.11-1.13 |
| Film properties | ||
| Shore hardness | 85°A | |
| 100% modulus | 650 psi | |
| Tensile strength | 1740 psi | |
| Elongation at break | 320% | |
| Angle tear strength | 228 lb / inch | |
| Cold flex temperature | -40°F | |
| Density | 0.95-1.0 g/ml | |
| Adhesion to concrete | > 435 psi on Traffideck primer | |
