Roofing Systems
Types for sustainable building
The green roof of Electronic Arts, Phase II. Musson Cattell Mackey Partnership, Vancouver. [1] Photo: Ed White
by Hugh Perry
Considerations to stay with conventional installation methods of roofing in order to keep initial cost low can no longer stand against improved installation methods and new products that provide long term returns.
Not surprisingly, higher initial investment in denser insulation, membrane thickness, good drainage and reflectivity brings the greatest return on investment. Such measures deliver longevity that lowers the life cycle cost of roofing by decreasing material sourcing, manufacturing energy cost and transportation. And a decrease in repeated installation and demolition reduces the need for raw materials, transportation and landfill space. The added benefits of energy savings are a bonus.
Duro-Last Roofing Inc., considers sustainability in roofing as requiring all of the Five E’s: Energy, Environment, Endurance, Economics, and Engineering. The time for short cuts and ‘it’s good enough’ attitudes may be finally outdated.
Recycling
The industry is in its infancy in re-using demolition material from conventional asphalt-based roofing. In some cases these petroleum-based materials are recycled into other products but are not yet being used for roofing materials. Attitudes toward virgin material vs recycled prevent action in this area.
However, companies such as Hydrotech have developed a two-coat hot fluid, rubberized asphalt system that incorporates a fabric layer that has 25% recycled content. In their green roofs application, the water retention layer is composed of recycled polypropylene fibres. Another example is Sika Sarnafil’s single-ply membrane that has 5% recycled content in the backing which is processed from all of their demolished products.
Integrated Solar Electric Roof
Photovoltaic panels [PV] have the advantage of providing free electricity and also offer the opportunity to sell excess electricity back into the grid.
The system provided by Sika Sarnafil consists of 10’ x 40’ photovoltaic roofing panels that are factory-laminated and hot-air welded to an installed reflective membrane. The result is a photovoltaic roofing system that protects the building interior from the elements while also generating power for its electricity needs. Soprema has a similar system applied to single-ply modified bituminous.
With such a system, LEED-EA c-2 offers three points for on-site renewable electricity when there is a reduction of overall consumption by 20%. Roofing membranes with high reflectivity and integrated PV panels could become the way of the future.
Cool Roof
The intent of the ‘cool roof trend’ is to offer alternatives to black heat-absorbing roof tops through options such as vegetable gardens and light-coloured roofing materials. The benefits for cool roofs involve both energy and social, yet neither application has value without sufficient insulation.
A cool roof is one that has relatively high solar reflectance [SR] or Albedo and high thermal emittance [TE]. Both properties are rated on a scale from 0 to 1, or 0% and 100%, where 1 is the most reflective or emissive. Most roofing materials can have emittance values above 0.85 [85%].
Another advantage for white cool roofs is the decrease in accumulated heat gain in city cores where the high concentration of dark roofs and other surfaces can increase the ambient air temperatures by 6ºC. This effectively causes air conditioning systems to work 30% longer. As reflective membranes age they lose their reflectivity due to soiling, however, single-ply surfaces can be cleaned. Materials that achieve reflectivity LEED ratings are single-ply membranes: EPDM, TPO and PVC materials with no coverings.
Green Roof
Green roofs or vegetable roofs are roofing systems that support vegetation and are built up of insulation, waterproof membranes, root barriers, drainage mat, filter cloth and growing mediums.
Besides providing relaxing space for occupants by providing green space in urban settings, they also offer cooling and heating energy savings, improved outdoor air quality, and reduced heat island effect within city cores. They contribute to storm water management by reducing or regulating the amount and rate of run-off which assist municipalities in managing peak rain falls. Water run-off is also well suited for water reuse, either for irrigation or for plumbing fixtures, which can earn LEED points.
There are two basic categories: [see issue # 7 of SAB mag for Green Roofs article]
l Intensive which have 20cm to 90cm of growing medium with weights of 240 - 1200 kg/sq.m.
l Extensive which have 7.5 cm to 15 cm of growing medium with weights of 90 - 180 kg/sq.m.
Energy Savings: Heating energy savings for green roofs are difficult to confirm due to variations in soil depth and soil types. Both factors affect the moisture content within the soil. When soil freezes it can remain colder than ambient air temperatures and acts as a heat sink. However, there is no question that energies required to cool the building are decreased.
Fire Rating: Determining fire performance is challenging due to plant types,
rainfall, wind and irrigation provisions. Dry vegetation on the roof can lead to increased fire spread ratings which are yet undetermined.
Structural Load: The building structural load is increased due to additional dead load of materials as well of live loads. These loads may increase based on type, depth, and moisture content of soil. In winter, snow loads on a conventional roof are generally low due to the wind reducing snow depth, whereas adding soil and plants encourages abuild-up of snow.
The social and environmental values supporting green roofs outweigh the generally higher quantity of materials needed to make them. When considering the added decrease of heating/cooling energy, HVAC equipment sizing, and the longer life expectancy of the green roof system, the savings can be worthwhile.
Convential roofs: insulation is key
In any installation, the resistance to the transfer of energy through the roof is the most sustainable practice, no matter which system of roofing is used. The MNECB [Model National Energy Code for Buildings] requires different roofing R-Values for different climatic zones, such as an average of R-15 for Ontario.
The LEED prerequisite calls for a minimum reduction in new buildings of 25% of energy over the MNECB and 10% in renovations. One way to achieve this is to increase roof thermal resistance to R-20 which is double the typical installation. Insulation becomes a vital consideration in achieving energy savings.
Built-up Roof:
The conventional roofing system composed of six basic components that can be assembled in a number of combinations: insulations, base ply, multiple layers of plies that are combined with either hot or cold bituminous adhesives, and finally the surfacing which is generally a thin layer of granular.
Ballast Single-ply Roof:
Consists of insulation laid on the roof structure covered by a single-ply of membrane with ballast. This method was introduced in 1970 and includes the following single-ply applications.
PMR - Protected Membrane Roof or Inverted Roof: The water proofing membrane is laid on the roof structure with the
polystyrene foam insulation laid above the membrane. With this installation the rainwater flows above and below the insulation. Ballast, such as stone or combinations of stone and pavers, offsets the buoyancy of the foam insulations. The structural support is increased in order to carry the loads and, as a result, the insulation R-value is often decreased to off-set these added costs. With the use of white aggregate or high reflectivity pavers related cooling energy is reduced.
Non-Ballast Single-ply:
Lends itself to unconventional shapes as well as flat surfaces. Large graphics can be applied to non bituminous systems.
Structural Roof Panel System: Incorporate rigid roof insulation manufactured with fibre-reinforced facers on one side and a variety of single-ply waterproofing membranes. Panels are designed to span steel joists without metal support pans. Some products are designed for spans greater than 2 m.
Modified Bituminous Membrane: Applied hot in a single-ply and has a coloured aggregate surface resembling the finish of residential asphalt shingles. It is still the most popular method used in Canada. The white-faced membrane is 30% reflective which is an improvement over darker colours.
EPDM - Ethylene Propylene Diene Terpolymer: EPDM is a rubber-based material secured with adhesive and has been traditionally black until the recent emphasis on reflectivity. The dark surfaces attract the sun’s solar energy while white decreases absorption. This raises a debate as to the energy efficiency of each, based on duration of the cooling/heating seasons, snow frequency and latitude.
TPO - Thermoplastic polyolefin: Plastic-based and come in a wide variety of colours. Installation includes adhesives with hot air welding seams and has the option of high reflectivity.
PVC - Polyvinyl Chloride: Applied mechanically or with adhesive and the overlaps are hot air welded. White roofs have high reflectivity, and the durability, long service life, and recyclability of PVC supports arguments for its green value. However, over the long term, many are looking for alternatives to PVC products.
Flat Metal and Low Slope Roofs:
Metal is generally used on low sloping steel structured buildings with the insulation rolled out prior to applying the steel waterproofing. The insulation value is usually minimal although many layers can be built up by using perpendicular steel channels, such as that provided by Vic-West.
Metal is known for its durability, high-recycled content and flexibility in colours. Metal insulated panels are also now available.
All roofing systems protect buildings from weather, however, the greatest need for change to the roofing assemblies is increasing insulation thickness. Focusing on reflectivity and vegetation without maximizing thermal resistance is a form of design green washing.
Hugh Perry is contributing editor of sabmag.
