Designing Modified Bitumen: Effective roofing assemblies for cold climate

by Katie Daniel | January 11, 2018 11:13 am

All photos courtesy IKO

By Dave Miller, RRO, CTR
For the past 50 years, the commercial roofing industry in North America has developed and adapted many roofing system options for low-slope buildings. Modified bitumen (mod-bit) membrane assemblies are well-suited to the Canadian climate, making the system a significant player in the commercial roofing marketplace. Industry-sponsored training programs, investments by manufacturers, and high levels of competition have contributed to the overall success of mod-bit. Nevertheless, design/construction professionals must understand the full range of considerations for achieving high-performing roofing systems.

Weather factors
Most of Canada’s population is concentrated in the southern parts of the country. The weather in these regions tends to be less severe than the rest of the country, but these areas are still subject to considerable variances in both warm and cool weather patterns. Temperatures can easily range from 30 to –30 C (86 to –22 F) throughout the year. Additionally, all of Canada’s major cities face relatively significant precipitation. St. Johns, NL, has the most snowfall (3350 mm [132 in.]) and Abbotsford, B.C., has the most rain (1538 mm [60.5 in.]). These conditions require resilient and durable membrane systems—mod-bit roofs answer the call by adapting to these diverse conditions.

Mod-bit is the preferred roofing membrane choice for extreme weather because of the nature of the rubberized polymers added to the asphalt, which is brittle by nature and does not effectively handle cold temperatures. Styrene butadiene styrene (SBS), or rubberized asphalt, forms a molecular bond with the asphalt to create asphalt bitumen that accommodates expansion and contraction in the cold temperatures. The polymer-modified asphalt properties of SBS add flexibility and durability to the membrane. An abstract from the Society of Petroleum Engineers (SPE) provides greater detail about this process.  (For more details, read “Polymer Modifications in Asphalt Roofing” at[2].)

Roofing system options
While a variety of membrane systems are used across the country, the combined modified bitumen and asphalt built-up roof (BUR) assemblies account for as much as 80 per cent of the annual Canadian industrial, commercial, and institutional (ICI) low-slope roofing market, according to a report by the Athena Sustainable Materials Institute. ( Roofing asphalt is prominent in three types of systems—traditional four-ply built-up roofs, two-ply mod-bit roofs, and rubberized asphalt BURs.

This photo shows the heat-welded application of styrene butadiene styrene (SBS) membrane roll on a commercial roofing project.

Built-up roofing
Also known as tar-and-gravel roofs, BURs use layers of bitumen and reinforcing fabrics to create a finished membrane. They are constructed with several-ply sheets of glass fibre or paper used as reinforcement. The bottom ply, or base sheet, is sometimes introduced and fastened mechanically, creating a multi-ply system referred to as a hybrid BUR. These membranes can be applied directly to the roof deck, but are often laid over exterior insulation and overlay boards in what is known as a conventional roof assembly.

In a BUR system, the most commonly used bitumen is asphalt-based—either hot-mopped or cold-applied in an adhesive form. However, there are a few systems that use a cold asphalt application form.

BUR systems offer good performance in absorbing building stress and thermal shock. This is accomplished through the unique application of successive layers of reinforcing material between layers of bitumen. This membrane has strength for proven long-term durability. When a mineral surface is applied, BUR also delivers Class A fire resistance from external flames in accordance with CAN/ULC S107M, Methods of Fire Tests of Roof Coverings. Surfacing options for BUR systems include:

In some cases, aluminum or elastomeric coatings may also be applied to the surface to provide a finishing coat and minimize the effects of ultraviolet (UV) radiation.

While BUR can be applied to almost every building, these systems do not lend themselves well to high-rises or areas with restricted access because there must be room for the asphalt to be pumped to the rooftop area. BUR materials are also heavy, labour-intensive, and can be noxious and dirty.

In this image, a skilled commercial applicator installs SBS membrane adjacent to a parapet wall.

Modified bitumen roofing
Applicable to almost any building scenario, mod-bit systems include factory-fabricated layers of asphalt, modified by employing a rubber or plastic polymer for increased flexibility in combination with a reinforcement for strength and stability. Bitumen (or asphalt) is a sticky, black, viscous liquid, or semi-solid form of refined petroleum distillate. It is found in natural deposits or may be a refined product primarily used for road surfacing and roofing. Bitumen displays a thermoplastic quality when softened by heat and offers excellent waterproofing capacity.

In modern mod-bit membranes, two primary modifiers are commonly in play: atactic polypropylene (APP) and SBS. They refer to the type of polymers added to the asphalt. A variety of reinforcements (e.g. fibreglass, polyester mats, and scrims) are used with the asphalt. Scrim is a reinforcing fabric made from continuous filament yarn in an open-mesh construction. Such reinforcements are seldom woven, but rather randomly dispersed to enhance the dimensional stability of the finished membrane.

Mod-bit membranes are conveniently packaged for the applicator and typically installed as a two- or three-ply system. The specific model of membrane chosen determines the method of installation. For example, a sand-back surface membrane is typically applied using hot asphalt or cold adhesives. Membranes with a thermos-fusible film back are applied by heat-welding. In either case, mod-bit also allows membranes to be mechanically attached over the substrate by using fasteners such as screws and plates.

Other systems
Single-ply membranes fall into two main categories: thermosets and thermoplastics. Thermoset membranes are made from rubber polymers, with ethylene propylene diene monomer (EPDM) being the most widely used material. Thermosets are known to effectively resist the effects of sun damage and chemical attack. Thermoplastic membranes are based on plastic polymers such as polyvinyl chloride (PVC) and thermoplastic polyolefin (TPO). These membranes have a track record of meeting the demands of numerous environmental conditions in the Canadian roofing market.

Strong, flexible, highly durable, and applicable to numerous building requirements, these roofs can be installed with a variety of attachment methods. Since they typically weigh less than BUR or mod-bit, they tend to be favoured for lightweight structures, and are competitively priced.

However, it is important to note single-ply roofing by nature is just that—a single ply of protection. This trait can be viewed as a limitation, as it represents a single layer of defence, forming the boundary of environmental separation.

Heat applications
High-performance mod-bit roofing systems are quickly installed, easy to maintain, and cost-efficient. The mod-bit membranes themselves are normally hot-mopped like traditional BUR but may also be applied as a torch-down product. The Manual of Low-slope Roof Systems by C.W. Griffin and R.L. Fricklas offers important points to consider with each installation method, including:

Cold-applied adhesives
These adhesives offer another installation method for mod-bit membranes and can be applied at a wide range of ambient temperatures. Bitumen can form the main ingredient of the base resin in several adhesives along with urethanes and polyurethanes, butyls, acrylics, and neoprenes. Other added components include polymer-modifiers, petroleum-based solvents, fibres, and fillers. These generally form two groups: solvent-based or non-solvent-based. As a point of safety, it is important to note solvents in the adhesives may be combustible and should not be placed near any source of heat or ignition.

Another consideration for cold-applied adhesives is they can take more time to fully bond than heated methods and require time to set and cure as sections are completed, so foot traffic and equipment must be avoided. Many cold adhesive systems also rely on hot-air-welding of the critical seam areas to ensure a solid bond. It is crucial to ensure solvent vapours released as the material dries do not filter into building ventilation systems.

An applicator carefully installs bitumen base sheet layer on a flat commercial roof and parapet wall.

Self-adhered membranes
This membrane adheres to both substrate and to itself at overlaps without the use of an additional adhesive. A release paper or film protects the undersurface of a self-adhering membrane to keep it from bonding to itself during shipping and handling.

Self-adhered membranes serve a suitable base for the application of a heat-welded cap sheet or as a flashing detail designed for use in flame-sensitive areas. Pre-adhesives in both solvent-based and low-volatile organic compound (VOC) products provide a quick-drying surface preparation for installing these membranes and conditioning the substrate for optimal membrane performance.

Workforce training and durability
Many Canadian-based roofing manufacturers have a long track record in mod-bit roofing systems. The market has widely accepted them for multiple reasons including available warranty programs, long life expectancy, and durability.  Market demand has motivated manufacturers to sponsor training programs, which have educated contractors and improved the labour force’s ability to effectively install these systems. Such training also ensures products are applied properly in order to fulfil the various requirements of manufacturers’ warranties.

While the labour force is trained in all application methods, each has its own consideration. Torch-applied methods increase the risk of fire, which may contribute to higher costs and danger. Installing self-adhered membranes is clean and safe, but requires attention to detail. Cold adhesive systems are designed to bridge the gap between heat-sensitive systems and flame-free membrane solutions, at relatively low risk to the contractor.

Commercial low-slope roofing applications face numerous challenges, including vapour control, thermal control, resistance to precipitation, weathering, durability, and life expectancy. While many system choices are available, identifying the right one for each building scenario is paramount.

Cost-effective, adaptable, and flexible, mod-bit assemblies lend themselves to a wide variety of construction scenarios. When properly designed, these membranes can resist the elements while being esthetically pleasing through a wide choice of colour granule surfacing options. Regardless of their appearances, mod-bit roofs are well-suited to Canada’s varied climatic zones and temperature fluctuations.

Dave Miller, RRO, CTR, is senior manager for codes and approvals for IKO Industries. His more than 40 years of construction experience includes foundation steel, electronic cabling, and commercial roofing. Miller co-chairs the Ontario chapter of the Roof Consultants Institute’s (RCI’s) technical committee; he is also an associate member of CSA’s Bituminous Roofing Committee and a member of the Canadian Roofing Contractors Association (CRCA). He can be reached at[6].

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