Designing Modified Bitumen: Effective roofing assemblies for cold climate

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:

  • hot-mopping is generally safer and faster than torching;
  • if hot asphalt is required for other roofing operations to adhere insulation or a base sheet, it is likely more economical to hot-mop the cap sheet as well;
  • heat-welding (also referred to as ‘torch down’) can evaporate surface moisture from the bonding area; and
  • heat-welding preheats and softens material, offering advantages in cold weather.

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.

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