SBS Definition and Terms

1 Definitions and Terminology

1.1 COMPOSITION

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Bituminous and modified bituminous flexible membrane systems are composite sheet membranes which are thermoplastic in nature but display variable elastomeric properties when modified.

These sheets generally consist of bitumen (oxidized or modified), reinforcement, and surfacing materials. The performance of these membranes may be compromised by subtle, seemingly inconsequential changes in composition.

The formulation of the bitumen is critical, particularly when its physical properties are modified by the addition of polymers. Bitumen is most commonly modified with styrene butadiene styrene (SBS) or atactic polypropylene (APP). The manufacturer must ensure that a suitable degree of compatibility exists between the bitumen and the polymer and that a thorough mix has been obtained. The careful selection and combination of bitumen and polymer (the type of modifier, the percentage of polymer used, etc.) determines the physical properties and long term performance of the modified bituminous “binder”. The binder provides the membrane with its waterproofing capability and may impart the following properties:

• low temperature flexibility
• elasticity
• high temperature resistance to flow
• resistance to aging

The bitumen utilized for oxidized bituminous membranes has been “blown”, similar to roofing asphalt (see Section Reference needed). This produces a higher softening point and, therefore, a more useful temperature range.

The physical properties of the binder are further enhanced by the imposition by saturation or coating onto a carrier or reinforcing mat, most commonly a textile such as:

• non-woven spun-bonded polyester
• woven polyester scrim
• fibreglass mats
• woven fibreglass cloth
• combinations of the above

(Some polyester modified membrane manufacturers use oxidized bitumen to saturate the reinforcing, particularly when “heavyweight” reinforcing is used.). The type of reinforcing selected depends on the end use and desired physical properties of the finished products. The weight, quality, tension during manufacturer, and method of saturation and coating of the reinforcing are some of the factors governing the final product's performance.

Membranes are usually surface treated with a parting agent so they do not stick in the roll. Surfaces that will be hot asphalt applied are usually sanded while torch-applied surfaces use polyethylene or other thermofusible films. Exposed surfaces may be embedded with mineral granules (usually ceramic chips or slate flakes) or laminated with metal foil for ultra-violet protection. As an alternative on roofs with minimal slopes, a pour coat and gravel surfacing may be installed over some membranes, although this makes the quality of application difficult to inspect and may lead to problems with membrane slippage due to the weight of the cap sheet and surfacing.The thickness of the membrane should be suited to the method of application. Torchable membranes must be thick enough to allow the underside to be melted without burning through to the reinforcing and thin enough to be manageable in winter conditions. Hot asphalt applied (mopped) membranes should be of sufficient thickness to perform adequately but should not act as a “heat sink”, lowering the temperature of the asphalt before a sufficient bond is achieved. Manufacturers often refer to membrane thickness in one of two ways. “Nominal Thickness” usually refers to the thickness including any granular finishes. As the granular surfaces do not contribute to the performance of the membrane (except as ultra violet protection and appearance), thickness measured “at the selvage edge” usually gives a more accurate indication of the physical properties of the membrane related to thickness.

1.2 PERFORMANCE

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The physical properties of the membranes available today vary greatly. Each membrane manufacturer “tackles” the problems of roofing technology differently, based on individual design criteria. The design authority, in consultation with the manufacturers, must identify the properties most important for the performance of the roof system on an individual project basis.

As a precaution, RGC has accepted the following performance limitations, based on the lowest expected in-service temperatures of the membrane:

• Temperatures warmer than -10°C (+14°F) require membranes with a minimum tensile strength of 150 N/50 mm.
• Temperatures warmer than -18°C (0°F) up to -10°C (+14°F) require a base sheet or cap sheet with a minimum tensile strength of 294 N/50 mm (CGSB 37-GP-56M Grade 1).
• Temperatures colder than -18°C (0°F) require a base sheet or cap sheet with a minimum tensile strength of 785 N/50 mm (CGSB 37-GP-56M Grade 2).

The design of a roof membrane system is contingent upon many factors and is not solely a function of design temperatures. Consult the membrane manufacturer for assistance with design considerations and membrane selection.

Detail BulletinRidging and Wrinkling of Modified Bitumen Membranes

Click on the tab to the right for further information about membrane performance

Reprint of CRCA Technical Bulletin, Volume 43, published October 1995:

Can roofs be “perfectly” constructed‌ Under most circumstances, the answer to that question would be ‘No’. Minor aesthetic imperfections, that have no bearing on performance, will always develop due to the materials used and the conditions under which they are installed.

Modified bitumen membranes, with their relatively thin protective surfacing (primarily ceramic granules or metal foils), can be particularly susceptible to the occurrence of these blemishes. Unlike traditional built-up roofs, these membranes are not “hidden from view” by a heavy top pour of asphalt and gravel. The reflective properties of their protective surfacing will allow many minor anomalies, such as deck or insulation unevenness, to readily telescope through to the roof surface. Over time, due to natural weathering and aging, these aesthetic imperfections will become increasingly visible. There are, however, distinct benefits to this aspect of modified bitumen membranes. Some manufacturing defects, such as poor surfacing embedment and uneven or insufficient backside coating, may be detected through an inspection of the rolls prior to installation. Serious errors in application will be apparent almost immediately allowing timely remedial action to take place.

The decision to repair a modified bitumen roof should, as with every type of roofing system, be made only after careful thought and should be based primarily on performance consequences. Open seams, fishmouths at laps, large unbonded areas of the membrane and areas where overheating has resulted in distortion to the reinforcing should be corrected as soon as possible. Trivial anomalies, on the other hand, such as minor wrinkling and ridging are to be expected as a function of the physical characteristics of these materials and their required application methods. Patching a minor blister or a small wrinkle because of visual impact alone is not justified and, in most cases, the repairs will look far worse than the blemish they were intended to correct.

As contractors have become more experienced with modified bitumen membrane installation, application techniques have evolved that have proven to reduce the occurrence and severity of these “blemishes”. These techniques, however, will not eliminate these minor anomalies entirely. In addition, it should be realized that they may significantly increase the cost of a roofing project due to added labour costs and lengthened construction schedules, with only marginal benefits for the owner and no meaningful increase in performance value.

Modified bitumen membranes have proven to provide satisfactory performance in most regions of our country. As with any product, however, they have their limitations. One should not forget that their primary purpose is to be a barrier to moisture. Although they are available in a variety of coloured surfaces and textures, their inherent physical properties make an absolute flawless appearance impossible under most circumstances. A reasonable level of expectation is required by those who specify these membranes together with a reasonable level of care and skill in their application.

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