Bituminous and modified bituminous membranes represent a large share of the flexible membranes that have replaced traditional built-up roofing felts. While offering the higher performance and versatility of a flexible membrane, these products also offer the practical advantages of bituminous materials (i.e. they are generally compatible with asphalt products and may be useful for re-roofing projects).
Styrene butadiene styrene (SBS) polymer modifiers are “thermoplastic elastomers”. They are elastomers, which behave similar to both thermoplastics and thermosets (or “elastomerics”). Thermoplastics soften with heat and harden when cooled, no cross-linking (vulcanization) of molecules occurs, and they tend to exhibit “plastic” properties. Thermosets are materials in which cross-linking of the molecules occurs and they tend to exhibit “elastomeric” or rubber-like properties.
Simply stated, flexible polybutadiene “chains” interconnect with the glass-like polystyrene “blocks” and, when cooled, the resulting network behaves as if the molecules were cross-linked. The process is repetitive in that the networks may be disrupted by heat or solvents but will reform upon cooling or removal of the solvents.
Bitumens modified with SBS can display improved physical and mechanical properties. The improved properties are only possible if the SBS polymer is properly dispersed within a suitable bitumen. A well dispersed, homogeneous polymer mixture will form a continuous rubber network throughout the bitumen.
SBS modification of bitumens can impart the following changes in properties:
- increased softening point
- improved low temperature flexibility
- greater elasticity
- improved aging characteristics.
SBS-modified roofing membranes, often referred to as "SBS roofing" or simply as "SBS", are relative new-comers to the world of modified bituminous roofing products. SBS-modified roofing membranes comprise a substantial portion of the flexible membrane market, which has largely replaced traditional built-up roofing felts. While offering the higher performance and versatility of a flexible membrane, SBS-modified roofing membranes are also generally compatible with asphalt products, and are therefore useful for re-roofing.
SBS-modified roofing membranes are just one type of modified bituminous roofing. They are flexible membranes manufactured in rolls and are thermoplastic in nature but display variable elastomeric properties, primarily because of the modifiers blended with the bitumen. These membranes are generally manufactured using three materials: (1) modified bitumen, (2) reinforcement fibers, commonly woven as a sheet and embedded within the membrane (these fibers may be glass, polyester, or a blend of the two), and (3) a surface coating, such as stone or ceramic granules. The performance of these membranes (for example, their flexibility or ability to retain their shape under higher temperatures) 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; see separate section in this Manual). 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
- high temperature resistance to flow
- resistance to aging
The bitumen utilized for oxidized bituminous membranes has been “blown”, similar to roofing asphalt. This produces a higher softening point and, therefore, a more useful temperature range.
The physical properties of the binder are further enhanced by applying it to or embedding it in 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
The type of reinforcing selected depends on the end use and desired physical properties of the finished products. The weight, quality, tension during manufacturing, and method of saturation and coating of the reinforcing are some of the factors governing the final product's performance. Some polyester modified membrane manufacturers use oxidized bitumen to saturate the reinforcement, particularly when “heavyweight” material is used.
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 (materials that can be bonded with heat). 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.
3 Material Composition
3.1 Modified Bituminous Membranes
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
- high temperature resistance to flow
- resistance to aging
The bitumen utilized for oxidized bituminous membranes has been “blown”, similar to roofing asphalt (see Section 3.9.1). 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.
3.2 Liquid Membranes
Liquid membrane flashing systems are manufactured from various bituminous and non-bituminous materials.
These manufactured proprietary flashing systems are specifically designed for use to flash unusual roof penetration shapes or roof details that are difficult to flash with traditional membranes.
Liquid membrane flashings are available as one or two part component systems that require reinforcement scrims for strength. To ensure quality installation, manufacturers’ proprietary application instructions must be strictly followed.
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.
|Ridging and Wrinkling of Modified Bitumen Membranes|
|Reprinted 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.
As with all flexible membranes, the quality of the installation is critical to the performance of bituminous and modified bituminous membranes. Although some of the application techniques for these membranes can be similar to those employed in built-up roofing, and the materials can be “familiar”, the differences are critical. It is important that the specifications, details, and installation techniques all conform to the membrane manufacturer's requirements.
One of the major concerns when applying these membranes with hot asphalt is to ensure the asphalt remains hot enough to fuse with the binder of the membrane. The asphalt temperature must conform to the membrane manufacturer's minimum application temperature and minimum +205°C (+400°F) as required by RGC Guarantee Standards. Asphalt should not be mopped more than 1 m (3') preceding the roll as it has been demonstrated that the temperature of mopped asphalt drops sharply once mopped out (see Section 22.214.171.124). In addition to asphalt temperature, particular attention should be paid to slope limitations, fastening requirements, type of asphalt, the amount of asphalt used, and membrane flashing requirements when using hot asphalt. Some manufacturers may require “torched” membrane flashings to ensure a proper bond and prevent problems with slippage or asphalt running down the vertical and collecting at the cant. Slippage can also be a problem when an excessive amount of asphalt is used. Only enough asphalt to meet the adhesive requirements is needed. The mopping asphalt has a lower softening point than the bitumen in the membrane and may result in asphalt running and slippage problems.
The major concern when installing “torch-on” membranes is an obvious one: SAFETY. This is discussed in detail in Section 5.0.1 Safety Precautions - Torching. In addition to safety, it is essential that the membrane be fully and evenly bonded to the substrate. As the roll is installed, the roofer must ensure the full width of the roll is melted and the preceding roll's side lap is preheated. A small “wave” or “bead” of melted bitumen in front of the roll usually indicates sufficient heat. Special attention to seams is required.
The polyester fabric used as reinforcing in many thermofusible (“torch on”) membranes is subject to dimensional changes at high temperatures. Care must be exercised by the applicator not to over-torch or over-heat membranes. Membranes must be designed and manufactured specifically for torch applications.
It is recommended that the cap sheet installation immediately follow base sheet installation in all cases. However, when it may be necessary to delay the installation of the cap sheet due to weather, project scheduling or the like, then the following criteria must be considered:
- ANY delay in the installation of a cap sheet will result in the requirement for a special inspection and thorough cleaning of the base sheet. The inspection is for mechanical damage from traffic or trades and cleaning is necessary to ensure good adhesion; this must be done to the satisfaction of the roofing contractor.
- Sanded base sheets should have a thin squeegee glaze coat of asphalt applied immediately after application of base sheet if any delay for cap installation is anticipated.
- Base sheets which are either sanded, glaze coated or thermofusible can be exposed for up to six months. It must be recognized that an exposed base sheet is vulnerable to mechanical damage and construction dirt. The base sheet should be well protected by carefully placed and maintained protection panels (plywood for example). This protection would be supplied, installed and removed by someone other than the roofing contractor.
Consult manufacturer(s) for specific construction details and specifications.
5.1.1 MODIFIED BITUMEN MEMBRANES IN HOT MOPPED ASPHALT
- The design authority should recognize that the appearance of wrinkling and ridging in modified bitumen cap sheets applied in hot asphalt may be generic to certain manufacturers of this system, and particularly when attempted in cooler temperatures. Although some manufacturers' literature suggests application may take place in temperatures as low as -25°C (-13°F), there are many subsequent limitations, the variables of which may result occasionally in severe wrinkling or ridging of the cap sheet as well as potentially poor adhesion.
- RGC recommends that, in order to minimize the potential for wrinkling and ridging, the use of mopped cap sheets not be considered or specified when ambient temperatures (including wind chill) may go below +5°C (+41°F).
- Many factors affect the behaviour of the finished prefabricated sheets including:
- The quality of the binder (waterproofing agent).
- The choice of carrier including suppliers of fibreglass, polyester, or a combination of the two.
- Saturation of the carrier.
- Engineering of the sheet (composition and placement of the components within the sheet).
- Quality control of the manufacturer.
- Expertise of the manufacturer's personnel (including field representation).
- Recommendations as to what is required for a completed system and the recommended application procedure.
- Installation in the field by a competent contractor.
- The designer's role in choosing the product(s) which satisfy each previous item described.
- The wrinkles or ridges are usually considered an aesthetic problem only, but may occasionally result in fishmouthing and intermittent bonding of the cap sheet. Both of these deficiencies affect the performance of the membrane.
- RGC recommends the use of torch applied thermofusible cap sheets in 2-ply modified bitumen systems in all cases, but particularly if roofing must be carried out in cool or cold weather. Appropriate safety precautions regarding torching must be addressed.
- Occasionally some mopped systems, particularly with heavy cap sheets, have exhibited a propensity for membrane slippage, frequently on insulated systems where there is a combination of slope, heavy inter-ply mopping and lack of mechanical fastening (back nailing) of sheets. (See SBS Roof Systems - 6.3.3 Mechanical Fastening for mechanical fastening requirements and slope limitations; see similar requirements for EPDM and TPO/PVC Roof Systems).
- It is also common for mopped systems to exhibit some asphalt bleed-out at the seams. Minimal or reasonable bleed-out is to be expected and is an aesthetic problem which may be minimized by the embedment of matching granules. Bleed-out which oxidizes and alligators with time does not normally affect the waterproofing efficiency of the roof.
5.1.2 APP MODIFIED BITUMEN MEMBRANES
- Cold Weather Application:
- Designers and contractors should recognize that APP modified bitumen membranes can be extremely difficult to work with in colder temperatures, and that even attempting to unroll the material in very cold temperatures may result in cracking of the sheet or other problems.
- Some manufacturers make different grades of material for application at different times of the year. Most manufacturers recommend an application temperature lower limit of approximately 5°C (40°F). DO NOT ATTEMPT to install APP modified bitumen membranes below these recommended temperatures.