Low-slope Membranes

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Low-slope Membranes

NOTE TO READER: Where applicable, the content of this page is replicated in the RoofStar Guarantee Standards sections of this Manual

1 General

A combination of technological and economic factors has brought about the increased popularity of flexible membrane roofing systems. As building and construction technology become more sophisticated, the demands on the roofing membrane become more severe. Large, highly insulated roof areas impose greater stresses on the membrane. Formerly expensive synthetic materials have become more cost competitive as the cost of petroleum products has increased.

2 Bituminous and Modified Bituminous

NOTE: Read the RoofStar Guarantee Standards for SBS Membrane Roof Systems and an Introduction to SBS Membranes.

Bituminous and modified bituminous membrane systems have the benefit of offering the physical properties of a flexible membrane while retaining the practical advantages of bituminous materials. Roofers tend to have a certain “familiarity” with the product and they can be used in conjunction with some traditional products and methods. This may prove especially advantageous in re-roofing over surfaces contaminated with bitumen.

2.1 COMPOSITION

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. 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.

2.2 PERFORMANCE

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, the RoofStar Guarantee Program 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.

2.3 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 RoofStar 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 roofing asphalt). 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 SBS Special Applications. 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.

2.4 APPLICATION CAUTION

MODIFIED BITUMEN MEMBRANES IN HOT MOPPED ASPHALT

  1. 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.
  2. the RoofStar Guarantee Program 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).
  3. 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.
  4. 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.
  5. the RoofStar Guarantee Program 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.
  6. 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 the INSULATION section of SBS Roof Systems for mechanical fastening requirements and slope limitations).
  7. 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.


APP MODIFIED BITUMEN MEMBRANES

  1. 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.

3 Hybrid Bituminous EPDM Roof Systems

Oxidized and modified asphalts are used as waterproofing compounds and as adhesives to adhere reinforcement scrims that are bonded to the underside of EPDM membranes to various substrates.

The use of manufacturers’ proprietary hot-melt modified asphalt requires the use of oil-jacketed kettles, equipped with accurate thermometers and a mechanical agitator to heat the asphalt.

4 Single-ply Thermoset and Thermoplastic Membranes

Due to the complexities involved in non-bituminous flexible membranes, “generic” application specifications and details are not possible. This section of the manual contains both an overview of the products, their composition, usage, and design considerations, and the accepted membrane manufacturer's written submissions. In all cases, the membrane manufacturer should be contacted for further recommendations and details.

In order to formulate materials to satisfy the requirements of a roof membrane, the manufacturer must ensure:

  • Strict Quality Control
  • Proper Mix of Ingredients
  • Consistency of Batches
  • Minimum Physical Properties are Obtained

The chemical formulations of specific flexible membranes can be complex, but the end product can generally be defined as either a THERMOSET or THERMOPLASTIC.

For more about 'single-ply' membranes, refer to Flexible Membrane Properties, with special thanks from the Single Ply Roofing Institute.

4.1 Thermoset (Elastomeric)

NOTE: Read the RoofStar Guarantee Standards for EPDM (Thermoset Membranes) Roof Systems
Thermoset flexible membrane systems are synthetic materials in which polymers chemically and irreversibly cross-link. This process is known as “curing” or “vulcanizing”. They may be manufactured in cured or uncured form. Once cured, the material cannot be bonded to itself except by using an adhesive and / or seaming tape. (See Manufacturer application standards).

Thermoset is defined as a material that solidifies or “sets” irreversibly when heated (vulcanized). This property is usually associated with cross-linking of the molecules induced by heat or radiation. Once the material is cured, it can only be bonded to itself by adhesives; new molecular linkages cannot form. Some thermosets are not cured during manufacture (ie. EPDM flashings) and are intended to cure over time after they have been installed. This allows the material to cure into the specific shape required for the roofing application.

The generic membrane types defined as cured thermosets include:

  • EPDM (Ethylene Propylene Diene Monomers)
  • Neoprene (Chloroprene Rubber)
  • CPE+ (Chlorinated Polyethylene)

Uncured thermoset membranes include:

  • CSPE (Chlorosulphonated Polyethylene)
  • CPE+ (Chlorinated Polyethylene)
  • PIB (Polyisobutylene)
  • NBP (Nitrile Alloy)

+CPE's may be formulated to be used as either a cured or uncured membrane.

EPDM (ETHYLENE PROPYLENE DIENE MONOMER)

EPDM is a thermoset, synthetic rubber membrane formulated from ethylene, propylene, and a small amount of diene monomer. Although most commonly available as a cured sheet, EPDM can be formulated as an uncured membrane for specific purposes (i.e., flashings).

EPDM roof systems are single-ply membranes available in thicknesses ranging from a nominal 1.1 mm (0.045") to 1.5 mm (0.060") and are usually black or white in colour. The membrane may be applied loose-laid and ballasted, mechanically-fastened, or fully-adhered. Seams are joined with contact splicing cement or seaming tape (or a combination of both), and are sometimes caulked (see manufacturer's requirements).

EPDM membranes should conform to CGSB 37-GP-52M 1984, “Roofing and Waterproofing Membrane, Sheet Applied, Elastomeric” which lists two types of membrane, according to manufacture:

  • Type 1
non-reinforced
  • Type 2
reinforced

and two classes, according to end use:

  • Class A
exposed
  • Class B
non-exposed

This standard covers requirements such as “thickness, tensile strength, breaking strength, lap joint strength, elongation, tensile set, low temperature flexibility, water absorption, dimensional stability after water absorption, heat aging, ozone resistance, resistance to accelerated weathering, dynamic impact test, tear resistance, and tearing strength.”

4.2 Thermoplastic (Plastomeric)

NOTE: Read the RoofStar Guarantee Standards for TPO, PVC (Thermoplastic Membranes) Roof Systems
Thermoplastic flexible membrane systems are synthetic materials in which cross-linking of the polymers does not occur. The membrane can be welded together by the use of solvents or heat. This process is repetitive and new welds may be formed at a later time. The strength of these welds often surpasses that of the base membrane.

Thermoplastic materials are defined as polymers which soften when heated and harden when cooled. This process is repetitive provided the material is not heated above the point at which decomposition occurs. No cross-linking or vulcanization occurs. In addition to heat welding, some materials can be solvent welded. These welds develop bonding strengths equalling or surpassing the strength of the base materials.

Thermoplastics materials used for roof membranes include:

  • PVC (Polyvinyl Chloride)
  • EIP (Ethylene Interpolymers)
  • ECB (Ethylene Copolymer Bitumen)
  • TPO (Thermoplastic Polyolefins)
  • EPR (Ethylene Propylene Rubber)


TPO

TPO single-ply membranes are made of flexible polyolefin polymers. TPO polymers are formed by chemically linking up olefin molecules that soften when heated. This characteristic permits TPO membranes to be heat-welded for seaming of sheets and membrane flashings.

The polymers used to form TPO membranes vary between manufacturers, polypropylene, polyethylene and isobutylene, as well as their derivatives are used in the various manufacturing processes. TPO’s do not contain plasticizers.

TPO single-ply membranes are reinforced with polyester, fibreglass or a combination of both fabrics. Membrane reinforcement provides dimensional stability as well as strength to resist in service stresses. Membrane thicknesses range from 1.14 mm (45 mils) to 1.52 mm (60 mils), and approvals from Factory Mutual (FM), UL and ULC may be available depending on the manufacturer.

TPO membranes must be applied in conformance with RoofStar Guarantee Standards for TPO/PVC Roof Systems and the membrane manufacturer’s printed instructions.

PVC

PVC is a thermoplastic material produced through the polymerization of vinyl chloride, resulting in a relatively hard, resinous material. This material is then formulated with compounds, such as plasticizers and stabilizers, depending on the physical properties required for end use. PVC formulations have been developed to suit numerous commercial and industrial applications.

PVC roof systems are single-ply membranes available either non-reinforced or reinforced with polyester fabric or glass fibres, and in thicknesses usually ranging from 1.2 mm to 2.0 mm, depending on the manufacturer. Non-reinforced PVC membranes may typically exhibit excessive shrinkage characteristics, thus their use is often discouraged. The membrane may be applied loose-laid and ballasted, mechanically fastened, or fully adhered. Seams are heat or chemically welded (see manufacturer’s requirements).

PVC membrane should conform to CGSB 37-GP-54M 1979, “Roofing and Waterproofing Membrane, Sheet Applied, Flexible, Polyvinyl Chloride” which lists two types of membrane, according to manufacture:

  • Type 1
non-reinforced
  • Type 2
reinforced

and four classes, according to end use:

  • Class A
non-exposed roofing
  • Class B
exposed roofing
  • Class C
waterproofing
  • Class D
flashing membrane asphalt compatible

This standard covers requirements such as “thickness, lap joint strength, permeability, tensile strength, elongation at break, low temperature flexibility, water absorption, resistance to accelerated weathering, dimensional change after stress relaxation, and cone penetration”

PVC membranes should be applied in conformance with RoofStar Guarantee Standards for TPO/PVC Roof Systems, with consideration given to CGSB 37-GP-55M 1979, “Application of Sheet Applied Flexible Polyvinyl Chloride Roof Membrane” and the membrane manufacturer’s printed instructions.



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