Flexible Membrane Properties

This section has been reprinted with the kind permission of:

The Single Ply Roofing Institute
104 Wilmont Road, Suite 201
Deerfield, Illinois 60015-5195

1 11.2.1 Physical Properties

Although it is difficult to directly correlate physical property data with actual performance or life expectancy, manufacturers typically utilize such data in their quality control programs to ensure the suitability of the compound for its end use.

The following is a list of twelve basic material properties which SPRI's Technical Committee has identified as being pertinent to all roofing membranes, regardless of chemical composition.

1 Thickness

2 Tensile strength

3 Ultimate elongation

4 Modulus

5 Tear resistance

6 Water vapour transmission rate

7 Water absorption

8 Dimensional stability

9 Factory seam strength

10 Low temperature resistance

11 Results after heat aging

12 Results after accelerated weathering

Test Procedures for Evaluation of Materials:

The test methods used to evaluate each of these properties vary depending upon the chemical composition and construction of the finished membrane. Different test methods are used for different generic types of material, as well as for reinforced and non-reinforced membranes. If physical property data are to be used to aid in product selection, the test methods should also be considered. Sometimes the difference in test method accounts for the difference in the particular results reported.

2 11.2.2 Significance of the Reported Physical Properties of Membrane Materials

2.1 11.2.2.1 Thickness

The distance between opposite surfaces of a material. Units of measure are millimetres, mils, or fractions of an inch.

The relationship of thickness to actual performance is not entirely clear, and membranes are available in thicknesses ranging from 0.75 mm to as many as 5.0 mm (30 mils to 200 mils). This rather significant variance may be accounted for by such factors as the polymer type and formulation, method of manufacture, physical construction of the finished sheet (e.g., surfacing, reinforcements, etc.), as well as the intended method of application. Thickness is related to quality control procedures in that the manufacturer must verify that a uniform thickness is maintained. The performance related factors usually associated with membrane thickness are its resistance to mechanical damage, hail, traffic and surface wear; although there are certainly other factors, such as compressibility of the substrate, which also contribute to all of these. In other words, the susceptibility of a membrane to damage does not in any way rely solely on the thickness of the material.

2.2 11.2.2.2 Tensile Strength

The maximum force or stress required to break a membrane sample. For non-reinforced membranes, strength is reported as a stress in Megapascals, or “Mpa” (pounds per square inch, or “psi”); for reinforced membranes, strength is reported as a force in Newtons (N), or pounds (lbs).

This physical property relates to the ability of a membrane to withstand stresses which might be imposed by such things as building movement, wind uplift, and thermal loading. The presence of reinforcing material and the type of materials used as reinforcement may also affect tensile strength.

2.3 11.2.2.3 Ultimate Elongation

The amount a membrane sample stretches during tensile testing before it ruptues, usually expressed as a percentage of the original length.

The elongation of a membrane may contribute to its ability to accommodate movement in the substrate or structure without rupturing. There is a broad range of elongation values exhibited by products which are appropriate for use as single-ply roofing membranes. The variance from product to product depends on chemical composition and sometimes on the presence of reinforcing materials. In some cases, a reinforcing material may break internally at a low strain level without affecting the integrity of the sheet, thereby allowing the membrane itself to stretch and achieve its elongation property. In other cases, the reinforcement has a high resistance to elongation and imparts this characteristic to the finished sheet, producing a membrane with a low elongation property. The selection is made by the manufacturer and is based largely on the manner in which the material will be installed.

It should be noted that elongation is most often performed on conditioned room temperature samples. As temperature greatly affects the ability of a material to elongate, elongation values will be greatly reduced at colder temperatures.

2.4 11.2.2.4 Modulus

Modulus is a measure of the stiffness of a polymeric sheet.

Since polymeric materials do not exhibit traditional elastic behaviour over their entire range of elongation, the modulus is not a constant; rather it is reported as the tensile stress required to produce a prescribed elongation. When the modulus at 50% elongation is reported for a number of products, it allows for a comparison of their relative stiffness. This is expressed as MPa or psi at a given percent elongation.

The presence of reinforcement affects the modulus of a material by significantly increasing its stiffness; it may also affect the elongation properties in the direction of the reinforcing medium. Like elongation, this property is an indicator of the suitability of the formulation for use as a roofing membrane, but is not a direct predictor of its performance once installed. However, modulus, in combination with other factors, such as coefficient of thermal expansion and dimensional stability, may have an effect on the manner of attachment of the membrane at terminations.

2.5 11.2.2.5 Tear Resistance

The load required to tear a material when the stress is concentrated on a small area of the specimen by the introduction of a prescribed flaw, expressed in kN/m, psi, or pounds-force.

This property indicates a membrane's ability to resist initiation and / or propagation of a tear. Recognizing that occasionally mechanical damage does occur which results in a tear or puncture, it is important that during installation - and also during membrane expansion and contraction due to structural or substrate movement or wind uplift pressures - the membrane be able to resist further tearing. Resistance to tear is also of importance in mechanically attached membrane systems in which the membrane is penetrated by fasteners, and wherever penetration of the membrane occurs at terminations. Different test methods are used to test the tear resistance of reinforced and non-reinforced membranes.

2.6 11.2.2.6 Water Vapour Transmission

A measure of the rate of transmission of water vapour through the membrane material under controlled laboratory conditions of temperature and humidity, expressed as grams / 24 hours / square metre or grains / hour / square foot.

This property, which is measured under prescribed testing procedures, determines the rate at which vapour passes through the membrane. The actual vapour transmission rate of a specific membrane is important in the design of the total roofing assembly with regard to the inclusion or exclusion of a vapour retarder.

2.7 11.2.2.7 Water Absorption

The amount of water absorbed by a material after immersion for a prescribed period of time, expressed as a percentage of the original weight of the material.

The membrane must be resistant to water absorption from continuous submersion in water due to ponding, whether because of poor drainage or snow and ice build-up. A significant loss or gain of weight during immersion would indicate that the membrane may not perform satisfactorily over a long period of time. This water absorption may affect dimensional stability and membrane thickness, and may cause internal stress which could lead to cracking.

2.8 11.2.2.8 Dimensional Stability

The change in length and / or width of a material that results from exposure to elevated temperatures over time, expressed as a percent.

Dimensional change which occurs after installation of the membrane may affect its watertight integrity and build up forces within the roof system. Such changes in sheet dimension can occur for a number of reasons: a) stress induced on the membrane during some manufacturing processes; b) stress introduced during the wind-up operation phase of some post-manufacturing processes; and c) the extraction of certain components of the compound due to contact with incompatible materials or through volatility of the compound.

The effect of all of the above conditions can often be accelerated by testing at elevated temperatures.

2.9 11.2.2.9 Factory Seam Strength

The force required to cause failure (in peel or shear) of a seam which has been created by the material supplier, expressed in MPa or psi or as a percentage of the strength of the sheet itself.

Not all manufacturers supply membranes containing factory seams. However, this property is considered to be as significant to the overall performance as are field seams. The most disruptive forces to which a membrane will be subjected occur during installation. The factory seam must resist unfolding, stretching, pulling, and fluttering by the installers during placement and final positioning of the sheet.

2.10 Low Temperature Resistance

(Also reported as “Low Temperature Flexibility”). The lowest temperature at which the material does not fracture or crack under prescribed impact and flexing conditions, expressed in °C or °F.

It is important for the membrane to be able to accommodate, without cracking, the combination of low temperatures and mechanical impact during application, structural movement, or rooftop traffic which occur in cold climates. However, there may not be a strong correlation between low temperature flexibility as tested in the laboratory and the actual temperature service range of the membrane on the roof.

2.11 Heat Aging

This test procedure is an attempt to accelerate the effect that solar heating will have on the properties of the installed roof membrane. The change(s) in physical properties (such as tensile properties) that result from exposure are then compared to those of the original unexposed material.

The results may provide some insight into, but no direct correlation with, the actual changes in physical properties which may occur during natural aging. It is particularly difficult to relate the exposure time during testing to real time during the life of the exposed membrane.

2.12 Accelerated Weathering

The process in which materials are exposed to a controlled environment where various phenomena, such as heat, water, condensation, and light, are altered to magnify their effects, thereby accelerating the weathering process. The physical properties that result from this exposure are then measured and compared to those of the original unexposed material.

These tests are an attempt to provide insight into the long term performance of the membrane under exposure to the climatic variables of sunlight and precipitation. Again, there is no clear correlation between the test results and actual performance, and the relationship between test exposure time and real time is difficult to determine.

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