Roof-System Performance II

Nov. 12, 2008
A second in a series of three articles that explore roof-system performance

The last issue of this column began exploring roof-system performance; this issue continues that discussion.

Performance of Roofing Components and Systems by Charles Hedlin was documentation for a series of seminars conducted by National Research Council Canada – Institute for Research in Construction (NRC-CNRC) during 1989 called “Roofs that Work.” This paper provided a comprehensive overview of this very complex issue.

Topics included:

  • Roof systems and assemblies, and their components.
  • Performance of roof components.
  • Main functions and properties.
  • Securing the components.
  • Decks and structure.
  • Vapor barrier functions.
  • Thermal insulation.
  • Additional factors affecting roof performance.
In any mathematical analysis, the number of possible variables greatly affects the results. If variable a has two possibilities, variable b has six, and variable c has 10, then we express this as Vtotal = a • b • c = 2 • 6 • 10 (or 120 possible results). Looking at Hedlin’s paper from this perspective, variables include:

Roof systems: (2) Conventional and Protected Membrane Systems
Roof membranes, top covers, and types of thermal insulation

  • Bituminous (BUR):  (8) (Asphalt types = 4; Coal-tar types =1; Hot applied, mop, torch, cold adhesive = 3)

Modified bitumens:  (14, at least)

  • APP, SBS, APO, asphalt = 4
  • Reinforcements woven, non-woven, combinations of both, thickness or weight per square meter in grams = 10

Single-ply membranes: (9 at least)

Polymer type (PVC, TPO, EPDM, CSPE, KEE, CPE) = 6
  • Thickness of sheets (45, 60, 90 mil) = 3
  • Reinforcements: (8, at least)

    • Glass fiber, polyester, woven, scrim, non-woven = 4

    Number of layers: one, two, three, more = 4

    Thermal Insulations possible

    • Wood fiber, perlite, cellular glass, glass fiber, urethane/isocyanurate, phenolic foam, polystyrene (extruded or expanded), mineral wool fiber, high density urethane cover board = 11
    • Differing densities, facers, thicknesses, cover boards = 12 (at least)

    Methods of attachment = 5 (at least!)

    • Thermal insulation attached, membrane adhered
    • Thermal insulation attached, membrane mechanically attached
    • Thermal insulation nominally attached to stay in place, membrane loose laid and ballasted
    • Protected configuration, insulation on top of membrane
    • Self-adhered

    Use of air and vapor barriers (yes or no) = 2
    Structural support

    • Continuity, strength, moisture content, rigidity (deflection), vibration

    Surfacing options = 4

    • Coatings
    • Factory-applied granules
    • Ballast
    • None

    As Hedlin points out, each component will have a full range of other properties, such as coefficient of thermal expansion; tensile, compressive, and shear strengths; vapor permeability and moisture accumulation rates; aging characteristics (including embrittlement and reduced thermal resistance); ductility; softening point for asphalt; and flammability. According to Hedlin, “Depending on the circumstances, all of the properties may come into play, anticipated or not. Their effects may be unimportant or significant, beneficial or damaging.”

    By now, the reader should get the point. By the time you add the variables for workmanship, abuse, desired aesthetic appeal, and, of course, life and cost, it’s easy to see why we have taken the prescriptive approach.

    Let’s fast forward. Recognizing the oil embargo of 1973-1974 was yet another variable not fully anticipated. In 1979, National Bureau of Standards (now National Institute of Standards and Technology) published Building Science Series (BSS) 167: Interim Criteria for Polymer Modified Bituminous Roofing Membrane Materials. Quoting from the abstract of this document:

    “… [it] presents the results of a study to develop interim criteria for the selection of polymer modified bituminous roofing membrane materials. The criteria are based on a review of existing standard specifications and related documents. They are intended for use by the construction agencies of the Department of Defense in specifying polymer modified bituminous roofing membrane materials until voluntary consensus standards are developed in the U.S. The suggested interim criteria generally are presented using a performance criteria format. The membrane characteristics for which performance criteria are suggested are dimensional stability, fire, flow resistance, hail impact, moisture content and absorption, pliability, strain energy, uplift resistance, and weathering resistance (heat exposure). Prescriptive criteria for five member characteristics are used to complement the suggested criteria. The approach of using complementary prescriptive criteria is taken to incorporate the performance criteria test methods, which can be relatively rapidly performed for characterization or identification of the membrane material. Other membrane requirements are listed for future development of criteria, but performance criteria for these requirements are not suggested at present. Lack of a consistent database in the existing standards and related documents precludes suggesting criteria at this time. It’s considered beneficial to present the needed criteria as a first step toward directing future research for standards development for polymer modified bituminous roofing membrane materials.”

    Suggested criteria for modified bitumens included:

    1. Dimensional stability – maximum 1 percent.
    2. Fire resistance – meet FM Global, UL, code.
    3. Flow resistance – no slippage.
    4. Hail impact – no leakage after impact 30J falling dart or 1.5-inch diameter hailstone
    5. Moisture content – 0.5 percent maximum.
    6. Water absorption – 1 gram for specified size of specimen.
    7. Low-temperature flex – no cracking at expected low temperatures.
    8. Strain energy – not less than 3 pound force (lbf) • inch/inch2 at 0 degrees F. (This is also called work-to-break, the area under the curve of stress and strain. Just as with the criterion of low-temperature strength for BUR, strain energy has been the most quoted criterion of the BSS 167 document.) At this time, this property is rarely cited, and most MB manufacturers are at a loss to explain why they have so many products in the marketplace with differing weights, amount, and type of reinforcement, or even whether combinations of polymer-modified cap sheets, for example with non-modified base sheets, makes sense or not.

    The good news is that, since BSS 167 was published, ASTM has managed to publish a number of prescriptive specifications for modified bitumens:

    • D 5602: Static Puncture Resistance of Roofing Membrane Specimens.
    • D 5635: Dynamic Puncture Resistance of Roofing Membrane Specimens.
    • D 5849: Evaluating the Resistance of Modified Bituminous Roofing membranes to Cyclic Joint Displacement.
    • D 6950 Application of APP Systems.
    • D 6135 Application of Self-Adhering MB Systems.
    • D 6509 APP Modified Base Sheet.
    • D 6222 APP with Polyester Reinforcement.
    • D 6223 APP Using Combinations of Glass & Polyester Fibers.
    • D 5849 Test Method for Cyclic Fatigue.
    • D 6298 SBS with Metal Foil Surfacing.
    • D 7051 Test for Thermal Cycle Shock of Foil-Faced Sheets.

    (In future columns in this series, we will look at existing standards for SPF, TPO, PVC, metal roofing, etc.)

    In 1979, Centre Scientifique et Technique du Bâtiment published FIT Classification for Roofing Systems. According to this document, “FIT is a performance-based classification for roofing systems. It’s mainly aimed at architects, engineers, and users and is, therefore, simplified in its design to be easy to understand and use . . . [it] should enable suitable roofing systems to be chosen for a given use based on major performance criteria regarding the stresses to which the systems are subjected."

    This is where the first significant strides were made on the performance concept: a group of experts, based on their experience and knowledge, have classified performance of roofs into levels of F, I, and T: F for fatigue, I for indentation (puncture), and T for temperature. For example, if a roof membrane is fully exposed to service traffic on the roof, it requires a higher degree of impact resistance than a protected membrane roof system would. Looking at indentation, the FIT systems looks at two conditions: static puncture, such as a concentrated load due to heavy A/C units placed on sleepers, and dynamic puncture, such as a dropped toolbox. (ASTM test methods for these conditions are now available.)

    For Static Load L:


    Failure Load, N

    Examples of systems



    Marginal roofing


    >5, <15

    Glass-reinforced MB


    >15, <25

    Polyester-reinforced MB



    Heavy polyester reinforced PVC or EPDM

    For Dynamic Impact:

    For Fatigue Resistance: (Level 5 is the highest level of resistance to damage)


    Examples of systems



    Marginal systems


    10 to 20

    BUR & MB min 4mm thickness
    PVC and EPDM min 1.2 mm thickness



    Special systems for car parks, roof gardens

    Bitumen type

    Reinforcement of Base Layer

    Cap Layer

    Fatigue Resistance Level


    180 g polyester




    60 g glass mat

    APP/180 g polyester



    200 g woven glass

    SBS/ 250 g polyester



    60 g glass mat

    SBS/180 g polyester



    180 g polyester

    SBS/180 g polyester


    Note in this FIT classification method: Any roof system that meets these requirements is permitted. It also provides a logical way to differentiate levels of performance based upon anticipated project conditions – something that individual modified bitumen manufacturers have been unable to do.

    The next column in this series will update you on the industry’s progress since the issuance of these documents.


    About the Author

    Richard L. Fricklas

    Richard (Dick) L. Fricklas received a Lifetime Achievement Award and fellowship from RCI in 2014 in recognition of his contributions to educating three generations of roofing professionals. A researcher, author, journalist, and educator, Fricklas retired as technical director emeritus of the Roofing Industry Educational Institute in 1996. He is co-author of The Manual of Low Slope Roofing Systems (now in its fourth edition) and taught roofing seminars at the University of Wisconsin, in addition to helping develop RCI curricula. His honors include the Outstanding Educator Award from RCI, William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.

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