NATIONAL BUREAU OF STANDARDS REPORT

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8632

A NEW APPROACH TO ROOF SYSTEM DESIGN

by

William C. Cullen and Thomas H„ Boone

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT NBS REPORT

421,04-12-4210447 19 February 1965 8632

A NEW APPROACH TO ROOF SYSTEM DESIGN

by

William C. Cullen and Thomas H. Boone

Organic Building Materials Section

Building Research Division

Institute for Applied Technology

Sponsored by

Office of the Chief of Engineers

Department of the Air Force

Bureau of Yards and Docks

NATIONAL BUREAU OF S

for use within the Government,

and review. For this reason, t

1 whole or in part, is not authi Bureau of Standards, Washinf

the Report has been specificall

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Approved for public release by the

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y the Government agency for which

copies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A NEW APPROACH TO ROOF SYSTEM DESIGN

by

William C. Cullen and Thomas H. Boone

ABSTRACT

Failures of bituminous built-up roofing membranes in service

which result from thermally and mechanically induced stresses

are related to some physical properties of the membrane such

as breaking strength, elastic modulus, and coefficient of

linear thermal expansion. A technique for preparing extremely

uniform built-up roofing membrane test specimens is described

„

The results of load-strain tests and linear thermal expansion

measurements for three bituminous built-up membranes are given at temperatures of 30°F (-1.1°C), 0°F (-17.8°G), and

“30°F (“34.4°C). A Thermal Susceptibility Factor based on these properties is proposed and values are given for the three

membranes. The Thermal Susceptibility Factor is related to

performance in service. The utilization of the factor as a guide to manufacturers, architect-engineers, and roofing

contractors in the manufacture, selection, and application

of components and in the design of a roofing system is proposed .

1 . INTRODUCTION

In 1964, the amount of built-up roofing components produced was sufficient

to cover over 1.7 billion square feet (17 million squares) of roof area

at a cost in excess of 450 million dollars. A large number of roofing

systems constructed from these components will perform adequately for 20 or more years. However, the small percentage which are destined to fail

prematurely will present serious and costly problems to manufacturers who

produce the components of the system, the architects who design the systems,

the contractors who apply them, and the building owners and occupants who

expect good service from them.

In the design and construction of a roofing system, the architect and roofing contractor have little flexibility. They must rely either on their

past experience with the performance of a specific system under known

exposure conditions or on the material and application specifications

recommended by manufacturers. In order to impart some flexibility to those

charged with responsibilities involving the design and construction of a

- 2 -

roofing system,, there is a need for criteria by which one can select the

suitable materials and proper design for a roofing system which will

perform adequately under specific exposure conditions.

The performance of a roofing system cannot be evaluated entirely on the

basis of the properties of the individual components. An understanding

of the interdependence and interaction between and among components is

of paramount importance and must be considered,. This is apparent from

the discussion, by various authors, of problems such as blistering,

wrinkle cracking and membrane shrinkage failures [1, 2, 3, 4, 5, 6]*,

frequently associated with bituminous built-up roofing systems. Nevertheless, this does not preclude that the properties of the individual

components can be ignored. Indeed, it is most important that in order

to comprehend the performance of the total system the function of each

individual component must also be understood. It was with this view in

mind that this research was conducted.

The object of this research was to obtain data on some engineering

properties of conventional built-up membranes and to relate these data

to the performance of roofing systems in service. Another object was

to propose the utilization of these data as a guide to the manufacturer,

the architect-engineer, and the roofing applicator in the manufacture,

selection, and application of components as well as in the design of a roofing system.

2. THERMALLY AND MECHANICALLY INDUCED STRESSES

Any discontinuity in the waterproofing element of the roofing system

generally leads to a failure of the total system from both a thermal and

weatherproofing standpoint. Recent experience has shown that splitting

of built-up roofing membranes is a frequent cause of this discontinuity

[7]. Initially it was believed that roofing splits resulted from

shrinkage of the membrane due to thermally induced stress alone. However, recent studies by Jones [8] and Cullen [9] on engineering properties of bituminous built-up membranes have indicated that this is not

the case and mechanically induced stresses must also be considered.

Mechanical stresses induced in the roofing membrane by differential move

merits between units of the substrate as evidenced by cracks or joints

have been discussed by Koike [10]. He derived equations describing the

conditions of rupture for a bituminous membrane in relation to some

'^Figures in brackets indicate literature references at the end of

this report.

3

-

physical properties of the membrane and the adhesive system used to bond

the membrane to the substrate . Further^ he confirmed the validity of the

equations by laboratory experiments. For example,, when the shear strength

(T a) of the adhesive between the substrate and the membrane is equal to or exceeds a quantity expressed as follows:

(E) <t a ) (tm> where ( 1 )

S = Tensile strength of bituminous membrane^ lb/ in.

G = Modulus of elasticity of adhesive in shear,, lb/in.

E = Modulus of elasticity of membrane , lb/ in.

t a = Thickness of adhesive,, in.

tm = Thickness of membrane,, in.

the membrane will fail when the width (W) of the separation in the substrate

is equal to or exceeds a quantity expressed as follows:

On the other hand,, when the shearing strength f$\) of the adhesive is less

than the value of equation (1), a shear failure will occur within the

adhesive and regardless of the width of the separation in the substrate,, no rupture will occur in the membrane.

In connection with thermally induced stresses in bituminous built-up

membranes^ Cullen [6] proposed a Thermal Susceptibility Factor wich will

prove useful to predict the ability of a membrane to tolerate movements

produced by rather sudden temperature changes. Again this factor is

related to some, engineering properties of the built-up membrane and can

be expressed as follows:

TSF = - , „ S

. ..... where (3)

E dL

TSF = Thermal Susceptibility Factor

E = Modulus of Elasticity,, lb/ in**

S = Breaking Load,, lb/ in.*

= Coefficient of linear thermal expansion per °F.

*The breaking loads and moduli of elasticity are expressed in lb/ in. rather

than the more conventional dimensions of lb./ in. 2 since a previous study [8]

has indicated the effects of the bonding bitumen on the strength characteristics of a bituminous built-up membrane are negligible.

4

Ass uming the relations proposed by Koike and Cullen are valid, it is

evident that the resistance of a roofing system to mechanically and

the.rmaJ.ly induced stresses can be predicted, in a qualitative manner at

least, by measuring some physical properties of the composite membrane

and the adhesive system used to attach the membrane to the substrate.

These measurements should be made, of course, at temperatures corres- ponding to the minimum temperatures expected at the exposure location.

Further, when a. correlation between these factors and performance in

service is established, it will be possible to assign numerical values

to these factors to predict acceptable performance of a specific roofing

system under specific climatic exposures. These values could be utilized

by the manufacturer, architect-engineer, and roofing contractor in the

manufacture of components and in the design and application of roofing

systems

3. MATERIALS AND SPECIMEN PREPARATION

Three types of bituminous saturated or impregnated felts and two types

of bitumen which are currently being used in the construction of built-up

roofs were selected for the study.

For the first series of tests, each specimen consisted of four plies of

the saturated felt adhered to each other without the use of additional

bitumen or mechanical fasteners. For the second series of tests, each

specimen consisted of four plies of the saturated felt adhered to each

other with the appropriate bitumen applied at a spreading rate of about

25 lb. per 100 sq. ft., which is comparable to standard roofing practice.

The samples are identified as Samples A, B, or C, depending on their

composition.

The four-ply specimens of felt alone were prepared by heating 4-in. X

8-in. sections of felt to about 248°F ( 1 20° G) for about 5 minutes and

while hot, four plies were pressed together in a laboratory press in

such a. manner as to utilize the bituminous saturant as an adhesive.

The four-ply composite membrane specimens were prepared by heating the

bituminous cement to 212 to 266 t’F (100-1 3Q°C) . The heated bitumen was

poured on an 8= X 4- inch section of the saturated felt placed on a larger piece of unlacquered cellophane and a second section of felt was placed on the bitumen to form a sandwich. This in turn was covered

with unlacquered cellophane and placed in an unheated laboratory press

using two 0.125-inch diameter rods, placed on either side and adjacent

to the specimen, as spacers. Sufficient pressure was applied so that

the platens of the press were separated from each other by the spacers.

When cool, the two-ply sandwich was taken from the press, the top piece

5

of cellophane removed and two additional plies were applied (one at a time) by repeating the above process using spacers having diameters of

0.218 inch and 0.312 inch for the third and fourth plies, respectively.

The specimens were chilled to about 32°F (0°G) and dumbbell, specimens were stamped out using a suitable die in the laboratory press. Figure 1 shows the die, the test specimen, and a cross section of the test specimen.

It is extremely important that felts in any single specimen be oriented

and identified in respect to longitudinal (machine) and transverse

(across machine) directions,,

4. LOAD" STRAIN PROPERTIES IN TENSION

The load-strain properties of the saturated felts and the composite

membranes were measured on triplicate specimens employing a tensile

testing machine equipped with a temperature chamber to control the

specimen temperature within ±5°F (±2„7°C) during the test. The loadstrain measurements were made at =30°F C~34.4°C), 0°F (- 1 7 „ 8°C) , and

+30°F (“1.1°C). The temperature was measured by inserting a copperconstantan thermocouple in a typical specimen. This was believed to be

a realistic temperature range for roofs exposed in many areas of the

United States where failures have occurred. The gage length as defined

by the distance between the jaws of the testing machine was 4.5 inches

and a straining rate of 0.05 inches per minute (1.1% per minute) was used in each determination.

The averaged results of the load-strain measurements for triplicate

4-ply specimens of felts and membranes (identified as Samples A, B, &

C) are reported to the nearest 10 lbs. in Tables 1, 2, and 3.

5. LINEAR THERMAL MOVEMENT

Linear thermal movement measurements were made on specimens prepared as described in Section 3 using a 5-inch gage length in accordance with the

procedure described elsewhere [9]. The coefficients of linear thermal

expansion over a temperature range of +30°F to -30°F (-1.1°C to -34.4°C)

are reported in Table 4.



TABLE 1. LOAD- STRAIN PROPERTIES - SAMPLE A

Specimen Temp.

°F.

Direction Breaking

Load

Ib/in

Breaking

Strain

1

Elastic

Modulus

Ib/in

4-Ply saturated -30 L 380 1.1 9.2 X 10 4

felt only T 270 1.3 6.1 X IQ4

0 L 400 2.2 7.3 X 1C) 4

T 280 3.1 4.9 X IQ 4

+30 L 270 2.0 5.8 X 10 4

T 150 3.1 2.7 X 10 4

4-Ply composite -30 L 480 1.7 4.3 X IQ 4

Membrane T 310 1.4 3.0 X 104

0 L 400 2.6 2.8 X 104

T 300 3.2 1.9 X IQ 4

+30 L 270 2.3 2.1 X IQ 4

T 180 4.0 1.3 X 104

* L - Longitudinal

T - Transverse

TABLE 2. LOAD- STRAIN PROPERTIES - SAMPLE B

Specimen Temp

*F.

Direction

* Breaking

Load

lb/ in

Breaking

Strain

X

Elast ic

Modulus

Ib/in

4-Ply saturated -30 L 170 1.2 1.8 X 10 4

felt only T 130 1.3 1.3 X 10 4

0 L 180 1.5 1.4 X 10 4

T 160 1.6 1.5 X ]04

4-30 L 150 1.7 1.2 X !04

T 120 1.6 1.0 X 10 4

4-Ply composite -30 L 230 1.4 2.0 X 10 4

Membrane T 170 1.4 1.7 X !0 4

0 L 200 1.4 1.7 X 104

T 200 1.7 1.5 X 104

+30 L 180 2.6 1.3 X ?G4

T 150 2.3 1.1

.1. 10*

* L - Longitudinal I T - Tr«m«Ter»e

TABLE 3. LOAD- STRAIN PROPERTIES - SAMPLE C

Specimen Temp

°F.

Direction

* Breaking

Load

lb/ in.

Breaking

Strain

X

Elastic

Modulus

lb /in

4-Ply saturated -30 L 390 1.6 3.7 X 10^

felt only T 180 1.1 2 .6 X 10*

0 L 350 2.1 2 . 7 X 104

T 180 2.6 1.4 X 104

+30 L 170 2.8 1 *4 X 10^

T 60 4.7 0 .4 x 10 ^

4-Ply composite -30 L 380 1.3 4. OX 104

Membrane T 210 1.2 2.6X tO 4

0 L 400 2.2 4 .9 X 10 4

T 180 2.8 2 .9 X 10 4

+30 L 240 2.5 3. OX IQ 4

T 120 3.0 1 .2 X 104

* L - Longitudinal

T - Transverse

- 6 -

TABLE 4. APPARENT LINEAR THERMAL EXPANSION COEFFICIENT OF 4-PLY

BUILT-UP MEMBRANES (TEMP. RANGE +30°F to -30°F)

Expansion Coefficient (°F)"^

Longitudinal Dir. Transverse Dir„

Sample A - 4-ply composite

Sample B - 4-ply composite

Sample C - 4-ply composite

membrane 7.8 X

membrane 11.7 X

membrane 19.4 X

o

6 O' 16.9 X

j o 10~ 6 14.6 X 10~6

10"6 39.6 X o

s ON

6„ THERMAL SUSCEPTIBILITY FACTOR

In order to establish a numerical value for thermal shock resistance, a Thermal Susceptibility Factor was calculated for each of the composite

membranes included in the program. The values were obtained for temperatures of = 30°F (-34.4°C), 0°F (-17.8°G), and 30°F (-1.1 °C) by substituting the data in equation 3 (see section 2). The factors rounded

off to the nearest fifty for the respective built-up membranes in both

the longitudinal and transverse directions of the reinforcing felts are reported in Table 5„

7. RELATION BETWEEN SPLITTING FAILURES AND THE

THERMAL SUSCEPTIBILITY FACTORS.

Investigations by Cullen [9] and Moseley [7] of splitting failures in

bituminous built-up membranes during recent years on roofs protecting

structures in the colder climates have revealed some factors which frequently contribute to such failures. The factors include, among others,

climate, orientation of the felt in the membrane and the thermal characteristics of the substrate.

It is interesting to develop a relation between the frequency of splitting

failures of membranes exposed to actual weathering conditions and the magnitude of the Thermal Susceptibility Factors reported in Table 5.

7 . 1 Climate

The incidence of splitting failures was greater in the colder climates

and the failures were frequently reported following periods of extremely

cold weather with little or no snow cover. It can be seen from Table 5 that with few exceptions the Thermal Susceptibility Factor decreases in

value as the temperature is decreased. It appears that a correlation is

established

TABLE 5. THERMAL SUSCEPTIBILITY FACTORS FOR

4-PLY COMPOSITE MEMBRANES

Sample Temp

e F.

Direction of Felt

Longitudinal

in Membranes

Transverse

A -30 1450 600

0 1850 950

30 1650 800

B -30 1000 700

0 1050 900

30 1200 950

C -30 500 200

0 400 250

30 400 250

7 7.2

Thermal Characteristics of Substrate

Splitting failures due to membrane shrinkage were more common in bituminous

built-up membranes applied to substrates having high thermal insulating

values. Since membranes placed over such substrates are subcooled to

a greater extent due to radiative cooling than similar membranes placed

over more dense substrates [ill], the conclusion can be made that the

Thermal Susceptibility Factor will have a lower value due to the lower

temperature. This is substantiated by the data reported herein.

7.3 Orientation of Reinforcing Felt

Splitting failures resulting in part from membrane shrinkage were observed more frequently in the transverse direction of the reinforcing

felt than in the longitudinal direction, thus indicating the anisotropic

behavior of the bituminous built-up membrane. It appears to be more

than coincidental that the Thermal Susceptibility Factor also demonstrates

this anisotropic behavior in that it is, in most cases, considerably

lower in the transverse direction than in the longitudinal direction.

7.4 Composition of Built-Up Membrane

The significant differences in the values of the susceptibility factors

among membranes identified as Samples A, B, and C would imply differences

in performance among the membranes in field service regarding splitting

failures. However, to our knowledge, a correlation has not been established to date since sufficient data on splitting failures in the field

in respect to composition of the membrane have not been developed.

8. APPLICATION OF THEMAL SUSCEPTIBILITY FACTOR

The differences in the values among the Thermal Susceptibility Factors

of the various membranes in relation to composition, temperature and

felt orientation are apparent and are indicative of the thermal shock

resistance of the respective membranes in service. The higher value

indicates the better thermal shock resistance. The question now arises

as to the application of this factor to roofing system performance in

the field.

We believe that a knowledge of the factor will prove useful in predicting

the ability of a built-up membrane to withstand stresses induced by

thermal movement. Further, the value of the factor assigned to a roofing

membrane will depend to a large extent on the climate to which exposed,

the position of the membrane in the roofing system, the orientation of

the felt in the membrane and to the area of the roofing unbroken by

expansion joints. For example, a higher Thermal Susceptibility Factor

will be required for a membrane exposed in an area experiencing an

average January temperature of 20°F than that required for another mem- brane to be exposed where the average January temperature is 30°F. Similar ly, a higher factor will be necessary for a membrane placed over a substrate having a. high thermal insulating value than that needed for a membrane placed directly on a concrete deck. We would also expect that

a higher Thermal Susceptibility Factor would be desirable for membranes

on roofs where large areas are involved as opposed to those having

smaller areas.

In conclusion, we are reluctant to suggest, at this time, even a range

of values for the Thermal Susceptibility Factor to describe an acceptable

roof membrane under given conditions because adequate supporting information from field service is not available. However, we believe that

the collection and utilization of these data by the manufacturer, the

architect-engineer, the roofing contractor, and the ultimate consumer will prove immediately beneficial and in the future may provide him

with the flexibility to manufacture., design and apply suitable roofing

systems for any combination of circumstances.

9 . ACKNOWLEDGMENT

The authors acknowledge with thanks the assistance of W. H. Appleton

and A. J. Turner in the preparation of the specimens and in making

the measurements reported in this paper.

10. REFERENCES

[ 1 ] C. E. Lund and R. M. Granum, Univ. of Minnesota Eng 0 Exp. Stn.

Bulletin No. 34 (May 1952).

[2] W. B, Warden, Building Research Institute Monograph No. 1 (I960).

[3] D„ E. Brotherson, Research Report 61-2, Small Homes Council-Building

Research Council, University of Illinois (1961).

[4] G. 0. Handegord, Technical Paper No. 182 of the Division of Building

Research, National Research Council, Canada (June 1964).

[5] F. A. Joy, Better Building Report No. 5, College of Engineering,

The Pennsylvania State University (September 1963).

[6] W. C. Cullen, NBS Technical Note No. 231 (December 1963).

[7] G. No Moseley, ’"An Explanation of Built-Up Roofing Tension Splits",

Building Research Institute 1964 Fall Conference (November 1964).

[8] P. M. Jones, Recent Research on Bituminous Materials, ASTM STP-347

(1963).

[9] W. C. Cullen, NBS Monograph No. 89 (1965).

[10] Michio Koike, BRI Occasional Report No. 15, Building Research

Institute, Japan (December 1963).

[11] W. C. Cullen, W. H. Appleton, The Construction Specifier _1_6, 5,

35 (October 1963).

USGOMM-NBS-DC