NATIONAL BUREAU OF STANDARDS REPORT

8309

THE EFFECTS OF THERMAL SHRINKAGE ON BUILT-UP ROOFING

by

William G 0 Cullen

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

1004- 12-10447

HAR 3 1 1964

NBS REPORT

8309

THE EFFECTS OF THERMAL SHRINKAGE ON BUILT-UP ROOFING

by

William G, Cullen

Sponsored by

Office of the Chief of Engineers

Department of the Air Force

Bureau of Yards and Docks

NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, the whole or in part, is not author Bureau of Standards, Washingti

the Report has been specifically

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

ts accounting documents intended

subjected to additional evaluation

listing of this Report, either in

i Office of the Director, National

the Government agency for which

>pias for Its own use.

II. S. DEPARTMENT OF COMMERCE

NATIONAL DUREAU OF STANDARDS

THE EFFECTS OF THERMAL SHRINKAGE ON BUILT-UP ROOFING

1 . INTRODUCTION

Asphalt and coal- tar-pitch built-up roofings are widely used by the U. S.

Department of Defense to protect large buildings having relatively flat

roofs (slopes less than 2 in. /ft.) located throughout the world. As a result of the widespread locations, these roofing systems are expected

to withstand various climatic conditions ranging from the hot, humid

Tropics to the cold Arctic and perform adequately for periods in excess

of 20 years. Experience has indicated that these expectations are not

fulfilled due to premature failure of components of the systems. For

example, an extensive survey of roofing in Alaska in 1957 revealed that

approximately 5G7, of the flat roofs,, the majority of which had no slope,,

installed since 1951 had failed and many of the failures occurred when

the roofs were only one or two years old [l]„* Similar experiences can

be cited for the performance of bituminous built-up roofings in other

climates. Obvious ly, the maintenance, repair, and replacement cost of

the prematurely failed roofs is quite large. Therefore, at the request

of agencies of the Defense Department a program was conducted under

Project 10447, Performance of Roofings, Tri-Service Engineering Investigations of Building Construction and Equipment, NBS, to identify the

factors involved in premature failures and to study the relation of the

pertinent factors to roof performance.

2. SOME CAUSES OF FAILURE

Field experience with built-up roof performance on military structures

has shown that many failures can be attributed to:

(a) Faulty workmanship.

(b) Faulty design.

(c) Application during inclement weather.

(d) Improper use of materials.

(e) Poorly designed and/or installed flashing systems.

-'Numbers in brackets refer to literature references at the end of this report
-
- 2 -

These causes and their effects are generally readily apparent and can be

eliminated to a large extent. However, the causes of other serious

failures common to built-up roofings, such as splitting, membrane

shrinkage, wrinkle formation, and wrinkle cracking are not as well known.

In our opinion, the primary causes of splitting and shrinkage failures

are different from those resulting in wrinkling and wrinkle cracking,

although some factors as solar heating and radiative cooling are common

to both.

A number of possible causes of wrinkling and subsequent wrinkle cracking

failures have been suggested by Joy [2] and Brotherson [3] as: watervapor pressure; absorption of moisture condensed on felts over insulation

joints; and movement in substrate (deck or insulation) with changing

moisture content. Cullen and Appleton [4] suggested the thermal cycle

produced by solar heating may contribute to the dimensional changes which

occur in the system components and result in the formation of wrinkles

and wrinkle cracking.

Although the seriousness of wrinkle formation and wrinkle cracking are recognized, it is believed that failures due to splitting and membrane

shrinkage result in more serious consequences. Field observations have

confirmed the widespread incidence of these failures, especially in cold

climates. For example, Figures 1, 2, and 3 represent some results of

membrane shrinkage. Figure 1 shows the number of splits which were

observed in the roof on one large structure, while Figures 2 and 3 indicate the amount of displacement of the flashings at an expansion joint

and at the gravel stop on the same structure, due to membrane shrinkage.

Although many membrane failures can be traced to the movement of a building component beneath the roof [5] which is transferred to the

membrane, ample evidence exists that thermal movement is also involved.

Therefore, in order to provide for the movement and minimize the

failures, the extent of the movement as well as other engineering

properties of a bituminous membrane should be known over the temperature

range to which the roofing may be subjected.

3. ENGINEERING PROPERTIES OF BITUMINOUS BUILT-UP MEMBRANES

In attempting to relate the engineering properties of a bituminous

built-up roof to splitting and shrinkage failures, three properties

should be considered: breaking load, breaking strain, and thermal

movement

.

0

o

o

o o

0

0

.«r-«os

0

©

t 4. w

FIGURE

1.

SPLITTING

FAILURES

ATTRIBUTED

TO

MEMBRANE

SHRINKAGE.

SECTIONS

ON

LEFT

WERE

APPROXIMATELY

3

YEARS

OLD.

SECTIONS

ON

RIGHT

WERE

APPROXIMATELY

9

YEARS

OLD.

FIGURE

2.

DISPLACEMENT

OF

FLASHING

AT

EXPANSION

JOINT

DUE

TO

MEMBRANE

SHRINKAGE.

FIGURE

3.

DISPLACEMENT

OF

GRAVEL

STOP

DUE

TO

MEMBRANE

SHRINKAGE.

- 3

-

3.1

Breaking Load

After a comprehensive study on some engineering properties of built-up

roofing^ Jones [ 6 ] reported the breaking load of a built-up membrane

(asphalt or coal-tar pitch - organic felt) tested in the length direction is double that of the cross direction,, while that of an asphaltglass membrane is equal in each direction and roughly the same as the

organic felt membrane in the cross direction. He further reported the

breaking loads of the organic felt membrane and of the glass felt

membranes increased 300% and 707, respectively when the temperature was decreased from 75°F to -20°F„

3.2 Breaking Strains

In the study referenced in paragraph 3.1, Jones [ 6 ] reported the breaking

strains for various bituminous membranes varied with temperature. For

example,, coal-tar pitch - organic membranes showed a marked decrease

from 1.7% at 75°F to 0.5%, at -20°F,, while the asphal t-organic felt

membranes decreased from 2.37<> at 75°F to 1 .2% at -20°F. The glass

membrane decreased from 1.7% at 75°F to 1.1% at -20°F.

3.3 Thermal Movement

Generally the dimensions of all substances increase as the temperature

increases and decrease as the temperature decreases. The extent of this

linear movement over a specified temperature range is described as the

coefficient of linear expansion. In order to relate thermal movement

to the performance of built-up roofing^, it is necessary not only to know

the expansion coefficient of the bitumen and the saturated felt.,, but also

that of the composite built-up membrane constructed from these components.

The coefficient of cubical expansion of asphalt and coal-tar pitch is

reported to be 3.5 to 3.9 X 10

~ 4 (32°F to 140°F) and 2.6 X 1(T 4 (2Q0 S F

to 300°F)/°F [7]. If the assumption is made that the linear expansion

is about 1/3 that of the cubical expansion^, the linear expansion co- efficient becomes 120 X 10 ~^ and 90 X 10“^ in,/in./ 0 F for asphalt and

pitch over the temperature ranges indicated.

Cullen [ 8 ] reporting on a limited study of the thermal movement in

relation to solar heating indicated the coefficient of linear expansion

was not linear with temperature and increases markedly as the temperature

is decreased. He further reported the expansion coefficient to be in

the range of 10 to 31 X 10“6 in./in./ e F for various composite bituminous

built-up membranes over a temperature range of -60®F to 0°F when measured

parallel to the machine direction of the reinforcing felt. However,, no published information~tLs available for the linear expansion coefficients

of roofing felts nor has data been published indicating the relationship

between linear expansion and direction of the felt. Therefore^, a program

was conducted to obtain these data.

4

-

4. MATERIALS AND TESTING PROCEDURE

Four types of bituminous-saturated felts and two types of bitumen which

are conventionally used in the construction of built-up roofs were

selected for study. Table 1 identifies the materials. In one case,

the specimens for tests consisted of four plies of felt which were mechanically fastened to each other without the use of bitumen,, while

in the other the specimens consisted of alternate plies of felt

cemented to each other with the appropriate bitumen (about 20 lb/sq.).

The thermal movement measurements were made on four 12-inch by 2-inch

specimens of each sample, two of which (duplicates) were prepared with

the 12-inch dimension perpendicular to the machine direction of the

felt and the remaining two (duplicates) were prepared with the 12-inch

dimension parallel to the machine direction of the felt. Brass reference

plugs, 1/4- inch in diameter, were inserted and sealed with an epoxy

adhesive into holes drilled into the specimen near each end and spaced

about 10 inches apart.

The specimens were placed unrestrained on a flat surface, dusted with

talc to reduce friction in the conditioning chamber. The temperature

of the chamber was then cycled from -60°F to 140°F and back down to

-60°F. At 20°F increments, when a control specimen reached equilibrium,

the distance between the reference plugs was measured to the nearest

ten thousandth of an inch with a Whittemore Strain Gauge, as shown in

Figure 4.

TABLE 1. MATERIALS USED FOR SPECIMEN PREPARATION

Bitumens

Asphalt

Coal-tar pitch

D3 12-44

D45Q-41, Type A

Felt, Asphalt-Saturated, 15 lb.

Felt, Asbestos, Asphalt-Saturated, 15 lb.

Felt, Coal-tar Saturated, 15 lb.

Felt, Glass fiber, Asphalt-Saturated, 8 lb

D226-60

D25Q-6Q

D227-56

D2178-63T, Type I

FIGURE

4.

THERMAL

MOVEMENT

MEASUREMENT

TECHNIQUE.

- 5

-

5. RESULTS

The averaged results expressed as the apparent linear expansion coefficient

of felts and of the composite membranes obtained both "in the machine" and

"across machine" directions are reported in tables 2 and 3, respectively.

The coefficients which were calculated from the data obtained during the

cooling cycle from 30°F to -30°F are expressed in inches per inch per

degree F.

Although the determinations were made over a much larger temperature range,

the coefficients were calculated over the range from +30°F to -30°F

since previous work [8] showed that the movements caused by temperature

changes are minimal at moderate and high roof temperatures and quite

criticaj. at low temperatures. Further it was believed that this was a realistic range for roofs exposed in many areas where splitting failures

have occurred.

pie results obtained compare favorably with previous results of thermal

nfiovement of composite built-up membranes in the machine direction which

were obtained under Project 10447 [9],

TABJJE 2. APPARENT EXPANSION COEFFICIENT OF ROOFING FELTS

(TEMPERATURE RANGE 30 ®F TO -30°F)

Description Expansion Coefficient. in./in./°F.

"With Machine" "Across Machine"

Asphalt- Saturated Organic

Asphalt- Saturated Asbestos

Asphalt- Impregnated Glass

Coal-Tar-Saturated Organic

6.3 X 10“6

6.3 X 10”6

14.5 X IQ-6

6.5 X 10“ 6

13.3 X 10-6

13.0 X 10-6

18.2 X 1Q~ 6

15.2 X 10“6

TABLE 3. APPARENT EXPANSION COEFFICIENT OF BUILT-UP MEMBRANES

(TEMPERATURE RANGE 30 a F to -3G°F)

Description

"With Machine" "Across Machine'

Asphalt - Asphalt-Sat. Organic 10.9 X 10-6 20.8 .X IQ" 6

Asphalt - Asphalt-Sat. Asbestos 8.3 X 10“6 20.3 X 10~ 6

Asphalt. - Asphalt- Imp. Glass 18.1 X 10“ 6 29.4 X 10~ 6

Coal-Tar Pitch - Coal-Tar-Sat. Org. 19.4 X IQ-6 26.1 X 10“ 6

- 6 -

6. SUMMARY AND DISCUSSION

The data indicate;

1. Bituminous built-up membranes are subject to rather large

shrinkage movements when the temperature is substantially decreased

at sub-freezing temperatures.

2. The thermal movement of bituminous built-up membranes is not

linear with temperature and increases as the temperature is decreased.

3. The thermal movement of the composite built-up membrane is

greater than that of the felt from which it was constructed 9 but con- siderably less than that of the bitumen used as the plying cement.

4. The thermal movement in the ““across machine"" direction of both

the bituminous saturated and impregnated felts and the built-up membrane

constructed of the felts is appreciably greater than that observed in

"“with the machine"" direction.

Field observations of splitting and membrane shrinkage failures by the

author and other investigators on roofs throughout the United States

have revealed some factors which are frequently common to these types

of failures,, as;

1. The failures are more apparent in built-up membranes placed

over insulation.

2. The cracking generally occurs parallel to the machine direction

of the felt.

3. The cracks or splits often coincide with the longitudinal joint

between the insulation boards. Seldom do they occur over the broken

joint.

4. The incidence of splitting failures is much greater in the

colder climates and the failures generally occur during periods of

extremely cold weather with little or no snow cover.

5. The roofs having large areas which are infrequently broken by

expansion joints are more susceptible to splitting and membrane shrinkage

failures than roofs of smaller area.

7

These observations would certainly suggest that thermal shrinkage of the

membrane may be involved in such failures . In this connection, the

results obtained in the laboratory confirm those obtained in actual

field exposure So For example, it has been demonstrated experimentally

in laboratory tests that roof membranes placed over insulation are subjected to greater temperature fluctuations [4] and as a consequence,

greater thermal movement [9] than similar membranes placed over more

dense substrates. Further it appears to be more than coincidental that

the splitting failures occur more frequently in the “'across machine"

direction of the membrane and that laboratory measurements have established

that not only is the thermal movement larger in the "across machine"

direction, but the strength is considerably less in this direction [6] 0

The incidence of cracks occurring over joints in the substrate may be

explained by a theory of stress concentration. For example, if a built-up membrane is subjected to a temperature change of 60 C F (+3G®F

to -30°F), it can contract as much as 0.1 870 as calculated from the

data reported, herein. Now based on the data of Jones [6], a similar

membrane has a breaking strain of 0.457, at “2Q°F. Therefore, it

appears safe to assume that if the roof membrane were free to move

no splitting failure would occur. On the other hand, if the membrane

were solidly adhered to the substrate (as it will be in many cases),

except for small areas over the joints between units of the substrate,

it is possible that an area of stress concentration may develop over

the joint and the strain may be of sufficient magnitude to cause failure.

The test data demonstrated that the thermal expansion coefficient of

bituminous roofings increase as the temperature decreases. Therefore,

it follows that such membranes are more susceptible to thermal shrinkage

when exposed in cold climates.

In conclusion, it is our opinion that the data regarding the engineering

properties of built-up roof membranes obtained in this and similar

programs in other laboratories will prove useful in explaining and predicting the performance of built-up roofing systems during exposure.

Further, we believe a better understanding of the fundamental factors

involved in roofing performance may lead to deviations from conventional

practices of the built-up roofing industry regarding roof application

and, thereby, increase the life of the roofing.

8

7. SUGGESTIONS

The data obtained during the investigation as well as the observations

of splitting and shrinkage failures in the field suggest that thermal

movement of the built-up roofing may be involved* The question,, therefore, arises as to the precautions that can be taken to avoid or reduce

the incidence of these failures. Obviously, the alteration of the basic

properties of bituminous roofing materials to overcome the effects of

thermal movement is not economically feasible. However, the innovation

of certain deviations from standard roof design and roofing procedures

based on data obtained in the studies of the engineering properties of

roofing materials may prove helpful in preventing premature roofing

failures involving splitting and membrane shrinkage. It is with this

intent that the following suggestions are made:

1. In climates where the average January temperature is 25®F

or less, the roof membrane and insulation should be separated by the

application of the membrane to the deck and the insulation beneath

the deck, where possible* This practice will serve to reduce the

large temperature fluctuations which occur in a roof membrane applied

over substrates of low density due to solar heating and radiative cooling.

2. When the use of insulation between the roof membrane and the

roof deck is necessary, as it will be in the majority of the cases

(metal decks, isolation of structural movement, etc.), the joints

between the insulation boards should be taped. This practice will

serve to eliminate areas of stress concentrations over the joints

between insulation boards. It is interesting to note that at least one major roofing manufacturer has recommended this practice for a number of

years.

3. The insulation should be applied to the deck so that the

longitudinal (continuous) joint is parallel to the short dimension of

the roof, while the roofing felts should be applied parallel to the

long dimension of the roof and perpendicular to the longitudinal

joints of the insulation. This practice would use the greater strength

of the reinforcing felts “in the machine88 direction to advantage while

eliminating the potential stress concentrations over joints due to the

greater thermal movement of felts in the “"across machine8 " direction.

4. The adhesive bond between the roof membrane and the substrate

should be of optimum strength, i.e., strong enough to hold the membrane

in place under conditions of exposure (wind uplift, etc.), but sufficiently vulnerable to failure as to permit the distribution of strains over larger areas of the membrane in the event of thermal shock. A

requirement for spot, sprinkle or strip mopping in lieu of solid

mopping would serve to accomplish this end. In addition, the use of a

laminated base sheet, in which both the elongation of the respective

laminants and the strength of the adhesive bond between laminants

could be controlled, may produce the desired effects.

5. The intelligent use of expansion joints in the roofing mem- brane (not structural joints) will of course reduce the incidence

of failure due to thermal shrinkage of built-up membranes. The

spacing of joints would vary with the climate to which the roof is

exposed. The average January temperature could be used as a guide

to establish spacing of expansion joints.

8. REFERENCES

[1] Report of Alaska Roofing Board, 16 August 1957.

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

The Pennsylvania State University, September (1963).

[3] D. E. Brotherson, Research Report 61-2, Small Homes Council - Building Research Council - University of Illinois (1961).

[4] W. C. Cullen, W. H. Appleton, The Construction Specifier, Vol.

16, 5, p. 35, October (1963).

[5] R. D. Marquis, Bulletin of Midwest Roofing Contractors, Vol.

VII, Issue 1, June (1963).

[6] P. M. Jones, ASTM Symposium on Recent Research on Bituminous

Materials, June (1963).

[7] Herbert Abraham, Asphalt and Allied Substances, Vol. IV, 169

(1962).

[8] W. C. Cullen, National Bureau of Standards Technical Note 231,

(1963).

[9] W. C. Cullen, National Bureau of Standards Report 7831,

30 March 1963.

USCOMM-NBS - DC
nsgilisSjori pti • „£