NBS MONOGRAPH 89

Effects of Thermal Shrinkage

On Built-up Roofing

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Effects of Thermal Shrinkage

on Built-up Roofing

William C. Cullen

National Bureau of Standards Monograph 89

Issued March 4, 1965

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 - Price 10 cents
Contents

Page

1. Introduction 1

2. Some causes of failure 1

3. Strength characteristics of bituminous built-up membranes 3

3.1. Breaking load 3

3.2. Breaking strains 3

3.3. Thermal movement 3

4. Materials and test procedure 4

5. Results 4

6. Summary and comment 5

7. Suggestions 5

8. References 6

Library of Congress Catalog Card Number : 64-62622
Effects of Thermal Shrinkage on Built-up Roofing

William C. Gullen

The effects of thermal movement of bituminous built-up roof membranes are related to some common failures of built-up roofing observed in field exposures. A procedure for determining the amount of thermal movement of built-up membranes is described. Data

are given for the thermal movement of various bitumens and reinforcing felts used in the

construction of built-up roofs and for the composite membranes over a temperature range

of +30 to — 30 °F. The data obtained are related to field experience and suggestions are presented which will prove beneficial in reducing the incidence of built-up failure due to thermal movement.

1. Introduction

Asphalt and coal-tar-pitch built-up roofings are used throughout the world to protect large buildings having relatively flat roofs (slopes less than

2 in. /ft). These roofing systems are exposed to

various climatic conditions ranging from the hot,

humid tropics to the cold polar regions. An ex- pected life of 20 years is not always obtained due

to premature failure of components of the systems.

For example, an extensive survey of military roofing in Alaska in 1957 in which the author participated revealed that approximately 50 percent of

the flat roofs had failed. The majority of these

roofs had no slope and were installed after 1950.

Many of the failures occurred when the roofs were

only 1 or 2 years old. Similar performance of

bituminous built-up roofings have been observed

in other climates. Obviously, the maintenance,

repair, and replacement cost of the prematurely

failed roofs is quite large. The object of this re- search was to identify and study some of the

factors involved in premature failures and to re- late the pertinent factors to roof performance.

2. Some Causes of Failure

The author's experience over many years with

built-up roof performance has indicated that many

failures can be attributed to the following:

(a) Faulty workmanship.

(b) Faulty design.

(c) Application of roofing materials during

inclement weather.

(d) Improper use of materials.

(e) Poorly designed or installed flashing

systems.

These causes and their effects are easily recognized and can be avoided to a large extent by the

use of available material and design criteria enforced by construction inspection. However, the

causes of other serious failures common to builtup roofings, such as splitting or wrinkle cracking,

are not as well known. The primary causes of

splitting failures are believed to be different from

those resulting in wrinkle cracking, although some

factors as solar heating and radiative cooling are common to both. Splitting failures are postulated to result from shrinkage of the roof membrane while wrinkle cracks are attributed to fatigue

of the roof membrane resulting from repetitive

movements during the development and subsequent regression of wrinkles in the membrane.

A number of possible causes of wrinkling and

subsequent wrinkle cracking failures have been

suggested by Joy [1]* and Brotherson [2] as:

•Figures in brackets indicate the literature references at the end of this monograph.

Water-vapor pressure; expansion of roofing felts

resulting from absorption of moisture condensed

on felts over insulation joints; and movement in

substrate (deck or insulation) with changing moisture content. Cullen and Appleton [3] 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 cracking is recognized, it is believed that splitting results in a more serious failure. The author's experience

with the performance of built-up roofs has con- firmed the widespread incidence of splitting fail- ures, especially in cold climates. Figures 1, 2, and

3 show schematically failures attributed to 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 displacement of the flashings at an expansion joint

and at the gravel stop, respectively, on the same

structure, due to membrane shrinkage.

Although many membrane failures can be traced

to the movement of a building component beneath

the roofing [4] which is transferred to the membrane, evidence exists that thermal movement of

the membrane is also involved [5]. Therefore, in

order to allow for the movement and minimize the

failures, the extent of the movement as well as the strength characteristics of a bituminous membrane must be known over the temperature range

to which the roofing may be subjected.

1
1350'

750' 600'

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Figure 1. Splitting failures attributed to membrane shrinkage.

The four sections on the left were about 3 years old. The remaining sections on the right were approximately 9 years old.

Figure 2. Displacement of flashing at an expansion joint due

to membrane shrinkage.
DOTTED LINES INDICATE DISPLACED

Figure 3. Displacement of gravel stop due to membrane

shrinkage.

3. Strength Characteristics of Bituminous Built-Up Membranes

In attempting to relate the strength characteristics of a bituminous built-up roof to splitting

failures, three factors should be considered : Breaking load, breaking strain, and thermal movement.

3.1. Breaking Load

After a comprehensive study on some strength

characteristics of bituminous built-up roofing,

Jones [6] reported the breaking load of an organicfelt built-up membrane tested in the length direction was double that tested in the across machine

direction, while that of an asphalt-glass felt membrane was equal in each direction and roughly the

same as that of the organic-felt membrane in the

across machine direction. He further reported

the breaking loads of the organic-felt membrane

and of the glass-felt membranes increased 300 and

70 percent, respectively, when the temperature of

the specimen was decreased from 75 to —20° F.

3.2. Breaking Strains

Jones [6] also reported that the breaking strains

for various bituminous membranes varied with

temperature. For example, coal-tar pitch and

organic-felt membranes showed a marked decrease from 1.7 percent at 75 °F to 0.5 percent

at — 20° F, while the asphalt and organic-felt membranes decreased from 2.3 percent at 75 °F to 1.2

percent at —20 °F. The asphalt and glass-felt

membrane decreased from 1.7 percent at 75 °F to

1.1 percent at -20 °F.

3.3. Thermal Movement

The dimensions of most substances increase as the temperature increases and decrease as the

temperature decreases. The ratio of the increase

in length of a body to the original length for a

specified temperature rise is known as the coefficient of linear thermal expansion. The coefficient

of cubical thermal expansion is defined in like

manner except that the quantities are now volumes instead of lengths. In order to relate thermal movement of built-up roofing to its performance, it is necessary not only to know the linear

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 was reported to be 3.5 to

3.9 X10-4 per °F over the temperature range of

32 to 140 °F and 2.6 X10"4 per °F over the range

of 200 to 300 °F [7], respectively. If the assumption is made that the linear expansion is about

)'% that of the cubical expansion, the linear ex- pansion coefficient is 120 X 10 -6

per °F for asphalt

and 90X10~6 per °F for coal-tar pitch over the

temperature ranges indicated.

Cullen [5] reporting on a limited study of the

thermal movement in relation to solar heating

indicated the coefficient of linear thermal expansion of composite built-up membranes increased

markedly as the temperature decreased. He further reported the expansion coefficient to be in the

range of 10 to 31X10-6

per °F for various composite bituminous built-up membranes over a temperature range of —60 to 0 °F when measured

parallel to the machine direction of the reinforcing

felt. However, no published information was

available for the thermal expansion coefficients

of roofing felts nor had data been found indicating

the relationship between linear expansion and direction of the felt.

3
4. Materials and Test 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 lists the types of

materials used. For the first test each specimen

was cut from a sample that consisted of four plies

of a felt fastened to each other, without the use

of bitumen, by applying staples at each end, while

for the second test the specimen consisted of four

plies of felt cemented to each other with the

appropriate bitumen.

The thermal movement measurements were made on four 12-in. long by 2-in. wide specimens

of each sample, two of which were prepared with

the 12-in. dimension perpendicular to the machine

direction of the felt and the remaining two were

prepared with the 12-in. dimension parallel to the

machine direction of the felt. Brass reference

plugs, % in. in diameter, were inserted and sealed

with an epoxy adhesive into holes drilled into the

specimen near each end and spaced about 10 in.

apart.

The specimens were placed in the conditioningchamber, unrestrained on a flat surface, which

was dusted with talc to reduce friction. A copperconstantan thermocouple was inserted into one of

the specimens to measure the specimen temperature during the test. The temperature of the

chamber was lowered to —60 °F and then cycled

from -60 to 140 °F and back down to -60 °F.

Table 1. Materials used for specimen preparation

Bitumens ASTM Specification

Asphalt. D312-44

Coal-tar pitch. D450-41, type A

Reinforcing felts

Felt, organic, asphalt-saturated, 15 lb

Felt, asbestos, asphalt-saturated, 15 lb

Felt, organic, coal-tar saturated, 15 lb...

Felt, glass fiber, asphalt-saturated, 8 lb

D226-60

D250-60

D227-56

D2178-63T, type I

At 10 °F increments during the cycle while the

specimens were in the chamber, the distance between the reference plugs was measured to the

nearest ten-thousandth of an inch using a Whittemore Strain Gage, as shown in figure 4. The

variation of specimen temperature during the

period when the measurements were made was

minimal due to the large mass of the conditioning

chamber.

The temperature limits and the cycling procedure used in this program were based on previous

work conducted at the National Bureau of Standards [5]. However, the apparent linear expansion

coefficients were calculated from the data obtained

over the range from 30 to —30 °F since it was

believed that this was a realistic temperature

range for roofs exposed in many areas where

splitting failures have occurred.

5. Results

The averaged results, expressed as the apparent

linear thermal expansion coefficient of felts and of

the composite membranes obtained both "in the

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

The results obtained compare favorably with

previous results of thermal movement of comFigure 4. Thermal movement measurement technique.

4

posite built-up membranes in the machine direction [5].

Table 2. Apparent linear thermal expansion coefficient of roofing felts

(Temperature range 30 to -30 °F)

Expansion coefficient per °F

Description

With machine Across machine

Felt, organic, asphalt-saturated 6. 3X10-« 13. 3X10-0

Felt, asbestos, asphalt-saturated 6.3X10-8 13. OX10-»

Felt, glass fiber, asphalt-saturated 14. 5X10-« 18. 2X10-9 Felt, organic, coal-tar-saturated 6.5X10"6 15.2X10-»

Table 3. Apparent linear thermal expansion coefficient of built-up membranes

(Temperature range 30 to -30 °F)

Expansion coefficient per °F

Description

With machine Across machine

Asphalt—asphalt-sat. organic felt 10.9X10-" 20.8X10-8 Asphalt—asphalt-sat. asbestos felt 8.3X10-8 20.3X10-8 Asphalt—asphalt-sat. glass felt 18. 1X10-8 26.1X10-8

Coal-tar pitch-coal-tar-sat. organic felt 19.4X10-8 29. 4X10-8
6. Summary and Comment

The data reported in this paper show that the

thermal movement of the composite membrane is larger than that of the felt from which it was

constructed but less than that reported for similar

bitumens used in the construction of built-up

roofs [7]. Further, the data show that the apparent coefficient of linear thermal expansion obtained in the across machine direction is appreciably greater than that obtained for in the machine

direction. Data obtained in a previous program

[5] showed that bituminous built-up membranes

undergo greater thermal movements than most

other components of a roof system at subfreezing

temperatures and that the thermal movement is

not linear with temperature but increases as the

temperature is decreased.

Investigations, by the author, of splitting fail- ures due to membrane shrinkage over a period of

7 years on roofs protecting structures throughout

the United States have revealed some factors

which appear common to this type of failure, as follows

:

1. The failures were most common in bitumi- nous built-up membranes placed over substrates

having high thermal insulating values.

2. The splitting generally occurred parallel to

the machine direction of the felt.

3. The splits often coincided with long unbroken

joints between the insulation boards. Seldom did

they occur over staggered joints.

4. The incidence of splitting failures was much

greater in the colder climates and the failures were frequently reported following periods of ex- tremely cold weather with little or no snow cover.

5. The roofs having large areas which were

infrequently broken by expansion joints were

more susceptible to splitting and membrane

shrinkage failures than roofs of smaller area. These observations would certainly suggest that

thermal shrinkage of the membrane may be involved in such failures.

It has been demonstrated experimentally that

bituminous built-up roof membranes placed over

insulation were subject to greater temperature

fluctuations [3], and as a consequence, greater

7. Sug

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. The alteration of the

basic properties of bituminous roofing materials to

overcome the effects of thermal movement is not

economically feasible. However, the initiation of

certain changes in standard roof design and roofing

procedures based on data obtained in the studies

thermal movement [5] than similar membranes

placed over other substrates. Further, it appeared to be more than coincidental that splittingfailures in the membrane occurred more frequently

parallel to the machine direction of the felt since

the results of laboratory measurements given here

establish that the thermal movement was larger

in the "across machine" direction. Further, the

strength of the composite membrane has been re- ported to be considerably less in this direction [6]. When a bituminous built-up membrane is subjected to a temperature change of 60 °F (+30 to — 30 °F), it can contract as much as 0.18 percent

as calculated from the data reported herein.

Further, it has been demonstrated experimentally

that a similar membrane has a breaking strain of

0.45 percent at -20 °F [6]. Therefore, it appears safe to assume that if the roof membrane

were free to move, no splitting failure would occur from thermal movement in the membrane alone.

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 unstaggered

joints, it is conceivable that the cumulative movement resulting from the thermal shrinkage of the

membrane and movement between units of the

substrate may produce a stress concentration in

the membrane over a joint of sufficient magnitude

to cause rupture.

The thermal expansion coefficient of bituminous

roofings increases as the temperature decreases [5]. Therefore, such membranes are especially susceptible to thermal shrinkage when exposed in cold

climates.

The data regarding the thermal and strength

characteristics of bituminous 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, it is believed

that a better understanding of the fundamental

factors involved in roofing performance may lead

to changes in conventional practices of the built-up

roofing industry regarding roof application and,

thereby, increase the life of the roofing.

estions

of the engineering properties of roofing materials

should prove helpful in preventing premature roofing failures involving splitting and membrane

shrinkage. It is with that intent that the following suggestions are made:

1. The intelligent use of expansion joints in the

roofing membrane (not structural joints) will re- duce 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 (Northern Hemisphere) temperature may be used as a

guide to establish spacing of expansion joints.

5
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, 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 machine" direction to

advantage while reducing the potential stress concentrations over joints due to the greater

thermal movement of felts in the "across machine" 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 distributed on finite areas of

contact 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. 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 also produce the desired effects.

The author acknowledges with thanks the assist- ance of W. H. Appleton and T. H. Boone in making

the thermal expansion measurements. The author

also acknowledges the advice and encouragement

given to him by W. W. Walton and the members

of the Tri-Service Committee.

The investigation was conducted under the TriService Engineering Investigations of Building

Construction and Equipment, NBS, sponsored by

the Bureau of Yards and Docks, U.S. Navy; the

Office of the Chief of Engineers, U.S. Army; and

the Directorate of Civil Engineering, U.S. Air

Force.

8. References

[1] F. A. Joy, Better Building Report No. 5, College of [4] R. D. Marquis, Bulletin of Midwest Roofing ConEngineering, The Pennsylvania State University tractors, Vol. VII, Issue 1 (June 1963).

(Sept. 1963). [5] W. C. Cullen, NBS Tech. Note 231 (1963).

[2] D. E. Brotherson, Research Report 61-2, Small Homes [6] P. M. Jones, ASTM Symposium on Recent Research

Council—Building Research Council, University of on Bituminous Materials (June 1963).

Illinois (1961). [7] Herbert Abraham, Asphalt and Allied Substances IV,

[3] W. C. Cullen, W. H. Appleton, The Construction 169 (1962).

Specifier 16, 5, 35 (Oct. 1963).

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