NATIONAL BUREAU OF STANDARDS REPORT

9476

THE SELECTION OF BUILDING SYSTEM BY A DECISION PROCESS

USING CLIMATOLOGICAL, ENGINEERING AND ECONOMIC DATA

by

William C. Cullen

Materials and Composites Section

Building Research Division

Institute for Applied Technology

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

Directorate of Civil Engineering, U.S. Air Force

Naval Facilities Engineering Command, U. S. Navy

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NiS PROJECT NBS REPORT

421.04-12-4212447 tuuJl /

c/(*7 9476

THE SELECTION OF BUILDING SYSTEM BY A DECISION PROCESS

USING CLIMATOLOGICAL, ENGINEERING AND ECONOMIC DATA

by

William C. Cullen

Materials and Composites Section

Building Research Division

Institute for Applied Technology

Sponsored by

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

Directorate of Civil Engineering, U.S. Air Force

Naval Facilities Engineering Command, U. S. Navy

IMPORTANT NOTICE NATIONAL BUREAU OF STi

(or rise within the Government. Approved for public release by the

and review. For this reason, the Director of the National Institute of

Whole or in part, is not author, Standards and Technology (NIST)

Bureau of Standards, Washmgtoi

the Report has been specifically on October 9, 2015

s accounting document* intended

subjected to additional evaluation

listing of this Report, either in Office of the Director, National

y the Government agency for wfctch

>pies tor its own use. <JjB§>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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THE SELECTION OF BUILDING SYSTEM BY A DECISION PROCESS

USING CLIMATOLOGICAL, ENGINEERING AND ECONOMIC DATA

by

William C. Cullen

I. INTRODUCTION

The selection of a roofing system by a decision maker is ofter based on

either past experience with the performance of a specific roofing system under

the service conditions in his area or on the results of simulated service tests

conducted in the laboratory 0 Frequently these sources of information are not

available especially in the case of new products or inovations in building

systems. The building industry in general and those involved with military

construction could profit by having a criterion or criteria to assist in the

selection of the more economical building systems which will perform

adequately under service requirements.

The object of this report is to propose a simple model to provide a

logical and systematic approach to the selection of a building system based

on a scientific method.

An attempt has been made to verify the model by deductively processing

currently available economic, climatic and engineering information to obtain

an indicator which will assist in the selection of one of two asphalt shingle

roofing systems. Further an attempt has been made to extrapolate present and

past information into the future in order to provide a basis for the selection

of a system.
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2. ASPHALT ROOFING SYSTEMS

Asphalt roofings account for over 80?o of the roof covering materials used

in the United States [1]*. In 1965 the number of shingles produced was

sufficient to cover about 4 billion square feet of roof area [ 2 ] which alone

is adequate testimony of their usefulness as a roof covering. However, they

possess, like most building materials, some inherent weaknesses. The most

frequent cause of failure of conventional (free- tab) asphalt shingles, which

in 1965 amounted to about two-thirds of the total production of asphalt shingles,

is damage by wind. To overcome this deficiency various types of wind-resistant

shingle is the self- sealing shingle which has been described by Cullen [3] . Suggested criteria for the performance of a wind-resistant shingle was proposed

by Cullen [4] and label service is currently being offered for this type of

roof covering by Underwriters' Laboratories Inc., based on these criteria [5].

The wind-resistant shingle may be classed as a premium product since the

purchaser of these materials must pay a slight additional cost for the

additional protection against wind which is afforded. The increased cost is

justified in high wind areas where conventional shingles are vulnerable to

damage but their use in areas experiencing low wind velocities is sometimes

questionedc The question now arises regarding the economic benefit to be

gained by the selection of a shingle commensurate with its projected exposure

conditions in service.

^Figures in brackets indicate literature references at the end of this paper.

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3. PERTINENT PARAMETERS

3.1 Economics

The cost of asphalt shingles including installation, like other building

systems, varies to a large degree depending on the type of shingle, the roof

area, the method of application, the labor market and the geographical area.

Therefore, an accurate assessment of costs is often difficult to make. However, for the purpose of this report and based on information from a number

of reliable sources, an approximate installed cost of $20.00 per square (100

square feet of roof area) has been used for conventional asphalt shingle

roofings. A cost differential of one dollar per square has been used for

wind-resistant shingles as compared to the use of the conventional type.

Assumingthe roof area of an average house to be 1500 square feet it follows

that the cost to apply a roof with conventional shingles will be about

$300.00. The premium which must be paid for using wind-resistant shingles

amounts to an additional penalty of $15.00 per house.

3.2 Engineering Properties

In a study of the wind resistance of conventional (free-tab) asphalt

shjLngles, Cullen [6] has reported that sustained winds of 30 miles per hour

or over are frequently sufficient to lift the unrestrained shingle tabs so that

the shingles may become vulnerable to damage by wind gusts of much greater

velocity. Therefore, we have used these data to select, for this study only,

the criterion of 30 miles per hour wind speed for predicting whether or not

conventional asphalt shingles may be vulnerable to damage by wind.

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3.3 Climatology

We have selected as our criterion the daily fastest observed one-minute

wind speed as reported in Local Climatological Data published by ESSA’s

Weather Bureau. The climatic condition of the Washington, Do C. area for

the year 1965 was used as the weather parameter in the model c These data

were obtained from Weather Bureau publications [ 7]

An examination of these

data revealed that the daily fastest observed one-minute wind speed exceeded

30 miles per hour on 30 of the 365 days in the Washington, D. C. area or on

8.2% of the total days in 1965.

4. THE DECISION PROCESS

Once the decision maker has arrived at the judgement that the new or re- placement roof will be asphalt shingles, he has two possible courses of action

regarding the selection of the type of shingles in respect to wind resistance.

One will be more advantegous under high wind conditions while the other will be

the better choice under less severe wind conditions. For example, if potential

damaging winds are infrequent to the area ir. question he can elect to use

conventional (free-tab) shingles. On the other hand, the selection of windresistant shingles appears to be the logical choice if the building is located

in a high wind area. The economics associated with these decisions will

depend to a large extent on the actual wind conditions which occur subsequent

to the application of the roofing.

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This can best be visualized with the aid of the matrix shown in Figure 1*

Wind Cost of Roofing

Conditions Choice A^ Choice A^

w. C , c

1 11 21

W C C

2 12 22

Figure 1. MODEL FOR DECISION PROCESS

The symbol C in Figure 1 represents the respective cost or loss associated

with the decision makers choice in respect to the wind conditions to which the

roofing may be subjected*

Let us assume that Choice is the more economical if wind conditions

are frequently experienced in the Area. On the other hand choice A2 appears to

be the better one where wind conditions W

2 prevail. Costs C^ and C^ would represent the minimum expected costs while C^ a^d C

^ would represent maximum

expected costs associated with the decision makers judgement. In the cases where

the decision maker correctly matches his choice to the wind conditions of the

area, the maximum economic benefit will be attained*

The application of the decision process to the selection of the roofing

system can be expressed on a cost-effectiveness basis. Let represent a wind condition where the daily fastest observed one-minute wind speed is less

than 30 miles per hour and let wind condition represent a daily wind condition

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in excess of the 30 mph value. Let Choice A-^ represent the selection of conventional (free-tab) asphalt shingles and Choice A^ the choice of the windresistant variety.

A review of the 1965 local climatological data [7] for the Washington, D.C.

area showed that the percentage of days the wind velocity fell below the 30 mph

criterion was 91.8% while the value was exceeded on 8 . 2% of the days* This can

be expressed as follows:

P(W

X

) = .918 and P(W

2

> = .082

The cost associated with the action can be expressed by the regret matrix

in Figure 2.

Action

Wind Condition Choice A^ (Free-Tab) Choice A^ (Wind-Resistant)

W

x (< 30 mph) Cn ($0) C21 ($15) P(w x )= .918

W^ (>* 30 mph) C

12 ($300) C22 ($15) P(W

2 )=.082

Figure 2. REGRET MATRIX

In areas where wind conditions W-^ are prevalent and Choice A^ is made, the

cost, C^, to the decision maker is zero. Under similar wind conditions and

Choice A

2

is decided upon the cost C^ is $15.00 per house or the additional cost

of the wind-resistant roofing over conventional shingles. However, if under wind

conditions W^ , Choice A-^ is made, the potential loss C is $300.00 per house, i.e.,

if the roof were heavily damaged by strong winds with subsequent damage to the

building and its contents. The cost C^ associated with Choice A^ is $15.00 per

house and subsequent wind conditions do not vary this cost.

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If the decision maker ignores or is unaware of local wind conditions in

respect to the performance of asphalt roofing shingles, he may elect for

various and sundry reasons to make either Choice all the time or to make

Choice A all the time. The expected costs C, and C

9 for always making Choices

A-^ and A2 respectively may be expressed as equations (1) and (2).

c

x = p(w

x

) c n

+ p(w2

) c

12 (1)

C

2 = P(W1

) C21 + P(W

2

) C22 (2) where:

P(W^) and P(W

2

) are fractions of the time when wind conditions ( 30 mph)

and W

2

( 30 mph) prevail in the selected location.

The costs associated with the decision makers choice may be calculated by

substituting values given in figure 2 in equation (1) 0 If he selects conventional

(free-tab) shingles all the time (Choice A^) his cost is:

C

l = (.918) (0) + (.082) (300) = $24.00 per house (3)

On the other hand, if he always chooses to use wind-resistant shingles his

expected cost will be from equation (2):

C2 = (.918) (15) 4- (.082) (15) = $15.00 per house (4) 0 Therefore if the assumptions made herein are correct the conclusion may

bej jnade that in the absence of an accurate long range forecast of wind conditions

the only rational decision for the Washington, D.C. area is to use wind-resistant

asphalt shingles all the time and accept the expected cost of $15.00 per house

as opposed to the expected ocst of $24.00 per house if the choice of conventional

shingles were made.

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If it were possible for the decision maker to obtain perfect wind information for the area in question he would obviously choose conventional shingles

when wind conditions were prevalent and the wind resistant type when wind

conditions W-> prevailed. Under these set of circumstances his cost (C ) can be

P

expressed by equation (5).

= P(W~ ) + P(W

2

) C22 (5)

by substituting values given in Figure 2 in equation (5) the cost is?

C = (.918) (0) + (.082) (15) = $1.25 per house.

P

The cost of $1.25 per house based on perfect wind information would be

$13.75 less than his best choice based on wind information obtained from

climatology. In aqtual practice the decision maker will not be able to obtain

perfect wind information since this product is not available, therefore he

will probably make the more economical choice for his area if he uses the

climatological data which is available from the ESSA's Weather Bureau for most

localities

.

5 . SUMMARY

Economic, climatic and engineering data can be integrated by means of a decision process so as to serve as a guide in the selection of the more

economical roofing system. In this report certain assumptions have been made

vrhich will not meet with universal agreement but are, nevertheless, necessary

for such an analysis. For example, the 30 miles per hour criterion based on

the daily fastest observed one-minute wind speed for conventional asphalt

shingles is probably not realistic for all classes of asphalt shingles and, of

course, the cost of the applied roof and the price differential between conventional and wind-resistant shingles will vary considerably from area to area.

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The utilization of the decision process proposed herein and using values

based on the best current information from engineering, economic, and

climatological sources has shown that the use of wind-resistant shingles

on residential structures in the Washington, D. C. area would effect cost

economies. On the other hand the use of similar data available for other areas

of the United States may indicate that the conventional shingles are thp more

economical choice e

The decision process provides a very useful tool to determine costbenefit rations based on the climate, price structure including labor and

materials, engineering properties and performance characteristics. For

example, if the wind resistance of conventional asphalt shingles were improved

by manufacturing processes or application procedures by increasing the value

of the criterion from the assumed 30 miles per hour daily fastest observed

one-minute mile to just 33 miles per hour, a toss of the coin would determine

the most economical choice. Further if advances in material and/or labor

costs increased the cost differential from $15.00 to $25.00 per house between

the conventional and wind resistant types of shingles, the choice of either

type would be optional on a cost-effectiveness basis.

In comparing cost-usefulness of using wind-resistant shingles in relation

to exposure location, the analysis indicated that the wind-resistant shingles

offer an economical advantage in those areas where the number of days the 30

miles per hour criterion is exceeded about 4% of the time.

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On the other side of the coin the decision process indicated that the

conventional shingles appeared to be the more economical choice when:

a) cost differential of wind-resistant shingles over the

conventional type is greater than $25.00 per house (15

squares) or

b) criterion for wind damage exceeds the 33 miles per hour

fastest observed one-minute wind speed, or

c) the fas test- observed one-minute wind speed criterion

occurs less than 4% of the days of the year.

5 . COMPLICATING FACTORS

The decision process used to assist in mailing a choice to use or not

to use the wind-resistant asphalt shingles is much more complex than

indicated herein. In actual practice many other factors play roles of

varied importance in respect to the wind-resistance of asphalt shingles.

For example, a number of the more important factors are:

a) type, weight and composition of shingle

b) the exposure, type and slope of roof deck

c) the number and placement of fasteners used to secure

shingles to the deck

d) the age and temperature of asphalt shingles.

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Ideally a probabilistic model should be constructed to take into account

all of these factors. However, these factors are either inherently complex

or their influence on the performance of a roofing in service is currently

unknown. Therefore the development of a model will to include all these factors

require a rather large effort both in the laboratory and in the field.

It should be noted that the complicating factors do not necessarily render

our simple model useless. On the contrary, it provides the manufacturer,

architect, roofing contractor or the home owner with a scientific method to

assist him in making a proper and economical choice of a roofing system based

on available economic, climatological and engineering information.

7 o ACKNOWLEDGMENT

The author acknowledges with thanks the suggestions of J. A. Joseph,

Technical Analysis Division NBS during the preparation of this report.

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REFERENCES

1. J. L. Strahan. Manufacture, Selection and Application of Asphalt Reefing

and Siding Products, Asphalt Roofing Industry Bureau. New York, N.Y.,

June 1964.

2. Current Industrial Reports - Asphalt and Tar Roofing and Siding Products,

Bureau of the Census, U. S. Department of Commerce, Summary for (1965).

3. W. C. Cullen. Sealing Systems for Asphalt Shingles, National Bureau of

Standards, Report No. 6442. June 1959.

4. W. C. Cullen, Building Research Institute, National Academy of Science,

New Building Research (1960).

5. Requirements for Wind-Resistant Asphalt Shingle Roofing. Underwriters 1

Laboratories Inc. (1961).

6. W. C. Cullen, Wind Resistance of Free- tab Asphalt Shingles, I;a:ional

Bureau of Standards Report No. 6522. September 1959.

7. Local Climatological Data. Washington, D. C. (1965).

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