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An Overv i ew o f Roo f i ng

C H A P T E R

1

Throughout history, mankinds need for shelter was second only tothe need for food. Prehistoric man took shelter under a roof of stone,

arguably the best protection from the elements.

Unfortunately, caves were not available everywhere, so early man

looked for a substitute roof. And one of the earliest and still most

important principles of roofing was discoveredlapping. When the

length of any covering for a building, including the walls, is too short

to protect the entire run of a wall or roof, it must be installed in rows,or courses, and each course, beginning at the top, must lap over the

next course below. As simple and obvious as this sounds, it is very

important for draining water off of a built struture.

In many parts of the world, abundant natural fibers, such as grass,

sticks, heather, and straw, were woven into effective shields against

the weather. Thatched, pitched roofs, which are still common in parts

of Africa, Asia, South America, and Polynesia, proved to be remark-ably efficient. Raindrops travel along each reed several inches and

then, before they can penetrate the bundle, are conveyed to the roof

eave by lapping the courses. From the eave, the raindrops fall harm-

lessly to the ground.

Americas first settlers from Europe continued to use thatched roofs

in this country, particularly in the East. As the settlers pushed westward,

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however, the available hay was needed for animal fodder and other pur-

poses. Settlers searching for an alternative roofing material found it, lit-

erally, at their feet. Kansas bricks, made of sod, soon became the roofingstandard. The sod bricks were heavy and difficult to manage, and, even

worse, they often leaked during heavy rainstorms.

Time marched on and so did the search for the perfect roof. Today

there are many fine roof coverings available. Asphalt shingles and roll

roofing, clay and ceramic tile, metal, slate, wooden shingles and

shakes, and cement panels and tiles are used mainly for residential

projects. Built-up roofing (BUR) and single-ply membrane roofs aretypically used on commercial structures.

Before we take a look at roof classifications and materials, lets con-

sider the three factors that architects, builders, designers, and property

owners must address before they choose a roof.

Fire Safety and Protection

Fire safety is a particularly important consideration, since the roof is

vulnerable to fire from overhead or airborne sources.

The fire resistance of roofing materials is tested by the Underwrit-

ers Laboratories, Inc. (UL), an independent, not-for-profit public

safety testing laboratory. UL established the standard for the testing of

roofing materials with the assistance of nationally recognized fire

authorities.Manufacturers voluntarily submit materials for testing. The materi-

als then are classified and labeled according to the classes below. The

American Society for Testing and Materials (ASTM) is a voluntary

organization concerned with the development of consensus standards,

testing procedures, and specifications.

Class A.The highest fire-resistance rating for roofing as per ASTME-108. This class rating indicates that the roofing material is able

to withstand severe exposure to fire that originates from sources

outside the building.

Class B. This fire-resistance rating indicates that the roofing material

is able to withstand moderate exposure to fire that originates from

sources outside the building.

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Class C. This fire-resistance rating indicates that the roofing material

is able to withstand light exposure to fire that originates from

sources outside the building.

Many communities require new roof coverings to meet at least the

UL Class C standard. This requirement has the backing of nationally

recognized authorities such as the National Fire Protection Associa-

tion and the International Association of Fire Chiefs.

Some roofing materials, such as slate and clay tile, offer natural fire

protection. Asphalt shingles are manufactured to meet the Class Cstandard or better. Readily combustible materials, such as wooden

shingles, do not meet the UL standard unless they have been chemi-

cally treated for fire retardancy (see Chap. 9).

Wind Resistance

UL also tests shingle performance against high winds. To qualify for

the UL wind-resistant label, shingles must withstand continuous test

winds of at least 60 miles per hour for two hours without a shingle tab

lifting. Wind-resistant shingles demonstrated their effectiveness under

hurricane conditions during the winds brought to Florida in 1992 by

Hurricane Andrew, which at the time was the worst hurricane ever

recorded in the United States.

Self-sealing asphalt shingles that bear the UL wind-resistant label

are manufactured with a factory-applied adhesive. Once the shingles

are applied, the sun activates the preapplied thermoplastic sealant and

each shingle is bonded to the one below it. Although self-sealers orig-

inally were developed specifically for high-wind areas, they are stan-

dard in most parts of the country today.

Estimating Life ExpectancyIn addition to fire and wind ratings, roof coverings have a life

expectancy. Manufacturers offer a guarantee or warranty for their

products. For example, most of todays asphalt shingles are designed

to provide satisfactory service for 15 to 25 years.

Generally, the longer the life expectancy, the more expensive the

shingle material. You might find, however, that a more expensive

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shingle is the most economical in the long run because the cost of

materials and labor is amortized over a longer period of time. Keep in

mind that while the labor cost to apply the shingles varies with theproduct, it is the same whether the life expectancy of the shingle is 15

or 25 years.

Estimate the probable annual cost of a new roof by adding the cost

of labor and materials and then dividing the total by the shingles

design life. The formula for determining the annual cost of use is

= annual cost of use

Classifying Roofs

The National Roofing Contractors Association (NRCA) classifies roofs

into two major categories: low slope roofs and steep-slope roofs.

Slope is defined as the degree of roof incline expressed as the ratio of

the rise, in inches, to the run, in feet (Fig. 1-1). For example, if thespan of a roof is 24 feet and the rise is 8 feet, the pitch is 824 or 13.

Expressed as a slope, the same roof is said to rise 8 inches per 12

inches of horizontal run. If the rise of the same roof span were 6 feet,

the pitch would be 1 4 and its slope would be 6 inches per 12 inches

of run. Whether a particular roof incline is expressed in pitch or slope,

the results of area calculations are the same.

Building Low-Slope Roofs

Low-slope roofs can have slopes as minor as 18 inch per 12 inches.

These roofs employ a waterproof roofing system and are found pri-

marily on commercial structures.

A low-slope roof system generally consists of a roof membrane,

insulation, and one of a number of surfacing options. To control theapplication and improve the quality of low-slope roofing, a variety of

specifications and procedures apply to the assembly of the roofing

components. These specifications and procedures are generally

accepted and used throughout the United States. Roofing systems that

meet these specifications normally can be expected to give satisfactory

service for many years.

Total cost (materials and labor)

Design life

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Climatic conditions and available materials dictate regional low-slope

procedures, which can vary greatly in different parts of the country. Low-

slope roofs are essentially a custom product. They are designed for a specificbuilding, at a specific location, and manufactured on the jobsite.

Membrane Components

Low-slope membranes are composed of at least three elements: water-

proofing, reinforcement, and surfacing. Some materials within the

membrane might perform more than one function. The waterproofing

agent is the most important element within the roof membrane. In BURand modified bitumen roofing (MBR), the waterproofing agent is bitu-

men. In single-ply roofing, the waterproofing agent is synthetic rubber

or plastic (for more information, see Chaps. 7 and 8).

The reinforcement element provides stability to the roof membrane;

it holds the waterproofing agent in place and provides tensile strength.

In BUR, reinforcement is typically provided by organic or glass-fiber

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F I G U R E 1 - 1 Degrees of slope.

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roofing felts. In MBR, the reinforcement is generally glass-fiber felt or

polyester scrim, which is fabricated into the finished sheet by the man-

ufacturer. Polyester and other woven fabrics are used as reinforcementsfor elastomeric and plastomeric, single-ply membranes. Some single-

ply membranes do not require reinforcement because the waterproof-

ing material is inherently stable.

The surfacing materials protect the waterproofing and reinforce-

ment elements from the direct effects of sunlight and weather expo-

sure. They also provide other properties, such as fire resistance, traffic

and hail protection, and reflectivity. Some single-ply membranes areself- or factory-surfaced. Aggregate, which is field-applied, and mineral

granules, which are usually factory-applied, are the most common

types of surfacing materials. Smooth-surfaced coatings, however, are

increasing in popularity.

Membrane Classifications

Low-slope roof membranes can usually be grouped, or classified, intothe general categories reviewed below. There are, however, hybrid sys-

tems that might not fit into a category, or that might be appropriate in

several categories. A brief overview of each category is provided here.

For specific information, please refer to the referenced chapter.

BUILT-UP ROOFING (BUR)

BUR, which uses asphalt or coal tar products, is by far the oldest of themodern commercial roofing methods. Many commercial buildings in

this country have BUR roofs. The large number of 20-, 30-, and even

40-year-old BUR roofs that are still sound attests to the systems dura-

bility and popularity. Roofing materials continue to evolve, however,

and improvements are continually being made to asphalt and coal tar

pitch, the basic bitumen components of BUR. Asphalt tends to be more

popular with most roofers than coal tar. (See Chap. 5.)

MODIFIED BITUMEN ROOFING (MBR)

Since the first MBR membranes were manufactured in the United

States in the late 1970s, they have become one of the roofing industrys

fastest-growing materials. The popularity and specification of MBR

membranes has increased steadily for more than two decades. Con-

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tractors have found the materials easy to use and easily inspected.

MBR systems provide a time-tested, high-performance, reliable roof.

See Chaps. 5 and 6.

SINGLE-PLY SYSTEMS

Since they first appeared in the 1950s, single-ply materials have

become increasingly popular in the United States. Whether imported

from Europe or produced domestically, these high-tech products have

proven themselves in a wide variety of climates during more than

three decades of use. As described in Chap. 6, there are many differentsingle-ply roofing products.

Constructing Steep-Slope Roofs

Steep-slope roofs have a pitch greater than 212 inches per 12 inches and

are generally found in residential homes. As a part of design, this

water-shedding roof system uses the roofs steep slope for water runoffand leakage protection. The slope of the drainage surface dictates the

type of roofing material that can be used.

Aesthetic Considerations

Unlike the case with low-slope roofing material, when designing the

steep roof, appearance is very important. Lets look at some of the

materials used to cover steep roofs. A brief overview of the most com-mon steep-slope materials is provided here. For more information,

please refer to the referenced chapter.

ASPHALT SHINGLES

Asphalt shingles are the most commonly used material for residential

roofing in the United States today. They are made in a variety of styles.

The most popular is the square-butt strip shingle, which has an elon-gated shape and is available with three, two, or one tab (without cutouts).

Less popular today are the hex shingle and individual shingles which are

available with interlocking or staple-down tabs. Of the three types, the

square-butt strip shingle provides the most attractive roof covering.

Todays generation of dimensional, or architectural, asphalt shingles

has elevated the art of roof design, especially in the residential reroofing

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and new construction markets. Thicker, stronger, and more natural

looking than typical three-tab shingles, dimensional shingles add a

visual effect of depth and distinction to homes. What is more, with theaddition of shadowlines and random, laminated tabs, architectural

shingles can capture with uncanny accuracy the old-fashioned warmth

and elegance of roofs constructed of wood shakes or shingles and slates.

Chapter 7 discusses asphalt products in detail.

WOOD SHINGLES

From a historical perspective, wood shingles and shakes could be con-sidered the most American of all roofing materials. The abundant sup-

ply of forested land on the newly settled continent made wooden roofs

prevalent in Colonial times, an era in which the most common roof

coverings in Europe were slate, tile, and thatch. Wood shingles from

trees as diverse as oak, eastern white cedar, pine, hemlock, spruce, and

cypress were used, depending on local availability. Today, wood

shakes and shingles are used for a variety of architectural styles andeffects. (See Chap. 8.)

SLATE ROOFS

As a roofing material, slate is long-lasting and extremely durable. Cer-

tain types of slate have a longevity of more than 175 years. No man-

made roofing material can make that claim. Like other shingles, slate

is bought by the square. Permanence does not come cheap, but main-taining a slate roof and replacing the few slates that might chip or

break is relatively inexpensive when compared to replacing other

roofs every 20 to 25 years. Chapter 9 discusses the cutting of slate and

its application.

CLAY TILE

Their simplicity of form and shape makes traditional tapered missiontiles ideal for funneling and shedding water from pitched roofs. Mod-

ern extrusion, pressed-form processes, and high-tech gas-fired kilns

have replaced the primitive method of shaping clay tiles over human

thighs and then baking them either in the sun or in wood-fired bee-

hive kilns. With these advances in manufacturing, tremendous

improvements in performance, quality, and product diversity have

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developed. The features and benefits of clay tiles are discussed in

Chap. 10.

FIBER-CEMENT SHINGLES

Because of manufacturing processes and the raw materials used

within them, fiber-cement products do not experience the natural

decomposition that can occur with wooden roofing: curling, cracking,

and splitting. And because of the absence of organic fibers, other prob-

lems like dry rot and the attraction of termites are nonexistent. Most

fiber-cement shingles and tiles are manufactured with an efflorescencepreventative, which inhibits the unsightly leaching of salts associated

with other manufactured products. Fiber-cement products are dis-

cussed with clay tiles in Chap. 10.

METAL ROOFING

The reasons for selecting metal roofing are both obvious and surpris-

ing. Metal roofing allows the owner or architect to make a design state-ment. When the roof is high-pitched and part of the integral building

design, the architectural possibilities of metal roofing are not attain-

able with any other material. When the roof is low-pitched and serves

solely as a functional water barrier, the weatherproofing capabilities of

metal roofing exceed those of any other materials available. Chapter 11

discusses the types of metal roofing available and their application

methods.

Steep-Slope Roof Styles

While low-slope roofs are generally limited to flat-roof styles and are

seldom found on residential structures, steep-roof styles vary greatly

(Fig. 1-2).

Of the steep-roof styles, the gable roof is the most common. It has a

high point, or ridge, at or near the center of the house or wing thatextends from one end wall to the other. The roof slopes downward

from the ridge in both directions. This roof style gets its name from the

gable, which is the triangular section of end wall between the rafter

plate and the roof ridge.

The roof on one side of the ridge is usually the same size and slope

as the roof on the other side. The gable roof of the saltbox house is an

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exception. An architecture common in New England, the saltbox has

different slopes and slopes of different lengths.

A hip roof also has a ridge, but the ridge does not extend from one

end of the roof to the other. The lower edge of the roof, or eave, is at a

constant height and the roof slopes downward to the eaves on all

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F I G U R E 1 - 2 (A

) Typical steep-slope roof styles. (B) Roofing terminology.

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sides. The point where two roof surfaces meet at an outside corner is

called a hip. The junction where two roof surfaces meet at an inside

corner is called a valley.A shed roof slopes in only one direction, like half a gable roof. The

roof has no ridge and the walls that support the rafters are different

heights. The shed roof has several variations. One is the butterfly roof,

where two shed roofs slope toward a low point over the middle of the

house.

In another variation, two shed roofs slope upward from the eaves,

but do not meet at a ridge. The wall between the two roofs is called aclerestory, and is often filled with windows to let light into the interior

of the house.

A gambrel, or barn roof, has double slopes: one pair of gentle slopes

and one pair of steep slopes. Like a gable roof, the gambrel roof slopes

in both directions from a center ridge. At a point about halfway

between ridge and eave, however, the roof slope becomes much

steeper. In effect, the lower slope replaces the upper exterior walls of atwo-story house. It is common to add projections through the roof,

called dormers, for light and ventilation.

Just as a gambrel roof is like a gable roof with two different slopes,

a mansard roof is like a hip roof. From a shorter ridge, the roof drops

in two distinct slopes to eaves that are the same height all the way

around the structure. Up to 40 percent of the building is roof with the

mansard roof design. In addition to typical residential applications,mansard roofs are often used for apartment complexes, commercial

buildings, and even institutions such as schools.

AN OVERVIEW OF ROOFING 1 1