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roof

roof

[roof, roof]
roof, overhead covering of a building with its framework support. Various methods of construction, such as are suited to different climates, have diversified exterior and interior architectural effects. A roof may be flat, as in hot, dry areas where the shedding of rain and snow does not present a problem, e.g., in ancient Mesopotamia, Egypt, and in the SW United States. Modern structural materials and methods have made flat-roof construction practical in nearly any climate, with the development of concrete slabs, efficient drains, and waterproofing materials. On the other hand, steeply sloping roofs are still commonly found in N New England, in the Scandinavian countries, and in other regions where it is necessary to shed snow. Variations of the pitched roof are in gable, gambrel, mansard, or hip (having four sides sloping from a short ridge or center) form. The pitched roof may be of the lean-to type, as in a simple shed, or it may achieve the dignity and aspiration of a dome or spire and embody such features as the dormer window, cupola, or minaret. Pointed-roof construction includes the tie-beam, trussed-rafter, collar-beam, and hammer-beam types. English churches and halls afford many examples of these various methods, some of which have highly decorative open-timber interiors. The simplest roof covering is thatch (of straw, palm leaves, or other fibers) used by the peasants of many lands. Other finishing materials include wood (usually shingles), tile, slate, tin, lead, zinc, copper, felt, and tar. A roof's ridge is the point where the rafters meet; its principals, the purlins, resting on center or side posts, support the rafters; a valley or trough is formed by the junction of two slopes (e.g., where an ell joins the main structure). The eaves, or overhang, carry gutters or themselves drain water beyond the walls, and in the chalet and bungalow they are very wide. The concave curve of East Asian roofs is said to follow the graceful lines of a sagging tent. The classical Greek roof was of marble slabs upon timber framing and sloped gently. Early Roman roofs also were timber framed (as in the basilicas), but vault and dome construction (as in the Pantheon) were prominent in later buildings. The pointed arch and vaulting gave the slope to the Gothic roofs of Europe, while roofs in Renaissance Italy, except those with domes, were concealed, but France and Germany of this period emphasized the gable. Stepped gables are characteristic of Dutch and German roofs. Cone-topped turrets are common on the steep roofs of French châteaus. Roof ornamentation consisted of finials, crockets, crestings, gable crosses, bosses, and fantastic gargoyles (that also served as waterspouts). Roof decoration was particularly elaborate in early Asian and Gothic architecture. In contemporary architecture, roofs can span great distances with little material and few supports because of advances in the methods of using concrete and steel. Green roofs, which have used mainly since the late 1980s, lessen the environmental impact of traditional roofs, especially in urban areas. The roof surface of a building is covered with soil or another growing medium that is planted with grasses, flowers, or other plants. Green roofs reduce storm water runoff, reduce roof heating (mitigating urban "heat islands" and lowering cooling costs) and insulate the building (lowering heating and cooling costs).

See T. Hamlin, Forms and Functions of Twentieth-Century Architecture (1952).

Covering of the top of a building. Roofs have been constructed in a wide variety of forms—flat, pitched, vaulted, domed, or in combinations—as dictated by regional, technical, and aesthetic considerations. Thatched roofs, usually sloping, were the earliest type and are still used in rural Africa and elsewhere. Flat roofs have historically been used in arid climates where drainage of water off the roof is not important, as in the Middle East and the southwestern U.S. They came into more widespread use in the 19th century, when new waterproof roofing materials and the use of structural steel and concrete made them more practical. Sloping roofs come in many different varieties. The simplest is the lean-to (or shed) roof, which has only one slope. A roof with two slopes that form a triangle at each end is called a gable roof. A hipped (or hip) roof has sloping sides and ends meeting at inclined projecting angles called hips. The gambrel roof has two slopes on each of its two sides, the upper being less steep than the lower. The mansard roof has two slopes on all four sides, a shallower upper part and a steeper lower part. Seealso hammer-beam roof.

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A roof is the covering on the uppermost part of a building. A roof protects the building and its contents from the effects of weather. Structures that require roofs range from a letter box to a cathedral or stadium, dwellings being the most numerous.

In most countries a roof protects primarily against rain. Depending upon the nature of the building, the roof may also protect against heat, against sunlight, against cold and against wind. Other types of structure, for example, a garden conservatory, might use roofing that protects against cold, wind and rain but admits light. A verandah may be roofed with material that protects against sunlight but admits the other elements.

The characteristics of a roof are dependent upon the purpose of the building that it covers, the available roofing materials and the local traditions of construction and wider concepts of architectural design and practice and may also be governed by local or national legislation.

The elements in the design of a roof are :-

The material of a roof may range from banana leaves, wheaten straw or seagrass to lamininated glass, aluminium sheeting and precast concrete. In many parts of the world ceramic tiles have been the predominant roofing material for centuries.

The construction of a roof is determined by its method of support and how the underneath space is bridged and whether or not the roof is pitched. The pitch is the angle at which the roof rises from its lowest to highest point. Most domestic architecture, except in very dry regions, has roofs that are sloped, or pitched. The pitch is partly dependent upon stylistic factors, but has more to do with practicalities. Some types of roofing, for example thatch, require a steep pitch in order to be waterproof and durable. Other types of roofing, for example pantiles, are unstable on a steeply pitched roof but provide excellent weather protection at a relatively low angle. In regions where there is little rain, an almost flat roof with a slight run-off provides adequate protection against an occasional downpour.

The durability of a roof is a matter of concern because the roof is often the least accessible part of a building for purposes of repair and renewal, while its damage or destruction can have serious effects.

Parts of a roof

There are two parts to a roof, its supporting structure and its outer skin, or uppermost weatherproof layer. In a minority of buildings, the outer layer is also a self-supporting structure.

The roof structure is generally supported upon walls, although some building styles, for example, geodesic and A-frame, blur the distinction between wall and roof.

Support

The supporting structure of a roof usually comprises beams that are long and of strong, fairly rigid material such as timber, and since the mid 19th century, cast iron or steel. In countries that use bamboo extensively, the flexibility of the material causes a distinctive curving line to the roof, characteristic of Oriental architecture.

Timber lends itself to a great variety of roof shapes. Moreover, because timber can be worked in a variety of ways, the timber structure can fulfil an aesthetic as well as practical function, when left exposed to view.

Stone lintels have been used to support roofs since prehistoric times, but cannot bridge large distances. The stone arch came into extensive use in the Ancient Roman period and in variant forms could be used to span spaces up to 140 feet across. The stone arch or vault, with or without ribs, dominated the roof structures of major architectural works for about 2,000 years, only giving way to iron beams with the Industrial Revolution and the designing of such buildings as Paxton's Crystal Palace, completed 1851.

With continual improvements in steel girders, these became the major structural support for large roofs, and eventually for ordinary houses as well. Another form of girder is the reinforced concrete beam, in which metal rods are encased in concrete, giving it greater strength under tension.

Outer layer

This part of the roof shows great variation dependent upon availability of material. In simple vernacular architecture, roofing material is often vegetation, such as thatches of different materials, the most durable being sea grass with a life of perhaps 40 years. In areas with an abundance of timber, wooden shingles are used, while in some countries the bark of certain trees can be peeled off in thick, heavy sheets and used for roofing.

The 20th century saw the manufacture of composition shingles which can last from a thin 20-year shingle to the thickest which are limited lifetime shingles, the cost depending on the thickness and durability of the shingle. When a layer of shingles wears out, they are usually stripped, along with the underlay and roofing nails, allowing a new layer to be installed. An alternative method is to install another layer directly over the worn layer. While this method is faster, it does not allow the roof sheathing to be inspected and water damage, often associated with worn shingles, to be repaired. Having multiple layers of old shingles under a new layer causes roofing nails to be located further from the sheathing, weakening their hold. The greatest concern with this method is that the weight of the extra material could exceed the dead load capacity of the roof structure and cause collapse.

Slate is an ideal, and durable material, while in the Swiss Alps roofs are made from huge slabs of stone, several inches thick. The slate roof is often considered the best type of roofing. A slate roof may last 75 to 150 years, and even longer. However, slate roofs are often expensive to install - in the USA, for example, a slate roof may have the same cost as the rest of the house. Often, the first part of a slate roof to fail is the fixing nails; they corrode, allowing the slates to slip. In the UK, this condition is known as "nail sickness". Because of this problem, fixing nails made of stainless steel or copper are recommended, and even these must be protected from the weather.

Roofs made of cut turf (known as Green roofs) have good insulating properties and are increasingly encouraged as a way of "greening" the Earth. Adobe roofs are roofs of clay, mixed with binding material such as straw or animal hair, and plastered on lathes to form a flat or gently sloped roof, usually in areas of low rainfall.

In areas where clay is plentiful, roofs of baked tiles have been the major form of roof. The casting and firing of roof tiles is an industry that is often associated with brickworks. While the shape and colour of tiles was once regionally distinctive, now tiles of many shapes and colours are produced commercially, to suit the taste of the purchaser.

Sheet metal in the form of copper and lead has also been used for many hundreds of years. Both are expensive but durable, the vast copper roof of Chartres Cathedral, oxidised to a pale green colour, having been in place for hundreds of years. Lead, which is sometimes used for church roofs, was most commonly used as flashing in valleys and around chimneys on domestic roofs, particularly those of slate. Copper was used for the same purpose.

In the 19th century, iron, electroplated with zinc to improve its resistance to rust, became a light-weight, easily-transported, waterproofing material. While its insulating properties were poor, its low cost and easy application made it the most accessible commercial roofing, world wide. Since then, many types of metal roofing have been developed. Steel shingle or standing-seam roofs last about 50 years or more depending on both the method of installation and the moisture barrier (underlayment) used and are between the cost of shingle roofs and slate roofs. In the 20th century a large number of roofing materials were developed, including roofs based on bitumen (already used in previous centuries), on rubber and on a range of synthetics such as thermoplastic and on fibreglass.

Insulation

Some roofing materials, particularly those of natural fibrous material, such as thatch, have excellent insulating properties. For those that do not, extra insulation is often installed under the outer layer. In developed countries, the majority of dwellings have a ceiling installed under the structural member of the roof. The purpose is to insulate against heat and cold, noise, dirt and often from the droppings and lice of birds who frequently choose roofs as nesting places.

Other forms of insulation are felt or plastic sheeting, sometimes with a reflective surface, installed directly below the tiles or other material; synthetic foam batting laid above the ceiling and recycled paper products and other such materials that can be inserted or sprayed into roof cavities.

So called Cool roofs are becoming increasingly popular, and in some cases are mandated by local codes. Cool roofs are defined as roofs with both high reflectivity and high emissivity.

Drainage

The primary job of most roofs is to keep out water. The large area of a roof repels a lot of water, which must be directed in some suitable way, so that it does not cause damage or inconvenience.

Flat roof of adobe dwellings generally have a very slight slope. In a Middle Eastern country, where the roof may be used for recreation, it is often walled, and drainage holes must be provided to stop water from pooling and seeping through the porous roofing material.

Similar problems, although on a very much larger scale, confront the builders of modern commercial properties which often have flat roofs. Because of the very large nature of such roofs, it is essential that the outer skin is of a highly impermiable material. Most industrial and commercial structures have conventional roofs of low pitch.

In general, the pitch of the roof is proportional to the amount of precipitation. Houses in areas of low rainfall frequently have roofs of low pitch while those in areas of high rainfall and snow, have steep roofs. The longhouses of Papua New Guinea, for example, being roof-dominated architecture, the high roofs sweeping almost to the ground. The high steeply-pitched roofs of Germany and Holland are typical in regions of snowfall. In parts of the North America such as Buffalo USA or Montreal Canada, there is a required minimum slope of 6 inches in 12 inches, a pitch of 30 degrees.

There are regional building styles which contradict this trend, the stone roofs of the Alpine chalets being usually of gentler incline. These buildings tend to accumulate a large amount of snow on them, which is seen as a factor in their insulation. The pitch of the roof is in part determined by the roofing material available, a pitch of 3/12 or greater slope generally being covered with asphalt shingles, wood shake, corrugated steel, slate or tile.

The water repelled by the roof during a rainstorm is potentially damaging to the building that the roof protects. If it runs down the walls, it may seep into the mortar or through panels. If it lies around the foundations it may cause seepage to the interior, rising damp or dry rot. For this reason most buildings have a system in place to protect the walls of a building from most of the roof water. Overhanging eaves are commonly employed for this purpose. Most modern roofs and many old ones have systems of valleys, gutters, waterspouts, waterheads and drainpipes to remove the water from the vicinity of the building. In many parts of the world, roofwater is collected and stored for domestic use.

Areas prone to heavy snow benefit from a steel roof because their smooth surfaces shed the weight of snow more easily and resist the force of wind better than a wood shingle or a concrete tile roof.

Solar roofs

Newer systems include solar shingles which generate electricity as well as cover the roof. There are also solar systems available that generate hot water or hot air and which can also act as a roof covering. More complex systems may carry out all of these functions: generate electricity, recover thermal energy, and also act as a roof covering.

There are different ways that solar systems can be integrated with roofs:

  • integrated in the covering of pitched roofs, e.g. solar shingles.
  • mounted on an existing roof, e.g. solar panel on a tile roof.
  • integrated in a flat roof membrane using heat welding, e.g. PVC.
  • mounted on a flat roof with a construction and additional weight to prevent uplift from wind.

Roof shapes

  • flat
  • lean-to
  • Skillion roof single-sloped, lean to, or shed roof
  • 入母屋(asian traditional style)
  • ridged
    • pitched or gabled
      • shaped gable
      • Dutch gable - a hybrid of hipped and gable
      • crow-stepped gable (also called corbie step) gable
      • salt-box
    • saddleback - a gabled roof atop a tower
    • hip roof includes a sketch of a Dutch gable (Australian terminology)
    • half-hipped
    • mansard - with the pitch divided into a shallow slope above a steeper slope
    • gambrel - as a mansard, but on only two sides of the roof
    • bell-cast - as a mansard, but with the shallow slope below the steeper slope
    • pavilion
  • conical
  • domical
  • catenary
  • pyramidal
  • saw-tooth

Commercially available roofing materials

The weather proofing material is the topmost or outermost layer, exposed to the weather. Many different kinds of materials have been used as weather proofing material:

  • Thatch is roofing made of plant material, in overlapping layers.
    • Wheat Straw, widely used in England, France and other parts of Europe.
    • Seagrass, used in coastal areas where there are esturies such as Scotland. Has a longer life than straw. Claimed to have a life in exccess of 60 years.
  • Shingles, called shakes in North America. Shingles is the generic term for a roofing material that is in many overlapping sections, regardless of the nature of the material. The word is also used specifically to denote shingles made of wood.
    • Redcedar. Life expectancy, up to 30 years. However, young growth redcedar has a short life expectancy. High cost. Should be allowed to breathe.
    • Hardwood. Very durable roofing found in Colonial Australian architecture, its use now limited to restorations.
    • Slate. High cost with a life expectancy of up to 200 years. Being a heavy material, the supporting structure must be very robust.
    • Ceramic tile. High cost, life of up to 100 years.
    • Metal shakes or shingles. Long life. High cost, suitable for roofs of 3/12 pitch or greater. Because of the flexibility of metal, they can be manufactured to lock together, giving durability and reducing assembly time.
    • Mechanically seamed metal. Long life. High cost, suitable for roofs of low pitch such as 0.5/12 to 3/12 pitch.
    • Concrete, usually reinforced with fibres of some sort.
    • Asphalt shingle, made of bitumen embedded in an organic or fiberglass mat, usually covered with colored, man-made ceramic grit. Cheaper than slate or tiles. Various life span expectancies.
    • Asbestos shingles. Very long lifespan, fireproof and low cost but now rarely used because of health concerns.
  • Membrane. membrane roofing is in large sheets, generally fused in some way at the joints to form a continuous surface.
    • Thermosetting plastic (e.g. EPDM rubber). Synthetic rubber sheets adhered together with contact adhesive or tape. Primary application is big box store with large open areas and little vertical protrusions.
    • Thermoplastic (e.g. PVC, TPO, CSPE). Plastic sheets welded together with hot air creating one continuous sheet membrane. Can be rewelded with the exception of CSPE. Lends itself well to both big box and small roof application because of its hot air weldability.
    • Modified bitumen - heat welded, asphalt adhered or installed with adhesive. Asphalt is mixed with polymers such as APP or SBS, then applied to fiberglass and/or polyester mat, seams sealed by locally melting the asphalt with heat, hot mopping of asphalt, or adhesive. Lends itself well to all applications.
    • Built-Up Roof - Multiple plies of asphalt saturated organic felt or coated fiberglass felts. Plies of felt are adhered with hot asphalt, coal tar pitch or adhesive.
    • Sprayed-in-Place Polyurethane Foam (SPUF) - Foam sprayed in-place on the roof, then coated with a wide variety of coatings, or in some instances, covered with gravel.
    • Fabric
  • Metal roofing. Generally a relatively inexpensive building material.
    • Galvanised steel frequently manufactured with wavy corrugations to resist lateral flexing and fitted with exposed fasteners. Widely used for low cost and durability. Sheds are normally roofed with this material. Known as Gal iron or Corro, it was the most extensively used roofing material of 20th century Australia, now replaced in popularity by steel roofing coated with an alloy of zinc and aluminium, claimed to have up to four times the life of galvanized steel.
    • Standing-seam metal with concealed fasteners.
    • Mechanically seamed metal with concealed fasteners contains sealant in seams for use on very low sloped roofs.
    • Flat-seam metal with soldered seams.

Gallery of significant roofs

See also

References

Further reading

  • Francis Ching; Building Construction Illustrated, Visual Dictionary of Architecture, ''Architecture: Form, Space, and Order."

External links

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