In most countries a roof protects primarily against rain. Depending upon the nature of the building, the roof may also protect against heat, sunlight, cold, or wind. Some structures, 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 will depend upon the purpose of the building that it covers, the available roofing materials, the local traditions of construction, wider concepts of architectural design and practice, and sometimes local or national legislation.
The elements in the design of a roof are
- the material
- the construction
- the durability
The material of a roof may range from banana leaves, wheaten straw, or seagrass to lamininated glass, aluminium sheeting, or 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, how the underneath space is bridged, and whether or not the roof is pitched, or sloped. The pitch is the angle at which the roof rises from its lowest to highest point. Most domestic architecture, except in very dry regions, uses pitched roofs. The pitch is partly determined stylistic factors, but more so by practicalities. Some types of roofing, such as thatch, require a steep pitch in order to be waterproof and durable. Other types of roofing, such as 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 major concern because the roof is often the least accessible part of a building and its damage or destruction can have serious effects.
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Parts of a Roof
There are two basic 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, such as 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 made of strong, fairly rigid material such as timber, 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 a practical function, when the timbers are left exposed to view.
Stone lintels have been used to support roofs since prehistoric times, but they cannot bridge large distances. The stone arch came into extensive use in the Ancient Roman period; in some forms it 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.
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 the girder greater strength under compression.
Outer Layer
Outer layers of roofs show great variation depending 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, which has 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 range from a 20-year shingle (the thinnest type) to a limited lifetime shingle (the thickest). 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. In fact, the slate roof is often considered the best type of roofing. A slate roof may last 75 to 150 years or even longer. However, slate roofs are often expensive to install. In the US, for example, a slate roof may have the same cost as the entire 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 made of clay, mixed with a binding material such as straw or animal hair, that is plastered on lathes to form a flat or gently sloped roof. Adobe roofs are usually found in areas that have low rainfall.
In areas where clay is plentiful, baked tiles have been the most common form of roofing material. 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, today tiles of many shapes and colors 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, oxidized to a pale green color, 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 lightweight, easily transported, waterproofing material. While its insulating properties were poor, its low cost and easy application made it the most accessible commercial roofing worldwide. 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; the cost falls somewhere between the cost of shingle roofs and that of slate roofs. In the 20th century a large number of roofing materials were developed, including roofs based on bitumen (used in previous centuries), on rubber, and on a range of synthetics such as thermoplastic and fiberglass.
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, cold, noise, and dirt.
Other forms of insulation include felt or plastic sheeting, sometimes with a reflective surface, installed directly below the tiles or other material; synthetic foam batting laid above the ceiling; 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.
The flat roofs 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 much larger scale, confront the builders of modern commercial properties, which often have flat roofs. Because of the great size of such roofs, it is essential that the outer skin is of a highly impermeable 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, have high roofs that sweep almost to the ground. The high, steeply pitched roofs of Germany and Holland are typical in regions that receive a lot of snowfall. In snowy parts of North America, such as Buffalo, New York, and Montreal, Canada, there is a required minimum slope of 6 inches in 12 inches, a pitch of 30 degrees.
There are regional building styles that contradict this trend. For example, the stone roofs of the Alpine chalets usually have a relatively gentle incline. These buildings tend to accumulate large amounts of snow that is seen as extra 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 can be damaging to the building. 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 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 steel roofs 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. 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
A vast variety of roof shapes can be found worldwide. These include the following shapes:
- flat
- lean-to
- skillion roof—single-sloped, lean to, or shed roof
- 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
- hipped roof—includes a sketch of a Dutch gable
- 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
- pitched or gabled
- conical
- domical
- catenary
- pyramidal
- saw-tooth
Commercially Available Roofing Materials
The weather-proofing material is the topmost or outermost layer, the one exposed to the weather. Many different kinds of materials have been used as weather-proofing material:
- Thatch. 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 excess 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. Since slate is 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 fibers 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. 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 few 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. The seams are 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. Polyester or synthetic fluoropolymer (PTFE) embedded in fiberglass
- Metal roofing. Generally a relatively inexpensive building material.
- Galvanized 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, but it has been replaced in popularity by steel roofing coated with an alloy of zinc and aluminium, which is 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.