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, sunlight, cold and 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 roofEdit
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.
- Main article: Roof construction
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. 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.
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, the most durable being sea grass with a life of perhaps 40 years. In many Asian countries bamboo is used both for the supporting structure and the outer layer where split bammboo stems are laid turned alternately and overlapped. 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 (modern ones known as Green roofs, traditional ones as sod 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 and pocketbook 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. 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.
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.
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 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, 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 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 metal 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.
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.
Solar systems can be integrated with roofs by:
- integration in the covering of pitched roofs, e.g. solar shingles.
- mounting on an existing roof, e.g. solar panel on a tile roof.
- integration in a flat roof membrane using heat welding, e.g. PVC.
- mounting on a flat roof with a construction and additional weight to prevent uplift from wind.
- Flat roof
- Terrace - a flat roof with balustrade, used as a living space
- Arched Roof
- Helm Roof - Rhenish helm - a pyramidal roof with gable ends found on church towers
- Pitched or gabled
- Hip roof
- Dutch gable
- 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
- Skillion roof single-sloped or shed roof
- Saw-tooth - admits natural light into a factory. The vertical surfaces are glazed and face away from the equator. The sloping surfaces are opaque, shielding the workers and machinery from direct sunlight.
Commercially available roofing materialsEdit
The weather proofing material is the topmost or outermost layer, exposed to the weather. Many materials have been used as weather proofing material:
- Thatch is roofing made of plant material, in overlapping layers.
- Shingles, Wood shingles longer than 16" are 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.
- Redcedar. Life expectancy, up to 30 years. However, young growth redcedar has a short life expectancy. High cost. Should be allowed to breathe, usually installed on lath strip for this purpose. The lath may rest on a roof deck with underlayment or be fastened directly to the rafters.
- Hardwood. Very durable roofing found in Colonial Australian architecture, its use now limited to restorations.
- Slate. Higher cost with a life expectancy of 50 to 200 years depending on the thickness and type of slate used. Being a heavy material, the supporting structure must be rated to support the weight load.
- Ceramic tile. High cost, life of 20–100 years.
- Imbrex and tegula, style dating back to ancient Greece and Rome.
- Mangalore clay tiles low cost, life of 20–80 years, used in India.
- Metal shakes or shingles. Long life. High cost, suitable for roofs of 4/12 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. Not suitable in climates that experience many freeze/thaw cycles during a year which will cause this type of material to form cracks and fail.
- Asphalt shingle, made of bitumen embedded in an organic or fiberglass mat, usually covered with colored, man-made ceramic grit. Cheaper and lighter than slate or tiles. Life expectancies vary from 20 to 50 years depending on the product. Sun is the enemy of asphalt shingles so longer life can be expected in cloudier locations or at higher latitudes.
- Asbestos shingles. Lifespans vary. Fireproof. Rarely used anymore because of health concerns. Abatement costs can be high when the old roof needs to be replaced and is subject to additional state and local environmental regulation and oversight.
- 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.
- 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.
- Bituminous waterproofing
- Building construction
- Building envelope
- Green roof
- Metal roof
- Metal Roofing Alliance
- Roof crush
- Roof garden
- Roofing felt
- Solar panel
- Tensile architecture
- Tented roof
- Thin-shell structure
- ↑ Thatching specifications
- ↑ Fleming, Honour, & Pevsner, A Dictionary of Architecture
- ↑ "The Machine Shop and the Works. Modern Principles of Design", The Times: Engineering Supplement, London, November 13, 1912, p.25
- ↑ Thatching Information
- ↑ 5.0 5.1 5.2 Robert Roskind (2000). Building Your Own House. Ten Speed Press. p. 353. http://books.google.com/books?id=Q3QCV5wzmJoC. Retrieved 2009-03-14.
- ↑ Hometips – Wooden shingle roofing, with good diagrams
- ↑ 7.0 7.1 Taunton Press Staff (1997). Roofing. Taunton Press. p. 11. http://books.google.com/books?id=VFTkJrl3WEEC&pg=PA11&dq=. Retrieved 2009-03-14.
- ↑ 8.0 8.1 Steven Bolt (1996). Roofing the right way. McGraw-Hill Professional. p. 7. ISBN 0070066507. http://books.google.com/books?id=iDjyitMjGHkC&pg=PA8&dq=. Retrieved 2009-03-14.
- ↑ HomeTips: Metal shingle roofing
- ↑ Asbestos and Your Health, Victorian Government
- ↑ Asbestos Diseases Advisory Service
- ↑ Ken Watson, Executive Director, National Association of Steel Framed Housing. Steel Framed Housing. p. 2. http://www.innovatek.co.nz/pdfs/Steel_Framed_Housing.pdf. Retrieved accessdate=2009-03-14.
- Francis Ching; Building Construction Illustrated, Visual Dictionary of Architecture, Architecture: Form, Space, and Order.
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