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Contractor may refer to General contractor organization or individual that contracts with another organization or individual the owner for the construction of a building road or other facility Subcontractor an individual or business that signs a contract to perform part or all of the obligations of another s contract School bus contractor a private company or proprietorship which provides school bus service to a school district or non public school Independent contractor a natural person business or corporation which provides goods or services to another entity under terms specified in a contract Contractor may also refer to Consultant a professional who provides expert advice in a particular domain or area of expertise See also Umbrella company Deadbeat a person who undertakes and operates a new enterprise or venture Permatemp a person who works for an organization for an extended period via a staffing agency Contractor is also a common surname among members of the Parsi community of South Asia Nari Contractor cricketer Behram Contractor columnist
A roof is the uppermost covering part of a building The purpose of the roof is to protect both the building itself and its living or material contents from the effects of weather 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 If the roof is the covering for a house then all these protective functions are called into play Other types of structure for example a garden conservatory might utilise 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 types of structures that require roofs range from the size of a letter box to that of a cathedral or stadium dwellings being the most numerous and the most socially significant The characteristics of a roof are dependant 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 various elements of a roof include its material it construction and its servicability 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 which 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 Thatching specifications 1 Other types of roofing for example pantiles are unstable on a steeply pitched roof but provide excellent weather protectection 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 serviceability of a roof includes its durability but also how well it functions in terms of its required purposes The durability of a roof is a matter of concern in both material and construction because in general the roof is the least accessible part of a building for purposes of repair and renewal while its damge or destruction can have serious effects Different roofing materials have very different properties and if possible these should be taken into account in constructing a roof to suit prevalent local weather conditions
Parts of a roof
In general there are two essential parts to a roof its supporting structure and its outer skin or uppermost weatherproof layer In a very small 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 lintel 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 compression
Construction of a ridged roof
A simple ridged roof consists of inclined rafters that rest on horizontal wall plates on top of each wall The top ends of the rafters meet at the horizontal ridge plate or ridge beam Horizontal purlins are fixed to the rafters to support the roof covering Heavier under purlin are used to support longer rafter spans Tie beams or ceiling joists are connected between the lower ends of opposite rafters to prevent them from spreading and forcing the walls apart Collar beams or collar ties may be fixed higher up between opposite rafters for extra strength Architecture Dictionary on Buffalo Architecture and HistoryThe rafters tie beams and joists serve to transmit the weight of the roof to the walls of the building There are a number of structural systems employed to facilitate this including the use of wall plates set at the top of the wall hammer beams which spread the weight down the wall and create an equilibrium between outward and upward thrust king posts which transfer the weight of the roof ridge and various types of trusses In cyclone and hurricane prone areas the main engineering consideration is to hold the roof down during severe storms Every component of the roof as of course the rest of the structure has to withstand the uplift forces of high wind speeds This is not normally a problem in areas not prone to high wind Modern roofing technologies apparent in the accompanying photo of a house under construction in a cyclone prone region of Northern Australia include the purpose made steel hook bracket which is bolted to the truss with M16 bolt The bracket is bolted to an M16 bolt cast in situ embedded 300 mm into the reinforced concrete block wall This system is typically in place every 900 mm around perimeter
Outer layer
a wattle and daub house roofed with banana leaves rice straw thatch slate terracotta tiles metal roof This part of the roof shows great variation dependant upon availabilty of material In simple vernacular architecture roofing material is often vegetative 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 anywhere 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 where it is abundant provides an ideal and durable material while in the Swiss Alps roofs are made from huge slabs of stone several inches thick The slate roof when properly installed is often considered the best type of commercially available roofing A slate roof may last 75 to 150 years and in extreme cases even longer although the durability depends on the quality of slate used and the levels of skill and care employed by the roofer However slate roofs are often very expensive to install particularly in areas where slate roofing is not common 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 usually recommended and even these must be protected from the weather Roofs of earth take a great number of forms Roofs made of cut turf have good insulating properties and known as Green roofs 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