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ROOFS
Roof is an overhead building construction, performing carrying, waterproofing and heat insulation functions. Roofs have been constructed in a wide variety of forms as dictated by technical, economic, or aesthetic considerations.
Two main types of roofs are flat roofs and sloping ones. The flat roof has historically been widely used in the areas where the climate is dry and the drainage of water off the roof is thus of secondary importance. Flat roofs came into widespread use in Europe when new waterproof roofing materials and the use of structural steel and concrete made them more practical. Flat roofs soon became the most commonly used type to cover warehouses, office buildings, and other commercial buildings, as well as for many residential structures. Besides, the flat roof also perform other functions: it can serve as a solarium, a garden, a sport ground or even as a car park.
Sloping roofs come in many different varieties. The simplest is the shed, which has only one slope. A roof with two slopes that form an "A" or triangle is called a gable, or pitched, roof. This type of roof was used as early as the temples of ancient Greece and is still a very common form of roof. A hip, or hipped, roof is a gable roof that has slopes instead of vertical ends. It was commonly used in Italy and elsewhere in southern Europe and is now a very common form in American houses. Gable and hip roofs can also be used for homes with more complicated layouts. The gambrel roof is a type of gable roof with two slopes on each side, the upper being less steep than the lower. The mansard roof is a hipped gambrel roof, thus having two slopes on every side. It was widely used in Renaissance and Baroque French architecture. Both of the above-mentioned roof types can provide extra attic space or other room without building an entire additional floor. They can also have a strong aesthetic appeal.
The external covering of a roof must prevent rainfall or other atmospheric precipitates from penetrating a building. There are two main groups of roof coverings. One group consists of a waterproof film that repels water. The tar that is used to coat roofing felt is the prime example of this type. The other group consists of pieces of a waterproof material that are arranged in such a way as to prevent the direct passage of water through the joints between those pieces. This group includes shingles made of various materials, tiles made of clay or slate, and corrugated sheets of steel, aluminum, lead, copper or zinc.
Flat roofs are normally covered with roofing felt and tar, while sloped roofs are generally covered with shingles or sheet metal.







TOWARDS INDUSTRIALIZED CONSTRUCTION
The destruction of urban Europe and Japan during the Second World War, combined with the post war baby boom, created a need for housing for millions of people. Builders have done a lot for restoring what had been destroyed during the war but they went further than rebuilding. The existence of a single large client (the state) and a need for rapid construction provided fertile soil for a boom in the development of industrialized construction techniques. New construction methods and materials, computers and robots are being developed to constantly improve productivity. These innovations are improving building quality. As construction evolves into an industrialized process, new construction methods and building systems are also being developed to assemble prefabricated components.
A number of social factors are also pushing the construction industry towards industrialization. The demand for high quality buildings is growing in the world. As consumers become more educated about energy conservation, lighting, indoor air quality, they want their houses to be built to the highest standards. Prefabrication of building components using mechanized, computer controlled tools is one way to achieve high quality. Almost all building materials will be manufactured industrially in the near future, and it is natural to expect more and more assembly of these materials in a controlled environment away from the building site.
Continued growth in communications technologies, consumer electronics and other creature comfort systems will make building construction more complex. Computer controlled heating and ventilation and multi room access to entertainment are some of the features that will be built in future "smart" houses. Assembling and installing these technologically advanced systems in an exposed site will not please the owner.
Computers have helped speeding up design considerably in recent years and can allow designers to analyze the building for different purposes before construction starts. Computer Aided Design (CAD) packages can be used for many tasks, ranging from analyzing buildings for heat loss to generating 3D interior mock ups thus reducing considerably the number of mistakes arousing through human errors.
Industrially produced buildings have come a long way from the repetitive boxes most people associate with the word "prefabrication". The move to industrial construction is inevitable, and designers and builders should pay attention to its opportunities now.







PREFABRICATION
The need to be competitive in the emerging global economy is a hot. Many prefabrication technologies deliver a better product because building is done in a quality controlled, sheltered environment. Besides, prefabrication can improve productivity greatly. An awareness of the current trends and latest innovations in prefabrication and industrialized construction is essential.
Prefabrication is an assembly of standardized building components under off-site conditions. Components are manufactured units, made to pre-determined sizes, to be used in building. Standardized components are joined together to form building elements. Units may include doors, stairs, window walls, wall panels, floor slabs, roof trusses, room-sized components, and even entire buildings.
Once fabrication of building components has started, it is difficult to make changes to the design. This means that the dimensional co-ordination between all the components is essential. For any component there will be a maximum and a minimum limit of size, between which the actual size must lie. The actual size must always be less than the basic size.
The greater the accuracy of the components to be joined, the less width will the joint between them require. The joint between components is the major problem in industrialized building systems. If external, its problems are those of weather exclusion, thermal insulation, and fire resistance. All joints should be of adequate strength and security. The aesthetic problem of large joints exposed is a serious one.
The design of the joints should be as simple and economical as possible, so the following basic requirements should be taken into account:
• in-situ concrete connections should be kept to a minimum;
• all joints should be able to transfer loads from one unit to another;
• all joints must have the same degree of fire resistance as the structure;
• all external panel joints should be insulated so that cold bridges will not be formed.
Advantages of prefabrication include the cost savings of mass production, the opportunity to use specialized equipment to produce components, and standardization of parts for quick assembly and erection.
The major drawback is the responsibility for quality control, but with the quick development of new technologies all this work can be done by robots. Robots are most suited for an industrialized construction, where prefabrication can be divided into standard repetitive tasks. Robots are also being developed for assembly of prefabricated components on site. They can inspect finished prefabricated components for drawbacks. The Japanese are installing rails on prefabricated panels to serve as guides for robots.



ASSEMBLY WORKS AND TIME-TABLING
The assembly of a prefabricated building has been divided into four sequences or stages.
Having completed the preparation of the site, the workmen begin the initial stage. This includes excavating the ground, constructing the concrete foundations, fixing the column base plates and erecting the steel columns. The workmen begin by excavating the ground. This precedes the construction of the concrete foundations because they require solid ground to support them. This is followed by the fixing of the column base plates. Finally the steel columns are erected.
Having completed phase 1, the workmen begin phase 2. This includes fixing the beams and bracing, putting the concrete floors in place, erecting the upper floor steel columns and putting the roof decks in place. The workmen begin by fixing the beams and bracing. This precedes the placing of the concrete floors because they require beams to support them. This is followed by the erecting of the upper floor steel columns. Finally, the roof decking is put in place.
Having completed phase 2, the workmen begin phase 3. This includes erecting the corner units, fixing the horizontal cladding panels, fixing the vertical cladding panels, fixing the balustrade fixing plates and laying the weatherproof roof membrane. The workmen begin by erecting the corner units. This precedes the fixing of the horizontal cladding panels because they are fixed to the corner units. This is followed by the fixing of the vertical cladding panels. Finally, the balustrade fixing plates are fixed and the weatherproof roof membrane is laid.
Having completed phase 3, the workmen begin phase 4. This includes erecting the partitions, installing the services in the ceiling void, fixing the suspended ceilings, decorating the building and laying the floor finishes. The workmen begin by installing the services in the ceiling void. This precedes the erecting of the partitions because the workmen require access to the ceiling void. This is followed by the fixing of the suspended ceilings. Finally, the building is decorated and the floor finishes are laid.
Many trades are involved in the construction of a building. They cannot all work on the building at the same time because some jobs cannot start until others have been completed. Consequently, the time-tabling of jobs is extremely important if the building is to be completed on schedule. Usually the sequence of events in a building program are shown on a bar chart. This is a technique for determining the most efficient way in which events can be time-tabled.
Here is the example of such a bar chart.

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SHELLS, TRUSSES AND SPACE FRAMES
А shell is a spanning and space-enclosing element of domed or other vault-like form but with a thickness less than was usual for the masonry and mass-concrete forms. Like the latter, a shell may be curved in two directions or in one only; but the two curvatures of the doubly-curved form may be of opposite sense and the singly curved form may be taken to include barrel-shaped and folded or corrugated forms that span along the length of the barrel or the folds, and act as deep beams. To achieve the reduction in thickness, tensile strength must be provided in the shell itself, or at the level of support, or in both places, in accordance with the requirements of the surface geometry, the pattern of loading, and the type of support.
The shell, together with the doubly-curved tensile membrane or cable net, will continue to play an important role where economy is not the main consideration. Whitney roof of the Terminal Building at Kennedy Airport demonstrates its versatility at the limits of practicality; Jorn Utzon's original proposal of sharply-ridged shells for the Sydney Opera House went beyond these limits and called for a different arched type of construction.
Trusses and space frames are assemblies of linear members that act primarily in axial tension or compression. The usual role in a building is carrying a roof — in place of the dome, vault, beam, or slab.
A truss is a structural member usually fabricated from straight pieces of metal or timber to form a series of triangles lying in a single plane. (A triangle cannot be distorted by stress.) A truss gives a stable form capable of supporting considerable external load over a large span with the components stressed in axial tension or compression. The individual pieces intersect at truss joints. The connected pieces forming the top and bottom of the truss are referred to as the top and bottom chords. The sloping and vertical pieces connecting the chords are referred to as struts and posts of the truss. The two systems most commonly used in trusses are the Pratt and the Warren; in the former, the sloping web members are parallel to each other, while, in the latter, they alternate in direction of slope.
The term space frame describes a three-dimensional assembly in which the interconnections are such that a load at any point is distributed in all directions through the assembly. The joints need not be rigid and, ideally, should allow free relative rotations of the members.
The most important space frames are lighter framed equivalents of domes and vaults, or of slabs spanning in two or more directions simultaneously. The framed dome is a very early form. But even in fully developed timber-framing systems, the ribs were aligned radially and circumferentially, and the system was then braced by the outer covering. Early iron-framed domes merely reproduced this timber form.
Further development of the framed dome or vault has taken place almost entirely in the 20th century and has lagged somewhat behind parallel developments in airframe structures, where there was a greater motive to seek the most efficient use of material to save weight.

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