types,roofs
a) Wooden Roofs The flat roof is the simplest of roof forms. When it is used in Dwellings’, a light frame construction is very suitable. This type of construction consists of beams, called joists, laid horizontally and supporting a cover of loading on which a membrane type of roofing s used. The joists have nominal widths o.2 in. and nominal depths from 6 in. t.
12 in. The joists span from wall to wall and are space.
16 in. t.
18 in. on centres. Flat roofs of timber construction are suitable for industrial plants, but because of the fire risk, they are constructed with the use of heavy timber beams, spaced 6 ft. to 8 ft. on centres, which support a plank roof cover. The flat roofs are often used in buildings of cities not only as coverings for play-grounds, tea-gardens, but in buildings where slates, tiles or stone slabs form the roof covering, the pitch should not be less than one-fourth of the span. They are largely constructed in eastern countries where the roof is often used for domestic purposes. Sloped roofs have such a construction in which the plane of the roof has a rather steep slope. The supporting sloping beams are called rafters and have the same proportions and sizes as the joists in the light frame construction. These roofs permit the use of unit types of roof covering when the slope is steep. The countries of moist climate require a sloped roof to carry off the rain and snow. There-are some other types of wooden roofs such as.
1) Open timber roof — in which the exposed trusses of frames have the function of a decorative element of roof construction. (The builders of the middle ages made such roofs).
2) Mansard roofs — in which the form of the roof provides additional usable area without adding a full story. Its cornice line is at the base of the upper floor.
3) Long-span timber roofs in which the length of the span was increased. b) Steel Roofs Steel is a material for roof structures having many good characteristics. It is strong and stiff; it can be prefabricated in a shop and rapidly erected on the site. Steel permits joining by means of rivets or welding without great loss in strength. Steel can be easily packed and transported, resist to fire and insects. Structural steel is used in straight lengths of many cross-sectional forms. For flat roots the open-truss steel joists supporting a steel deck and protected by a plastered ceiling are suitable. The steel joists have the depths from 8 in. t.
20 in. and may be spaced up t.
3 ft. on centres on spans .varying fro.
16 ft. t.
36 ft. Other types of structural roof covers may include wood planks or various types of composite materials consisting of a cementing material usually Portland cement, and a light aggregate having such good properties as insulation, resistance to fire, holding power for nails and strength. Industrial buildings requiring large .areas of free space are usually built with steel roof trusses supporting purlins which carry the structural roof caver. For flat roofs the chord members (top and bottom) are parallel; but they are not parallel for long spans where the top chord is a little sloped from the middle span to the ends to facilitate draining of the roof surface. For aesthetic and economical reasons a sloping edge of the roof may be made at the outer walls, because it reduces the height of the wall and the height of the building. The use of concrete as a structural material has its beginning in the era of the Roman Empire. The cementing material was a mixture of hydrated lime and volcanic ash known to-day as a slag or puzzolan cement. With this cementing material a mortar was made which had the characteristics of Portland cement mortar. The introduction of reinforcing steel and the progress in the technology of concrete since the middle of th.
19-th century have resulted in its increasing use as a structural material for roofs. Concrete is plastic and may be formed into various shapes having flat or curved surfaces. It has an excellent quality of continuity when lit is reinforced with steel which permits to construct monolithic structures. For roof structures concrete is usually used with reinforcing steel. In modern practice there is a tendency to use lighter roof structures using thin-shell forms reinforced with steel rod where tension may take place, because concrete resists compression.
