roof

A roof is the uppermost part of a building whose main function is to enclose the space and to protect the same from the effects of weather elements such as rain, wind, sun, heat and snow. A good roof is just as essential as a safe foundation. As a well-designed foundation secures the building against destruction starting at the bottom, similarly a good roof affords protection for the building itself and what the building contains and prevents deterioration starting from the top. To fulfil this main function efficiently, the roof should satisfy the following functional requirements in its design and construction.

1. Strength and stability: The roof structure should be strong and stable enough to take up the anticipated loads safely.
2. Weather resistance: The roof covering should have adequate resistance to resist the effects of weather elements such as wind, rain, sun and snow.
3. Heat insulation: The roofs should provide adequate insulation against heat, particularly in the case of single-storeyed buildings where the roof area may exceed that of walls with a consequent greater heat loss.
4. Sound insulation: The roof construction for all buildings should provide adequate degree of insulation against sound from external sources.
5. Fire resistance: The roof should offer an adequate degree of fire resistance in order to give protection against the spread of fire from any adjacent building and to prevent early collapse of the roof. The form of construction should also be such that the spread of fire from its source to other parts of the building by way of roof cannot occur.

The roofs should be well designed and constructed to meet the requirements of different climates and the covering materials available. From experience it is found that pitched or sloping roofs are very suitable in coastal regions where rainfall is heavy and flat roofs are suitable in plains where rainfall is low and temperatures are high.

The roofs may be classified as follows:

1. Pitched or sloping roofs
2. Flat roofs
3. Shell roofs
4. Domes

18.1 TECHNICAL TERMS

1. Shed roof or lean to roof: This type of roof slopes in one direction only and is used for smaller spans.
2. Gable roof: This roof slopes in two directions so that the end formed by the intersection of the slopes is a vertical triangle.
3. Hip roof: This roof slopes in four directions such that the end formed by intersection of slopes is a sloped triangle.
4. Gambrel roof: This roof like the gable roof slopes in two directions but there is a break in the slope on each side.
5. Mansard roof: This roof like the hip roof also slopes in four directions but there is a break in slopes.
6. Ridge: It is an apex line of a sloping roof.
7. Ridge piece or ridge beam or ridge board: This is a wooden piece or board, which runs horizontally at the apex (highest point on the roof). The common rafters are fixed to this piece and are supported by it.
8. Common rafters or spans: These are inclined wooden members supporting the battens or boarding to support roof covering. They run from a ridge to the eaves (edges). They are normally spaced at 30–45 cm centre to centre depending upon the roof covering material.
9. Hip: It is the line produced when two roof surfaces intersect to form an external angle, which exceeds 180°. Hipped end is a portion of the roof between two hips.
10. Jack rafters: These are common rafters shorter in length, which run from a hip to the eaves or from a ridge to a valley. A hip or valley is formed by the meeting of jack rafters.
11. Valley rafters: These are sloping rafters which run diagonally from ridge to the eaves for supporting valley gutters. They receive the ends of the purlins and ends of jack rafters on both sides.
12. Valley: A valley is the reverse of a hip. It is formed by the intersection of two roof surfaces having an external angle, which is less than 180°.
13. Eaves (edges): These are the lower edges of the inclined or pitched roof from which the rainwater from the roof surface drops down. Normally, gutters are fixed along the eaves to collect and drain the rainwater.
14. Eaves board: This is a wooden board fixed to the feet of the common rafters at eaves. The ends of the lower most roof covering material rest upon it. The eaves gutter can also be secured against it. Normally, eaves board is 15–20 cm wide and 20–25 mm thick.
15. Barge boards: These are wooden planks on boards fixed on the gable end of a roof. They connect the ends of ridges, purlins and wall plates.
16. Battens: These are thin strips of wood which are fixed on the common rafters or on the top of ceiling boards to support the roofing materials.
17. Cleats: These are small blocks of wood or steel that are fixed on the principal rafters to support the purlins.
18. Purlins: These are horizontal wooden or steel members laid on principal rafters on wall to wall to support common rafters of a roof when the span is large.
19. Wall plates: These are long wooden members, which are embedded from the sides and bottom in masonry on top of walls, almost at the centres of their thickness. This is essential to connect the walls to the roof. The feet of the common rafters are fixed to the wall plates by means of simple notching and nails.
20. Truss: A roof truss is a framework of triangles designed to support the roof covering or ceiling over rooms. The use of interior columns is avoided.
21. Span: A span or clear span is the clear horizontal distance between the internal faces of wall or supports. The effective span is the horizontal distance between the centres of walls or supports.
22. Rise: This is the vertical height measured from the lowest to the highest points. In the case of pitched roof it is the vertical distance between the wall plate and the top of the ridge.

18.2 PITCHED ROOF OR SLOPING ROOF

The following are the different types of pitched roofs.

18.2.1 Lean to roof

This is the simplest type of pitched roof and consists of rafters that slope in one direction only. Generally, it is used to cover the verandah of a building and projects from the main wall of the building. At the upper ends the rafters are fixed by nails to the wooden wall plates, which are placed on the corbel of the main wall. The lower ends of the rafters are notched and nailed to the wooden post plate. The post plate is of timber section, which runs parallel to the wall and is supported on the intermediate columns or posts. Battens are placed and fixed over the rafters and it is finally covered by suitable roof covering materials. It is suitable for spans up to 2.5 m (Figure 18.1).

18.2.2 Couple roof

In this type of roof, each couple or pair of common rafters is made to slope upwards from the opposite walls and they are supported at the upper ends by the ridge piece or ridge board in the middle. The lower ends of the common rafters are fixed to the wall plates embedded in the masonry on the top of the walls. The use of this form of roof is not much favoured as it has a tendency to spread at the feet and thrust out the walls. The couple roof is therefore adopted only for a maximum span of 3.5 m (Figure 18.2).

18.2.3 Couple close roof

This type is similar to a couple roof except that the legs of the common rafters are closed by a horizontal tie known as tie beam. This tie beam is connected at the feet of the common rafters to check their tendency of spreading outwards and hence saves the walls from the danger of overturning. The tie may be a piece of wood or steel rod in tension. The connection between the ties and the feet of rafters is usually obtained by means of dovetailed halved joint, but for inferior work the ties are just spiked to the rafters. Under normal loading conditions this type of roof can be used for a maximum span of 4.5 m. However, for increased spans or greater loads the rafters have a tendency to sag in the middle. To check this tendency a couple close roof is supported by a central vertical rod known as king rod or king bolt between the ridge piece and the centre of the tie beam (Figure 18.3).

Figure 18.1 Lean to roof

Figure 18.2 Couple roof

18.2.4 Collar beam roof

It is used for spans between 4 and 5.5 m. A collar of the same width as the rafter is fixed to every pair of rafters and it is attached at a height of half to one-third of the vertical height between the wall and the ridge. The collar is dovetailed with the rafter and the bolts can be used for additional safety. It is desirable to place the collar as low as possible to provide maximum strength to the roof (Figure 18.4).

18.2.5 Collar and tie roof

It is used when the roof spans exceed 5.5 m. It is a combination of collar beam roof and couple close roof. The rafters are supported by purlins and the purlins rest at the ends on walls. A collar and strut are employed to support the purlins and rafters. Its use is recommended when purlins may be supported at the ends with reasonable economy.

Figure 18.3 Couple close roof

Figure 18.4 Collar beam roof

18.2.6 King post truss

For spans greater than 4.8 m, when no intermediate supporting walls for the purlins are available, framed structures known as trusses are used. The spacing between trusses is guided by the load coming on the roof, material of the truss, span and the location of cross walls.

In a king post truss, the central vertical post called as king post provides a support for the tie beam. The inclined members are known as struts and are used to prevent the principal rafters from bending at the centre. A king post truss can be used economically for spans 5–8 m.

The joint between the king post and the tie beam is an ordinary mortise and tenon joint. An iron stirrup is also provided to strengthen the joint further. For joining principal rafters and the king post, a tenon is cut in the principal rafter and the corresponding mortice into the head of the king post. A bridle joint is provided to connect the principal rafter with the tie beam. Joints between the king post and the strut are also mortice and tenon joints (Figure 18.5).

Figure 18.5 King post truss

18.2.7 Queen post truss

It can be used for spans 9–14 m. It varies from the king post truss in having two vertical members known as queen posts. The heads of the queen posts are put apart by a horizontal member known as straining beam. The head of the queen post is made wider to receive the principal rafter and the straining beam. The top end of the principal rafter and the end of the straining beam are tenoned into the widened head of the queen posts. A three-way iron or mild steel strap is fixed to further strengthen the joint. The bottom end of the queen post is tenoned into the tie beam and a steel stirrup strap is fixed by jibs and cotters to make the joint stronger. The tenon of the inclined strut is inserted into the splayed shoulder of the queen post. The other joints in this truss are similar to that of the king post truss (Figure 18.6).

18.2.8 Mansard truss

It is a combination of king post truss and queen post truss. The upper portion has the shape of a king post truss and the lower portion resembles a queen post truss. The truss has two pitches. The upper pitch varies from 30 to 40° and the lower pitch varies from 60 to 70°. This type of truss is economical and in the span an extra room may be provided. This type of truss is now rarely used due to its ugly appearance. The construction of various joints is similar to that of the king post trusses.

18.2.9 Belfast roof trusses

This truss is in the form of a bow and is also called bow string or latticed roof truss. It is made of thin sections of timber. This truss can be used for big spans up to 30 m provided light roof coverings are used. The central rise in this type of truss is usually kept about one-eighth of the span.

18.2.10 Steel trusses

The use of steel trusses has become economical for spans greater than 12 m. Various standard shapes and sizes of rolled steel are available for the fabrication of steel trusses. This type of truss is designed in a manner that members are either in compression or in tension and bending stress is not allowed to develop in them.

Figure 18.6 Queen post truss

The size and type of the truss depends upon the roof slope, span, centre-to-centre distance of the trusses and the load coming over the roof. T-sections are best suited for use as principal rafters, whereas angle iron or channel section is used as struts. The tension members should preferably be of a flat or round section. The different members of the truss may be fabricated with two or more sections joined together. The members of a truss are joined by rivets or bolts or by welding the plates known as gusset plates. The minimum spacing of the rivets should not be less than 3 times the diameter and the maximum spacing is limited to 15–20 cm in compression and tension members.

The minimum number of rivets to be used at any joint should not be less than two. Gusset plates are designed for the forces coming at the junction but the least thickness should be adopted as 6 mm. The ends of the trusses are placed on bed plates provided on the walls. The bed plate maybe of stone or concrete. The ends of the truss are bolted down with lewis or rag bolts which hold down the truss firmly. The small trusses are pre-fabricated in the workshop on the ground and are then placed in the required position. The bigger trusses are pre-fabricated in smaller parts and then erected in the required position and fixed by gusset plate and riveting or welding.

The relative advantage of steel roof trusses over timber sloping trusses are as follows:

1. Steel sections forming the roof truss are light in weight and can be fabricated in different shapes and sizes. It suits the structural as well as architectural requirements.
2. Steel trusses being made of mild steel sections are free from the attack of white ants and dry rot.
3. Steel trusses are much stronger than timber trusses and they are equally strong in tension and compression.
4. These trusses have a greater resistance against fire and hence are especially suited where fireproof construction is desired.
5. Timber trusses can only be used up to a minimum span of 14 m or so, whereas there is no span restriction in case of steel trusses. Steel trusses are used for structures requiring large spans such as industrial buildings, large sheds, assembly halls, hangers and auditoriums.
6. The various sections forming a steel truss can be easily machined and shaped in the workshop and subsequently packed and transported to the site for assembling. Moreover, there is no wastage in cutting.
7. The erection of steel trusses from the rolled sections is very easy, rapid and economical.

18.3 FLAT ROOFS

A roof which is approximately flat is called a flat roof. It is becoming more popular with the introduction of suitable building materials. It may be constructed in reinforced cement concrete (RCC), flag stone supported on rolled steel joist, bricks, concrete or tiled arches. This roof is provided with a slight slope in one direction to drain off the rainwater easily. The construction of flat roofs is similar to the construction of floors except that the top surface is protected against rainwater.

18.3.1 Advantages of flat roof

1. The roof can be used as a terrace for playing or for sleeping or for other domestic purposes.
2. The construction and maintenance of the flat roof is simpler.
3. It provides better architectural appearance to the building.
4. It is easier to make the flat roof fire resistant.
5. It possesses good insulating properties.
6. It avoids the need for a false ceiling.
7. The construction work of upper floors can be readily taken up in the case of flat roof, whereas in the case of pitched roof the entire roof has to be dismantled before construction.
8. Pitched roof needs much more area of roofing material than flat roofs.

18.3.2 Disadvantages of flat roof

1. A flat roof cannot be used for long spans without using columns and beams.
2. In areas of heavy rainfall, flat roofs are not suitable.
3. The initial cost is more.
4. Due to greater variations in the temperature cracks sometimes develop on the surfaces of the roof, which is difficult to repair.
5. The speed of construction is slower than that of a pitched roof.
6. If proper slope is not provided on the roof to drain off the rainwater, pockets of water are formed on the surface of the roof, which leads to leakage in the roof.

18.3.3 Types of flat roof

The various types of flat roof constructions include the following.

18.3.3.1 Madras terrace roof

Bricks are the major constituent and they are supported on wooden and steel joists.

1. Wooden joints are kept over rolled steel joists with a furring piece in between them.
2. A course of thoroughly burnt terrace bricks (15 × 8 × 2 cm) is laid on the edge diagonally across the joists in 1:1.5 lime mortar.
3. After the bricks are completely set, a 10 cm thick layer of brick bat concrete is laid over the course of brick. It is beaten down to 7.5 cm by wooden hand beaters. The beating is continued till the beater fails to make an impression on the roof surface.
4. Two layers of flat tiles (15 × 10 × 1.2 cm) are laid over the layer of concrete in 1:1.5 lime mortar.
5. Finally, three coats of lime plaster are applied to the surface and it is rubbed to a polished surface. Generally, a slope of 1:36 is provided to the roof by giving slope to the joists and not by increasing the thickness of terracing.

18.3.3.2 Jack arch flat roof

The brick or concrete arches are constructed with rolled steel joists, which are supported on the walls of the rooms. Some inert materials fill up the spandrels between the arches. Over it, a 10 cm thick average lime concrete (LC) terracing is provided to make it waterproof. Some waterproofing material may be used with lime concrete and the LC terracing is beaten thoroughly by wooden beaters to make it more compact.

18.3.3.3 Reinforced concrete slab roof

An average of 10–13 cm thick LC terracing with some waterproofing compound is provided over the RCC slab to make the roof leak proof. The lime concrete is thoroughly beaten by several labourers with wooden beaters for several days to make the terracing more compact and impervious. At the junctions with wall, the lime terracing is taken inside the wall for 10–15 cm depth and the corner is given a smooth and round shape such that water may not accumulate. The lime terracing is provided with a slope (1 in 60 to 1 in 100) to drain off the rainwater easily from the roof to the gutters provided for the purpose.

18.4 SHELL ROOF

Shell roofs are very useful for covering large structures, e.g., assembly halls, recreation centres, libraries, theatres and factories. RCC shell roofs are becoming very popular these days. Very less quantity of materials are required to build up a shell roof as compared to other conventional methods of roofing for the same span. The design of the shell is made as thin as practical requirements will allow, such that the dead load is decreased and the shell acts as a membrane free from large bending stresses. Least quantity of materials is used to the maximum structural advantage.

The following are the common types of shell roof:

1. North light shell roof which is used mostly in factories, workshops and places where good daylight is desired.
2. If good day lighting is not a requirement, long multiple cylindrical shells with feather edge beams may be useful.
3. Double curved shells are structurally more efficient than single curved shells, but it presents more difficulties in preparing the centring for it. Though consumption of materials is less, sometimes the costs of formworks make the shell roofs quite expensive. It proves to be more costly when only a few similar units are to be constructed. Thin shell roofs are economical when many identical units are to be built and the forms can be reused several times. The forms are usually fabricated from timber battens lined with steel sheets or plywood. Sometimes, plastic forms are also used to obtain special surface textures. The materials of formwork and the lining are selected in consideration of the number of reuses in a particular project. Economy may be achieved in two ways for the formwork. Firstly by using moveable formwork when the shell is to be cast in situ. The second way is to use the precast shells.

18.5 DOMES

A dome is a special type of shell roof of semi-spherical or semi-elliptical shape. The modern thin shell dome may be considered as an evolution of a structural form known and used by man from ancient ages.

Dome structures may be divided into two main divisions:

1. Smooth shell domes
2. Ribbed domes

Smooth shell domes may be divided into:

1. Domes with shells of uniform thickness.
2. Domes with shells of uniformly varying thickness.

Smooth shell domes are constructed by brick, stone, concrete or tile. Ribbed domes may be built in steel, concrete or wood. A dome may be constructed with or without a lantern.

The structure of the dome is such that within certain height and diameter ratios very small thickness is required. They are used where architectural treatment is required such as in monumental structures or where roofs have to be constructed on buildings circular in plan or hexagonal in plan.

18.6 SELECTION OF ROOF COVERING MATERIALS

The following factors require due consideration in selecting a roof covering for a building:

1. Climatic conditions of the locality: The climatic conditions have a marked influence on the performance and durability of roof coverings. Strong winds may damage the roof covering such as slates, tiles and AC sheets by blowing it off, if not properly fixed in position. Extreme temperature changes may cause the sheets to crack and the joints to leak, if not properly protected. Atmospheric effects of fog, salt air, smoke and other gases may result in corrosion of metal roofing if not protected by painting; clay tiles, slates, AC sheets and built-up roof coverings are unaffected by atmospheric action.
2. Slope of roof: The flatter the slope the greater would be the effect of rains, causing leakage in the roof. However, the steeper the slope the greater would be the effect of wind action, requiring heavier supports and more covering material. Thus, the slope of the roof influences both the strength and the economy of the roof and is decided by considering climatic conditions of the locality and the material of roof covering. Thatch, tiles and slate usually require steeper slopes to prevent water infiltration whereas corrugated sheets and concrete terraces require flatter slopes.
3. Nature of the building and of the roof deck: A roof covering must be in conformity with the nature of the building as well as of the roof deck. It is the roof deck to which all the types of roof covering are fastened. The decks are supported on principal supporting members such as girders, trusses and rigid frames. Sometimes the deck serves as a principal supporting member in shell roofs.
4. Initial cost: The initial cost of roofing material is not a definite value and depends upon the time and place and materials availability. For example, a roofing of slates or wood may be cheaper in hilly regions. In the cost analysis of roof covering, it should be noted that the cost of supporting structures and the deck should be accounted in the initial cost because the materials, which are cheap and heavier, may require a strong and costly supporting structure as in the case of slate roofing in hilly regions.
5. Maintenance cost: While considering the overall economy of the roof covering, the maintenance cost may become as important as the initial cost. Wood shingles and tiles need less maintenance and repairs than thatch roofing. Asbestos sheets and slates may be broken and require replacements occasionally. Galvanized iron sheets require only periodic painting to check its tendency to corrosion.
6. Durability: The durability or life is an important factor in assessing the economic value of roof coverings. The life of a roof covering is affected by its quality, its suitability for the purpose used, climatic conditions, workmanship in fixing and laying, degree of maintenance and many other factors. Under normal weather conditions, when all roof covering are laid in the best possible way, clay tiles, slates and galvanized iron (GI) sheets are said to have longer life.
7. Resistance to fire: The roof covering should offer sufficient resistance in the event of fire. From the fire resistance point of view, roof covering of slates, AC sheets, GI sheets, etc. are considered to be quite satisfactory, whereas thatch and shingles are unsatisfactory for this purpose.
8. Heat insulation: The roof covering should offer adequate insulation against heat so that the inside of rooms can be kept cool and comfortable for living. This is particularly important in tropical countries.
9. Weight of roofing material: The weight of roof covering affects the design, weight and the cost of roof deck as well as supporting structure and frame work. Heavier roof covering requires stronger supporting structure, which adds to the cost.
10. Appearance: It is an important factor for residential and other public buildings but is of less significance in the case of industrial buildings. The appearance depends upon the architectural style of the building and the class of occupancy.

18.7 ROOF COVERING MATERIALS

Roof covering material provides protection to the roof and the structure. It prevents rainwater, moisture, heat, dust, etc. from entering into the building from top. The roof covering does not share load in the building. It is rigidly fixed to the roofing structure with various types of fittings and fixtures. The various types of roof covering materials used include the following.

18.7.1 Thatch covering

This form of covering is extensively used in sheds, low-cost houses and village buildings. It is considered suitable for rural areas because it forms the cheapest and the lightest material as a roof covering.

The frame work for supporting the thatch consists of round bamboo rafters spaced at 30 cm and tied with split bamboos or bamboo reapers laid at right angles to the rafters. The thatch is tightly secured to the framework or battens with the help of ropes or twines dipped in tar. Sometimes fire-resisting properties are imparted to the thatch by soaking it in specially prepared fire-resisting solutions that are very costly. For adequate drainage of rainwater the thatch covering should be at least 15 cm thick and laid with a slope of 45°.

18.7.2 Shingles

The use of wood shingles as a roof covering is generally restricted to hilly areas where wood is easily and cheaply available in abundance. Wood shingles are nothing but the sawn or split thin pieces of wood resembling slates or tiles. These sawn shingles, which are obtained from well-seasoned timber, are dipped in creosote to impart preservative qualities. Shingle strips are driven on rafters and shingles are nailed on their top. Shingles are commonly obtained in length varying from 30 to 50 cm and in width varying from 5 to 25 cm.

18.7.3 Tiles

The use of tiles is one of the oldest method of roof covering. The tiles are named according to their shape and pattern and they are manufactured in a similar manner as bricks. The clay tiles are of various types such as flat tiles, pan tiles, pot tiles or half-round country tiles and patent tiles such as Mangalore and Allahabad tiles. Sometimes cement concrete tiles are also used, but is limited on account of high cost and the difficulties in their manufacture. Clay tiles have been widely used as a roof covering material for residential buildings.

18.7.3.1 Advantages of clay tiles as a roof covering

1. Clay tiles, being non-conductors of heat, prevent the building from extreme changes of temperature outside and keep them cool.
2. These tiles provide quite a durable roof covering when made of well-burnt good materials.
3. They are quite strong and pleasing in appearance.
4. If properly selected and laid they have good resistance against fire and moisture penetration.
5. These tiles provide a very economical roof covering with aesthetic values and hence are used for urban and rural houses.

However, these tiles suffer from the limitation of being heavy in weight. The weight of the roof covering is further increased as the rafters are kept closer to reduce the span of timbers and to throw off the rainwater. The average weight of a tile roof is about 75 kg/m2.

Flat or plain tiles are manufactured in rectangular shapes (size, 25 × 15 cm to 28 × 18 cm) in thickness varying from 9–15 mm. Tiles are not perfectly flat but they have a slight camber usually 5–10 mm. These tiles have two small projecting nibs and two or more nail holes at one end of their surface. These nibs and holes help to fix the tiles on the battens of the roof truss. The tiles should be laid at proper gauge and overlap both at the sides and edges as it is important for their strength, durability and imperviousness.

Curved pan tiles are shorter, less curved, heavier, stronger and more durable than the pot tiles. These tiles are moulded flat first and then given the required curvature. These tiles are about 30–35 cm long and are about 20–25 cm wide.

Pot tiles or half-round country tiles are very commonly used for rural houses as they offer a very cheap roof covering. These tiles are laid in pairs of under tiles (concave upwards) and over tiles (convex upwards) with a proper overlap of at least 8 cm on all the sides. The under tiles are flat with a broad head tapering towards the tail while the over tile has a wider tail and a narrower head which is segmental in section. Country tiles are laid in two layers one over the other and the roof is called doubled-tiled roof. This type of roof requires heavy supporting timbers of greater strength than the usual ones.

Patent tiles are generally rectangular in plan with face corrugations so arranged that the corrugations of tiles fit in or interlock with those of other tiles. These interlocking tiles, which are machine made, provide a lighter roof covering with a decent appearance. In ordinary works the groundwork for these tiles consist of battens only. In superior type of construction the tiles are laid on boarding covered with a protective coat. Boarding is directly nailed to purlins and tiles are on battens nailed on the boarding.

18.7.4 Asbestos cement sheets

Asbestos cement is a material which consists of Portland cement and asbestos fibres (about 15 per cent). Roof covering made of this material is cheap, tough, durable, watertight, fire resisting and light in weight. Asbestos does not require any protective coat. Asbestos cement roof coverings are supplied in flat corrugated and ribbed sheets in various sizes.

18.7.5 Corrugated GI sheets

These are used for the roof coverings of workshops, factories and temporary sheds. GI sheets are available in various sizes. Generally 22 gauge sheets are used. End lap of 15 cm and side lap of two corrugations are provided at the time of fixing the GI sheets at the top of the roof. It is light and simple to fix. The only disadvantage with the use of these sheets for roof covering is that during summer season, the rooms under the roof are heated too much. To protect against the sun, sometimes a layer of ordinary tiles are provided at the top of GI sheeting.

REVIEW QUESTIONS

1. What are the main functions of roofs?
2. To fulfil the main functions, what are the functional requirements to be satisfied by roofs in its design and constructions?
3. How are the roofs generally classified?
4. Define hip roof, gabled roof, ridge, eaves, valley, purlins and rise.
5. Write short notes on
   1. Sloping roof
   2. Couple cross roof
   3. Colour beam roof
6. Explain briefly the king post truss.
7. Explain briefly the queen post truss.
8. What are the advantages of steel roof truss over timber sloping truss?
9. What are the advantages and disadvantages of flat roofs?
10. Explain the different types of flat roofs.
11. What is a dome and what are the main divisions of dome structures?
12. What are the factors to be considered in selecting a roof covering for a building?
13. What are the different materials used for roof covering?