What is Roof | Types Of Roof | Pitched Or Sloping Roofs | Flat Roofs | Shell Roofs | Domes

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..

i. Strength and stability: The roof structure should be strong and stable enough to take up the anticipated loads safely.

ii. Weather resistance: The roof covering should have adequate resistance to resist the effects of weather elements such as wind, rain, sun and snow.

iii. 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.

iv. Sound insulation: The roof construction for all buildings should provide an adequate degree of insulation against sound from external sources.

v. 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 the 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

 

TECHNICAL TERMS

1. Shed roof or lean-to the 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 the 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 the 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°. The 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 that run diagonally from ridge to the eaves for supporting valley gutters. They receive the ends of the purlins and the 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 lowermost roof covering material rest upon it. The eaves gutter can also be secured against it. Normally, the 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 the wall to wall to support common rafters of a roof when the span is large.

19. Wallplates: 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.

ALSO READ: Roof Covering Materials | Types Of Roof Covering Materials | Roofing Materials

21. Span: A span or clear span is the clear horizontal distance between the internal faces of walls 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 a pitched roof, it is the vertical distance between the wall plate and the top of the ridge.

 

PITCHED ROOF OR SLOPING ROOF

The following are the different types of pitched roofs.

 

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 (below figure).

 

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 (below figure).

 

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 a 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 maybe 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 (below Figure).

 

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 (Below Figure).

 

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 rest of the purlins at the ends on the 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 a reasonable economy.

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, the 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 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 (Below Figure).

 

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 a 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 (Below Figure).

 

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.

 

Belfast roof trusses

This truss is in the form of a bow and is also called a bowstring 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.

 

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.

The size and type of the truss depend 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 bedplate may be 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 advantages of steel roof trusses over timber sloping trusses areas follows:

a. 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.

b. Steel trusses being made of mild steel sections are free from the attack of white ants and dry rot.

c. Steel trusses are much stronger than timber trusses and they are equally strong in tension and compression.

d. These trusses have greater resistance against fire and hence are especially suited where fireproof construction is desired.

e. Timber trusses can only be used up to a minimum span of 14 m or so, whereas there is no span restriction in the case of steel trusses. Steel trusses are used for structures requiring large spans such as industrial buildings, large sheds, assembly halls, hangers and auditoriums.

f. 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.

g. The erection of steel trusses from the rolled sections is very easy, rapid and economical.

 

FLAT ROOFS

A roof that 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.

 

Advantages of the flat roof

a. The roof can be used as a terrace for playing or for sleeping or other domestic purposes.

b. The construction and maintenance of the flat roof are simpler.

c. It provides a better architectural appearance to the building.

d. It is easier to make the flat roof fire resistant.

e. It possesses good insulating properties.

f. It avoids the need for a false ceiling.

g. The construction work of upper floors can be readily taken up in the case of flat roof, whereas in the case of the pitched roof the entire roof has to be dismantled before construction.

h. Pitched roof needs much more area of roofing material than flat roofs.

 

Disadvantages of flat roof

a. A flat roof cannot be used for long spans without using columns and beams.

b. In areas of heavy rainfall, flat roofs are not suitable.

c. The initial cost is more.

d. Due to greater variations in the temperature cracks sometimes develop on the surfaces of the roof, which is difficult to repair.

e. The speed of construction is slower than that of a pitched roof.

f. If the 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.

 

Types of flat roof

The various types of flat roof constructions include the following.

 

Madras terrace roof

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

i. Wooden joints are kept over rolled steel joists with a furring piece in between them.

ii. A course of thoroughly burnt terrace bricks (15 x 8 x 2 cm) is laid on the edge diagonally across the joists in 1:1.5 lime mortar.

iii. After the bricks are completely set, a 10 cm thick layer of brickbat 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.

iv. Two layers of flat tiles (15 x 10 x 1.2 cm) are laid over the layer of concrete in 1:1.5 lime mortar.

v. 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.

 

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.

 

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 leakproof. 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 the 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.

 

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 little quantity of materials is 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. The 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 daylighting 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. The 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.

 

DOMES

A dome is a special type of shell roof of a 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.

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