BUILDING PLANNING & CONSTRUCTION (FESIII-02T)

Roofs

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Dr. Dhananjay Singh

Assistant Professor

National Fire Service College Nagpur

Academic Year: - 2020-2021

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Definition

A roof may be defined as the uppermost part of the building, provided as a structural covering, to protect the building from weather.

Requirements of a Roof

The requirements of a good roof are summarised below :

1. It should have adequate strength and stability to carry the super-imposed dead and live loads.
2. It should effectively protect the building against rain, sun, wind, etc., and it should be durable against the adverse effects of these agencies.
3. It should be water-proof, and should have efficient drainage arrangements.
4. It should provide adequate thermal insulation.
5. It should be fire resistant.
6. It should provide adequate insulation against sound. Most forms of roof construction provide for majority of buildings an adequate insulation against sound from external sources.

Types of Roofs

Roofs may be divided into three categories

1. Pitched· or sloping roofs,
2. Flat roofs or terraced roofs, and
3. Curved roofs.

Pitched Roofs : Basic Elements

A roof with sloping surface is known as a pitched roof. Pitched roofs are basically of the following forms

1. Lean-to-roof
2. Gable roof
3. Hip roof
4. Gambrel roof
5. Mansard or curb roof
6. Deck roof

Lean-to-roof: This is the simplest type of sloping roof, provided either for a of small span, or for the verandah. It has slope only one side.

Gable roof: This is the common type of sloping roof which slopes in two directions. The two slopes meet at the ridge. At the end face, a vertical triangle if formed.

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Hip roof: This roof is formed by four sloping surfaces in four directions. At the end faces, sloped triangles are formed.

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Gambrel roof: This roof, like gable roof, slopes in two directions, but there is a break in each slope. At each end, vertical face is formed.

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Mansard roof: Mansard roof, like a hip roof, slopes in the four directions, but each slope has a break. Thus, sloping ends are obtained.

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Deck roof: A deck roof has slopes in all the four directions, like a hip roof, but a deck or plane surface is formed at the top.

Various Elements of Pitched Roof

Span: It is the clear distance between the supports of an arch, beam or roof

Rise: It is the vertical distance between the top of the ridge and the wall plate.

Pitch: It is the inclination of the sides of a roof to the horizontal plane. It is expressed either in terms of degrees (angle) or as a ratio of rise to span.

Ridge: It is defined as the apex: line of the sloping roof. It is thus the apex of the angle formed by the termination of the inclined surfaces at the top of a slope.

Eaves: The lower edge of the inclined roof surface is called eaves. From the lower edge (eaves), the rain water from the roof surface drops down.

Hip: It is the ridge formed by the intersection of two sloping surfaces, where the exterior angle is greater than 180°.

Valley: It is a reverse of a hip. It is formed by the intersection of two roof surfaces, making an external angle less than 180°.

Hipped end: It is the sloped triangular surf ace formed at the end of a roof.

Verge: The edge of a gable, running between the eaves and ridge, is known as a verge.

Types of Pitched Roofs

Pitched roofs may be broadly classified into the following

1. Single roofs
1. Lean-to-roof (verandah roof)
2. Couple roof
3. Couple-close roof
4. Collar beam roof or collar tie roof

b) Double or purlin roofs

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c) Triple-membered or framed or trussed roofs

1. King-post roof truss.
2. Queen-post roof truss.
3. Combination of king-post and queen-post trusses,
4. Mansard roof truss.
5. Truncated roof truss.
6. Bel-fast roof truss or latticed roof truss.
7. Composite roof trusses.
8. Steel sloping roof trusses.

Single Roof

Single roofs are those which consist of only the rafters which are supported at the ridge and at the eaves. Such roofs are used only when the span is limited to 5 m, otherwise the size of the rafters will be uneconomical

1. Lean to Roof

• This is the simplest type of sloping roof, in which rafters slope to one side only.
• It is also known as Pent roof or Aisle roof
• The wall to one side of the room (or verandah) is taken higher than the wall (or pillars) to the other side.
• A wooden wall plate is supported either on a steel corbel or a stone corbel, which are provided at 1 m centre to centre.
• The wall plate (or post plate) is embedded on the other side, to the wall or pillars.
• The difference in elevation between the two wall plates is so kept that the desired slope is obtained. Usual slope is 30°.

2. Couple Roof

• This type of roof is formed by couple or pair of rafters which slope to both the sides of the ridge of the roof.
• The upper ends of each pair of rafter is nailed to a common ridge piece and their lower ends are notched and nailed to the wooden wall plates embedded in the masonry on the top of the outer walls.
• Such a roof is not very much favoured because it has the tendency to spread out at the feet (Wall plate level) and thrust out the walls supporting the wall plates.
• Due to this, the couple roof is used when the span is limited to 3.6 metres.

3. Couple Close Roof

• A couple close roof is similar to the couple roof, except that the ends of the couple of common rafters is connected by horizontal member, called tie beam, to prevent the rafters from spreading and thrusting out of the wall.
• The tie beam may be a wooden member or a steel rod.
• A couple close roof is economically suitable for spans upto 4.20 m

4. Collar Beam Roof

• When the span increases, or when the load is more, the rafters of the couple close roof have the tendency to bend.
• This is avoided by raising the tie beam and fixing it at one-third to one-half of the vertical height from wall plate to the ridge.
• This raised beam is known as the collar beam ( or collar tie).
• Thus, a collar beam roof is similar to a couple close roof, except that in the latter case a tie beam is provided at the level of wall plates while in this case a collar beam is provided at the raised level.
• This roof is suitable for spans upto 5 metres.
• A lower collar position gives stronger roof
• A collar beam provides roof greater height of the room.

Double or Purlin Roofs

These roofs have two basic elements :

1. Rafters
2. Purlins.
1. The purlins give intermediate support to the rafters, and are supported on end walls.
2. The intermediate supports so provided in the form of purlins, reduce the size of the rafters to the economical range.
3. Such a roof is also known as rafter and purlin roof.
4. The rafters are provided fairly close (40 to 60 cm c/c).
5. Each rafter is thus supported at three points :
1. at the bottom; on the wall through wall plate,
2. at the top, by the ridge ham
3. at the centre by a purlin

Trussed Roofs

• When the span of the roof exceeds 5 m and where there are no inside walls to support the purlins, framed structures, known as trusses are provided at suitable interval along the length of the room.
• Spacing is generally limited to 3 metres for wooden trusses.
• In this system, the roof consists of three elements :
• rafters to support the roofing material (i.e tiles etc.),
• purlins to provide' intermediate support to rafters,
• trusses to provide support to the ends of purlins. The trusses span in the same direction in which the couple of rafters run.

1. King-Post Truss
1. The spacing of the kingpost truss is limited to 3 m centre to centre.
2. The truss is suitable for spans varying from 5 to 8 m.

2. Queen-Post Truss

• A queen-post truss differs from a king-post truss in having two vertical posts, rather than one.
• The vertical posts are known as queen-posts, the tops of which are connected by a horizontal piece, known as straining beam.
• Two struts are provided to join the feet of each queen-post to the principal rafter.
• These trusses are suitable for spans between 8 to 12 metres.

3. Combination of King-Post and Queen-Post Trusses
1. Queen-post trusses are suitable for spans upto 12 metres.
2. For greater spans, the queen-post truss can strengthened by one more upright member, called princess-post to each side.
3. Fig. show the resulting combination, of king-post and queen-post trusses, which are suitable upto 18 m span.

4. Mansard Roof Truss

• This roof truss, named after its designer Francois Mansard, a French architect, is a combination of king-post and queen-post trusses.
• It is a two-storey truss, with upper portion consisting of king-post truss and the lower portion of queen-post truss.
• The entire truss has two pitches. The upper pitch (king-post truss) varies from 30° to 40° while two lower pitch (queen-truss) varies from 60° to 70°.
• The use of this truss, results in economy in space, since a room may be provided between the two queen-posts.
• However, it has become obsolete because of odd shape.

5. Truncated truss

• A truncated truss is similar to Mansard truss, except that its top is formed flat, with a gentle slope to one side.
• This type of truss is used when it is required to provide a room in the roof, between the two queen-posts of the truss.

6. Bel-Fast Roof Truss

• This truss, in the form of a bow, consists of thin sections of timber, with its top chord curved.
• If the roof covering is light, this roof truss can be used upto 30 m span.
• The roof truss is also known as latticed roof truss.

7. Composite Roof Trusses

• Roof trusses made of two materials, such as timber and steel, are known as composite roof trusses.
• In a composite truss, the tension members are made of steel, while compression members are made of timber.
• If tension members are made of timber, their section becomes very heavy because of reduction of section at the joints.
• Special fittings are required at the junction of steel and timber members.
• The joints in composite trusses should be such that cast or forged fittings can be easily used

8. Steel Roof Trusses

• When the span exceeds 10 m, timber trusses become heavy and uneconomical.
• Steel trusses are more economical for larger spans.
• However, steel trusses are more commonly used these days, for all spans - small or large, since they are :
• more economical,
• easy to construct or fabricate,
• fire-proof,
• more rigid,
• permanent.
• Steel trusses are fabricated from rolled steel structural members such as channels, angles, T-sections and plates.
• Most of the roof trusses are fabricated from angle-sections because they can resist effectively both tension as well as compression, and their jointing is easy.

Roof Coverings for Pitched Roofs

• Roof covering is an essential component of pitched roof, to be placed over the Roof frame work, to protect it from rain, snow, sun, wind and other atmospheric agencies.
• Various types of roofing materials are available, and their selection depends upon

1. type of building,
2. type of roof framework,
3. initial cost,
4. maintenance requirements
5. fabrication facilities,
6. appearance and special features of the locality,
7. durability,
8. availability of the material itself,
9. climate of the locality.

The following are the roof-covering materials commonly used for pitched roofs:

1. Thatch covering
2. Wood shingles
3. Tiles
4. Asbestos cement sheets
5. Galvanised corrugated iron sheets
6. Slate roofing
7. Light weight roofing.

Thatch Covering

Wood Shingles

Tiles

Asbestos Cement Sheets

Galvanised Corrugated Iron Sheets

Slate Roofing

Light Weight Roofing

Flat Terraced Roofing

Flat roof is the one which is either horizontal, or practically horizontal with slope less than 10°. Even a perfectly horizontal roof has to have some slope at top so that rain water can be drained off easily and rapidly. Similar to the upper floor, the flat roofs can be constructed of flag stones, R.S.J. and flag stones, reinforced cement concrete, reinforced brick work, jack arch roof or precast cement concrete units. However, the flat roof differ from the upper floor only from the point of view of top finish, commonly called terracing, to protect it from adverse effects of rain, snow, heat etc.

Advantages of flat roofs

1. The roof can be used as terrace for playing, gardening sleeping and for celebrating functions.
2. Construction and maintenance is easier.
3. They can be easily made fire proof, in comparison to pitched roof.
4. They avoid the enclosure of the triangular space. Due to this, the architectural appearance of the building is very much improved.
5. Flat roofs have better insulating properties.
6. They require lesser area of roofing material than pitched roof.
7. They are more stable against high winds.
8. They do not require false ceiling, which is essential in pitched roofs.
9. Flat roofs are proved to be overall economical.
10. In multi-storeyed buildings, flat roof is the only choice, .since overhead water tanks and other services are located on the terrace.
11. The construction of upper floors can be easily done over flat roofs, if so required in future.

Disadvantages of flat roofs

1. The span of flat roof is restricted, unless intermediate columns are introduced. Pitched roofs can be used over large spans without any intermediate columns. .
2. The self weight of flat roof is very high. Due to this, the sizes of beams, columns, foundations and other structural members are heavy;
3. They are unsuitable at places of heavy rainfall. ,
4. They are highly unsuitable to hilly areas or other areas where there is heavy snow fall.
5. They are vulnerable to heavy temperature variations, specially in tropics, due to which, cracks are developed on the surface. These cracks may lead to water penetration later, if not repaired in time.
6. It is difficult to locate and rectify leak in flat roof.
7. The speed of flat roof construction is much· slower than the pitched roof.
8. The initial cost of flat roof is more than pitched roof.
9. The flat roof exposes the entire building to the weather agencies, while the projecting elements (such: as eaves etc.) of pitched roof provide some protection to the building.

Types of Flat Terraced Roofing

Following are the commonly used terraced roofing

1. Mud-terrace roofing.
2. Brick-jelly or Madras terrace roofing.
3. Mud-phaska terracing with tile paving.
4. Lime concrete terracing.
5. Lime concrete terracing with tile paving.
6. Bengal terrace roofing.
7. Light weight flat roofing.

Mud-Terrace Roofing

Madras Terrace Roofing

Mud-Phaska Terracing with Tile Paving

Lime Concrete Terracing

Bengal Terrace Roofing

Light Weight Flat Roofing