Roofing

ROOFING

A roof is defined as the uppermost part of a building which is constructed in the form of a frame work to given protection to the building against rain, heat, snow, wind. A roof basically consists of structural elements provided at the top of building for the support of roof coverings.

Requirements of a roof

1. It should be structurally stable and sound it should be capable of taking loads likely to come over it

2. It should be durable against the adverse effects of various elements such as rain, wind, snow, sun etc...

3. It should have water proofing and drainage arrangements

4. It should be fire resistant

5. It should provide adequate insulation against heat and sound

Factors that govern the selection of roof covering:

1. Shape and plan of the building

2. Climatic conditions

3. Type of construction materials available

4. Cost

5. Fire resistance

6. Insulation against heat and sound

 

BASIC ELEMENTS OF A ROOF

Span: The horizontal distance between the internal faces of walls or supports is known as span or clear span.

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

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

Ridge: It is defined as the apex line of the sloping roof.

Eaves: The lower edges of a roof which are resting upon or projecting beyond the supporting walls are known as eave.

Hip: The angle formed at the intersection of two roof slopes is known as hip.

Valley: When two roof surfaces meet together and form an internal angle, a valley is formed. Verge: The edge of a gable, running between the eaves and  ge is known as a verge

Common rafter: These are the intermediate rafters, which give support to the roof coverings.

Principal rafter: These are the inclined members of a truss.

Hip rafter: These are provided at the junction of two roof slopes.

Purlins: The wooden pieces which are placed horizontally on principal rafters to carry the common rafters are known as purlins.

Battens: Thin strips of wood which are fixed on rafters or ceiling to support the roof ceiling. Cleats: Small blocks of wood which are fixed on truss to prevent the sliding of purlins.

Gable: The triangular upper part of a wall' formed at the end of a pitched roof is known as gable.

 

TYPES OF ROOF

1) Pitched roof: A sloping roof is known as pitched roof. These are suitable in those areas where rainfall/ snowfall are very heavy. The pitched roofs are available in various forms:

Gable Roof: The triangle produced when the two pitched portions of the roof meet are referred to as a gable. This is the common type of sloping roof which slopes in two directions. The two slopes meet at the ridge.

Hip Roof: A conventional hip roof is made up of four slopes of equal length that meet at the ridge to make a simple ridge.

Gambrel Roof: This roof is like gable roof, which slopes in two directions but there is break in each slope. A gambrel roof is a symmetrical two-sided roof with a shallow upper portion and steeper lower slope on either side, most typically found in barns.

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

Mansard or club roof: This roof like a hip roof, slopes in four directions but each slope has a break. The roof slope is greater at the bottom, thus the pitch of the roof hardly begins.

The different types of pitched roof are explained as follows:

(i) Single roof: In this type of roofs, common rafters are provided to each slope without any intermediate support. The following are the varieties of single roof.

(a) Lean to roof: It is the simplest form of a pitched roof and it is also known as pent roof or Aisle roof. In this type of roof, one wall is carried up sufficiently higher than the other to give necessary slope to the roof. A lean-to roof is generally used for sheds, out-houses attached to main buildings verandah etc. This is suitable for a maximum span of 2.40m

(b) Couple roof: In this type of roof the common rafters slope upwards from the opposite walls and they meet on a ridge piece in the middle. A couple roof is suitable for spans upto about 3.6m.

(c) Couple close roof: This roof is just similar to couple roof except that the les of the common rafters are connected by a tie beam. The tie beam prevents the tendency of rafters to spread out and thus danger of overturning of the walls is avoided. This roof can be adopted economically upto the span of 4.2m.

(d) Collar beam roof: The tie beam is raised and placed at a higher level which is known as collar or collar beam. This beam roof is adopted to economize the space and to increase the height of a room. This roof can be adopted upto a maximum span of 4.8m.

(ii) Double roofs: This type is also known as purlin roofs. When the span exceeds 2.4m, the necessary size for the rafters becomes uneconomical. Hence in order to reduce the size of rafters, intermediate supports called purlins are introduced under the rafters. This roof can be adopted economically upto 4.8m.

(iii) Trussed roofs: When the span exceeds 4.8m and when there are no inside supporting walls or partitions for purlins, framed structure known as trusses are provided on the roof. The spacing is 3m for wooden trusses. Trusses carry the ridge piece and purlins on which the common rafters rest. Some of the usual forms of roof truss are discussed below.

(a) King post truss: In this type of truss, the central post known as king- post forms support for the tie beam. The inclined members known as struts, prevents the principal rafters from bending in the middle. A king-post truss suitable for roofs should be of span varying from 5 to 8 m.

(b) Queen post truss: This truss is different from a king-post truss by having two vertical members known as queen posts. The upper ends of the queen posts are kept in position by means of a horizontal member known as straining beam. Additional purlins are supported on the queen posts. A queen post truss is suitable for roof spans varying 8 to 12 m.

(c) Mansard truss: Mansard truss is a type of truss which is the combination of the king post and queen post truss. It is designed with two story truss with upper portion consisting of the king post truss and the Lowe portion of Queen post truss. Mansard truss effectively used in pitched roof with consisting of two pitches, upper pitch in King post truss which is varies from 30° to 40° and the lower pitch in queen post varies from 60° to 70°.

(d) Truncated truss: Truncated truss is the type of pitch roof truss which is similar to the mansard truss except that the top is effectively finished flat with a gentle slope to one side that is effluently used when a room is necessary in the roof.

(e) Bel fast truss: Bel fast truss is a thin section of timber, associated with its top chord curved as a form of a bow which is suitably adopted for long span of 30 metres, this type of truss is effectively carry for light roof covering, this truss is also called as latticed roof truss. The bell fast roof truss is consist form of a bow which is made with a thin section of member, this truss roof is also called bow string or latticed roof truss.

(f) Steel trusses: Steel trusses is designed with the angles riveted or welded together through the plates for the provide facilities the roof construction. This type is economically suitable for spans greater than 12 metres.

(g) Composite trusses: Composite trusses are the type of pitch roof truss which are composed of steel or wrought iron and wooden members that is light in weight and economical, in this type of truss the steel is used for resist the tensile stresses. In the composite trusses, fitting are necessary at the junction of steel and timber.

2. Flat Roofs: These roofs are nearly flat. The construction of flat roof is same as that of floors except that the top surface is made slightly sloping in case of flat roofs to drain out the rain water. All types of upper storey floors can serve as flat roofs. Many times top of these roofs are treated with water proofing materials-like mixing water proofing chemicals in concrete. With advent of reliable water proofing techniques such roofs are constructed even in areas with heavy rain fall. Flat roofs are used in plains where rainfall is less and climate is moderate.

The advantages of flat roofs are:

(a) At any latter stage the roof can be converted as a floor by adding another storey.

(b) The roof can be used as a terrace for playing and other utilities.

(c) They can suit to any shape of the building.

(d) Over-head water tanks and other services can be located easily.

(e) They can be made fire proof easily compared to pitched roof.

The disadvantages of flat roofs are:

(a) They cannot cover large column free areas.

(b) Leakage problem may occur at latter date also due to development of cracks.

(c) The dead weight of flat roofs is more.

(d) In places of snow fall flat roofs are to be avoided to reduce snow load. (e) The initial cost of construction is more.

3. Curved Roofs: These are the just the modifications of pitched roofs and are frequently employed in modern age to cover large areas shed/roofs and domes are the varieties of curved roofs. They are useful for big structures such as factories, monumental works etc. curved roofs may be constructed of timber or R.C.C. the latter material being very common now-a-days. They may be different forms like shell, dome, barrel arch, cone and hyperbolic paraboloid.

Advantages of Curved Roof

(a) It can be constructed in short time.

(b) It does not require skilled supervision.

(c) Cheap in construction.

(d) Requires less frame work.

(e) Low maintenance roof.

Disadvantages of Curved Roof

(a) As the complexity of the design of curved roof increases, the cost of the roof also increases.

(b) Height, curve and materials used for covering the roof are generally considered while estimating, while increases the cost.

 

ROOF COVERING MATERIALS

Various types of covering materials are available for pitched roofs and their selection depends upon the climatic conditions, fabrication facility and availability of materials and affordability. Commonly used pitched roof covering materials are discussed below:

(a) Thatch Covering: These coverings are provided for small spans, mainly for residential buildings in villages. Thatch is a roof covering of straw, reeds or similar materials. The thatch is well-soaked in water or fire resisting solution and packed bundles are laid with their butt ends pointing towards eves. Thickness varies from 150 mm to 300 mm. They are tied with ropes or twines to supporting structures. The supporting structure consists of round bamboo rafters spaced at 200 mm to 300 mm over which split bamboos laid at right angles at close spacing. The advantage of thatch roof is they are cheap and do not need skilled workers to build them. The disadvantages are they are very poor fire resistant.

(b) Shingles: Wood shingles are nothing but the split or sawn thin pieces of wood. Their size varies from 300 mm to 400 mm and length from 60 mm to 250 mm. Their thickness varies from 10 mm at one end to 3 mm at the other end. They are nailed to supporting structures. They are commonly used in hilly areas for low cost housing. They have very poor fire and termite resistance.

(c) Tiles: Various clay tiles are manufactured in different localities. They serve as good covering materials. Tiles are supported over battens which are in turn supported by rafters/trusses etc. They give good appearance also. They may be of different types such as Plain tiles, Curved tiles and Interlocking tiles.

(d) Slates: A slate is a sedimentary rock. Its colour is gray. It can be easily split into thin sheets. Slates of size 450 mm to 600 mm wide, 300 mm long and 4 to 8 mm thick are used as covering materials of pitched roofs in the areas where slate quarries are nearby. A good slate is hard, tough and durable. They are having rough texture and they give ringing bell like sound when struck. They do not absorb water.

(e) A.C. Sheets: Asbestos cement is a material which consists of 15% of asbestos fibres evenly distributed and pressed with cement. They are manufactured in sufficiently large size. The width of a A.C. sheet varies from 1.0 to 1.2 m and length from 1.75 to 3.0 m. To get sufficient strength with thin sections they are manufactured with corrugation or with traffords. They are fixed to the steel purlins using J-bolts. The roofing is quite economical and waterproof. They are commonly used as covering materials in ware houses, godowns or for larger halls. In auditorium if these sheets are used, false ceilings are provided to get good thermal resistance.

(f) G.I. Sheets: Galvanised iron corrugated sheets are manufactured in the sizes 1.0 to 1.2 m wide and 1.65 m length. Galvanization of iron makes them rust proof. They are fixed to steel purlins using J-bolts and washers. They are durable, fire proof, light in weight and need no maintenance. They are commonly used as covering materials for ware houses, godown and sheds.