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Roof and Roof Covering

Nov 08, 2014



Roof types and roof covering
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A roof may be defined as the uppermost part of the building, provided as a structural covering, to protect the building from weather (ie from rain, sun, wind etc).• A roof consists of structural elements which support

roof coverings.• The structural elements may be trusses, portals, beams,

slabs(with or without beams), shells or domes.• The roof coverings may be A.C sheets, G.I sheets,

wooden shingle, tiles, slates or slab itself.

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• It should have adequate strength and stability to carry the super-imposed dead and live loads.

• It should effectively protect the building against rain, sun, wind, etc. and it should be durable against the adverse effects of these agencies.

• It should be waterproof and should have efficient drainage arrangements.• It should provide adequate thermal insulation.• It should be fire resistant.• 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.

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Roofs may be divided into three categories:• Pitched or sloping roofs• Flat roofs or terraced roofs• Curved roofs

The selection of the type of roof depends upon the shape or plan of the building, climatic conditions of the area and type of constructional materials available.

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A roof with sloping surface is known as a pitched roof.

They are basically of the following forms:• lean-to-roof• Gable roof• Hip roof• Gambrel roof• Mansard or curb roof • Deck roof

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LEAN-TO-ROOF: This is the simplest type of sloping roof, provided either for a room 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 is 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.

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

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

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• SPAN: It is the clear distance between the supports of an arch, beam or roof truss.

• 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 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.

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

• 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 degrees.

• HIPPED END: It is the sloped triangular surface formed at the end of a roof.

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

• RIDGE PIECE, RIDGE BEAM OR RIDGE BOARD: It is the horizontal wooden member, in the form of a beam or board, which is provided at the apex of a roof truss. It supports the common rafters fixed to it.

• PURLINS: These are horizontal wooden or steel members, used to support common rafters of a roof when span is large. Purlins are supported on trusses or walls .

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• COMMON RAFTERS OR SPARS: These are inclined wooden members running from the ridge to the eaves. They are bevelled against the ridge beam at the head, and are fixed to purlins at intermediate point. They support the battens or boarding to support the roof coverings. Depending upon the roof covering material, the rafters are spaced 30 to 45 cm centre to centre

• HIP RAFTERS: These are the sloping rafters which form the hip of a sloped roof. They run diagonally from the ridge to the corners of the walls to support roof coverings. They receive the ends of the purlins and ends of jack rafters.

• VALLEY RAFTERS: These are the sloping rafters which run diagonally from the ridge to the eaves for supporting valley gutters. They receive the ends of the purlins and ends of jack rafters on both sides.

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• EAVES BOARD OR FACIA BOARD: It is a wooden plank or board fixed to the feet of the common rafters at the eaves. It is usually 25 mm thick and 25 mm wide. The ends of lower most roof covering material rest upon it. The eaves gutter, if any, can also be secured against it.

• BARGE BOARD: It is a timber board used to hold the common rafter forming verge.

• WALL PLATES: These are long wooden members, which are provided on the top of stone or brick wall, for the purpose of fixing the feet of the common rafters. These are embedded from sides and bottom in masonry of the walls, almost at the centre of their thickness. Wall plates actually connect the walls to the roof.

• JACK RAFTERS: These are the rafters shorter in length, which run from hip or valley to the eaves.

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• BATTENS: These are thin strips of wood, called scantlings, which are nailed to the rafters for lying roof materials above.

• BOARDINGS: They act similar to battens and are nailed to common rafter to support the roofing material.

• TEMPLATE: This is a square or rectangular block of stone or concrete placed under a beam or truss, to spread the load over a larger area of the wall.

• CLEATS: These are short sections of wood or steal (angle iron), which are fixed on the principal rafters or trusses to support the purlins.

• TRUSS: A roof truss is a frame work, usually of triangles, designed to support the roof covering or ceiling over rooms.

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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 metres, otherwise the size of the rafters will be economical.

They are of four types:• Lean-to-roof, verandah-roof or shed roof• Couple roof• Couple close roof• Collar beam roof

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• 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.• The difference in elevation between the two wall plates is so kept

that the desired slope is obtained. Usual slope is 30 degrees.• This type of roof is suitable for maximum span of of 2.5 m.• These are provided for sheds, out-houses attached to main building,

verandahs, etc.

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• This type of roof is formed by couple or pair of rafters which slope to both the sides of the ridge of the roof.

• Such a roof is not very much favored 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.

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• 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.

• A couple close roof is economically suitable for spans up to 4.20 m.• For increased span or for greater loads, the rafters may have

tendency to sag in the middle.• This can be checked by providing a central vertical rod, called king

rod or king bolt which connects the ridge piece and the tie beam.

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• 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).• This roof is suitable for span upto 5 metres.• A low collar position gives stronger roof.• A collar beam provides roof greater height of the room.

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It is similar to the collar roof, except that two collar beams, crossing each other have an appearance of scissors.

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• These roofs have two basic elements: (i) rafters and (ii) purlins.

• The purlins give intermediate support to the rafters, and are supported on end walls.

• Purlins reduce the size of the rafters to the economical range. Such a roof is also known as rafter and purlin roof.

• Each rafter is thus supported at three points: (a)at the bottom; on the wall though wall plate, (b) at the top, by the ridge, and (c) at the centre by a purlin.

• For larger roofs, two or more purlins may be provided to support each rafter.

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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.

The roof consists of 3 major elements:a) Rafters: to support the roofing materialb) Purlins: to provide intermediate support to the raftersc) Trusses: to provide support to the ends of the purlins

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The various types of trusses are as follows:• King-post truss• Queen-post truss• Combination of king-post and queen-post truss• Mansard truss• Truncated truss• Bel-fast truss or Bow- string truss• Steel truss• Composite truss

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A king-post truss consists of the following components:• Lower tie beam • Two inclined principal rafters - supports the purlins• Two struts - supports the principal rafters• A king-post - prevents sagging of tie-beam at the centre• Two purlins - placed at right angles to the principle rafters and

supports the common rafters• Common Rafters - supports the roof covering• Cleats - prevent the purlins from tilting

The spacing of king-post truss is limited to 3 m centre to centre and it is suitable for spans varying from 5 to 8 metre.

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A queen-post truss has two vertical posts known as Queen-posts unlike king-post where there is only one.

A queen post roof truss has the following components:• Straining Beam - horizontal beam that connects the top of the queen posts• Two struts - join the feet of the queen post to the principle rafter• Straining sill - to counteract the thrust from the struts which are in

compression• Purlins with cleats• Principle rafters and common rafters• Tie beams

Queen post roof truss are suitable for spans between 8-12 m.

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Queen post trusses are suitable for spans up to 12 metres. For greater spans, the queen-post truss can be strengthened by one more upright member, called princess-post to each side.

A combination of the king-post and queen-post is suitable up to 18 m span.

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This truss is named after its designer and is a combination of king-post and queen-post truss. • It is a 2- storey truss, with the upper portion consisting of king-post truss

and lower portion of queen-post truss.

• Mansard truss has two pitches. The upper pitch varies from 30⁰-40⁰ while that of lower pitch varies from 60⁰ - 70⁰.

• The use of this truss results in economy in space but it has now become obsolete because of it odd shape.

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It is similar to Mansard truss, except that its top is formed flat, with a gentle slope to one side.

Generally used when it is required to provide a room in the roof, between the two queen posts of the truss.

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• It is also called Bow-string roof truss since it is in the form of a bow, with its top chord being curved.

• This can be used up to a span of 30 m if the roof covering is light.

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Roof made of two materials, such as timber and steel, are known as composite roof truss.

Normally the tension members are made of steel and the compression members of timber.

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• When the span exceeds 10m, timber trusses become heavy and uneconomical.

• Steel trusses are used economically over larger spans.

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• Consist of a roof truss supported over steel stanchions

• Transversely braced

• Roof trusses provided over columns provide structural roof system for industrial buildings

• Used in factories or mills

• Asbestos corrugated and Trafford cement sheets are commonly used as roof covering material

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• Section comprising of steel trusses are readily available in required dimensions resulting in minimum wastage of material.

• They are light in weight, easy to construct and can be fabricated in any shape depending upon structural and architectural requirements.

• Steel trusses are stronger and more rigid in comparison to timber trusses.

• They can be used for any span.• Steel trusses are fire-proof• Steel trusses are termite proof• They are more resistant to other environmental agencies and have

longer life.• They are economical.

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• Open trusses

• North light trusses

• Bow string trusses

• Arched rib trusses and solid arched ribs

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Roof covering is an essential component of pitched roof to be placed over the framework

Function: • Protect the framework

from atmospheric agencies.

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• Type of building• Type of framework• Initial cost• Maintenance requirements• Fabrication facilities• Appearance and special features of locality • Availability of material• Climate of locality

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• Thatch covering• Wood shingle roofing• Tile roofing• Asbestos cement sheet roofing•Galvanised corrugated iron sheet roofing• Eternit or slate roofing• Light weight roofing

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• Consists of bundles of reed or bamboo laid with their butt end pointing towards the eave after soaking in water

• Framework consists or round bamboo rafters spaced 20 to 30 cm apart

• Thatch is secured to the framework using ropes or twine dipped in tar

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• Very cheap• Very light• Easy to lay.

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• Highly combustible• Unstable against high winds• Absorbs moisture• Liable to decay• Harbors burrowing animals• Gives bad smell in rainy


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• Shingles are thin slabs of well seasoned wood• They are 10 mm thick at the butt end and taper to 3 mm

or less at the head• Used in hilly areas• Light weight• Not resistant to fire and termite

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Used for residential buildings and country houses

Different types:• Plain or flat tile• Curved or pan tiles• Pot tiles• Spanish tiles• Italian or allahabad tile• Inter locking tiles

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• Made of clay or concrete• Manufactured in rectangular shapes of size

varying from 25 cm x15 cm to 28 cm x 18 cm• They have slight camber of 5 to 10 mm in their

length• Stubs project on bed or underside at head to

facilitate hanging from battens• Tiles are nailed through the hole formed at about

25 mm from head and 48 mm from edges

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• Common rafters are first laid at 20 to 30 cm apart• Battens are the fixed across rafters at

a spacing of 4 to 6 cm• Tiles are then laid over it with

sufficient overlap on sides and edges

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• They are 33 to 36 cm long,22.5 to 25 cm wide and12 to 19 mm thick

• Flat longitudinally and curved transversely

• They are unbonded having continuous ide joints from eave to ridge

• Single lapped with head of longitudinal lap varying from 7.5 to 10 cm

• Two diagonally opposite are splayed off to the depth of lap to permit close fit between tiles

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Half-round country tiles• Laid in pairs of under tilles and over tiles

• Undertiles are laid with concave surface upward and over tiles with convex surface upward

• End sections are semicircular and the diameter tapers longitudinally

• Used in villages

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Spanish tiles• Laid in pairs of under

tiles and over tile

• Overtiles taper down from tail to head while the undertiles from head to tail

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Italian or Allahabad tiles• It consists of flat broa bottom undertile that alternate with convex curved


• Undertile is flat,tapered,with flanges at the side

• The overtile is half round and tapered in plan with diameter tapering from 16 cm at tail to ly 12 cm at the head

• The taper in the overtile allow the tile in next course to fit in

• Vertical battens of size 2.2cmx7.5cm are fixed between sides of adjacent undertiles and to these haif round overtiles

• Overtiles are fixed to vertical battens with 75 mm nails

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Interlocking tiles

• Machine made

• Available with patent locking devices to prevent their dislodgement

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Granny bonnet hip tiles• Special tiles used for hip

• They are well bedded with mortar on the back of the tile below and is secured with long nail to the hip rafter

• Bonded with general plain tiles

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Asbestos cement sheet roofing:•Manufactured from asbestos, fibre and

portland cement

• Used for industrial buildings, factories, sheds, cinema houses, auditorium, residential buildings

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Advantages• Cheap• Light weight• Tough• Durable• Water tight• Fire resistant• Vermin resistant• Available in bigger units• SupportinG f framework is cheaper, easier and lighter• Do not require protective paint• Does not require elaborate maintenance• Fast construction

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Available in three formsEverest big six corrugated sheet • have 7.5 corrugations per sheet• overall depth of corrugationis 55 mm • sheets are fixed with smooth surface to steel or wooden purlins Everest standard corrugated ac sheet • have 10.5 corrugations per sheet • overall depth of corrugation is 25 mm• end lap is 150 mm and side lap is 100 mm Everest trafford ac tiles• standard width is 1.09 m• each sheet has four 50mm deep corrugations alternating with flat portions• end lap is150 mm and side lap 74 mm

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Procedure for laying• laid at the end opposite to the direction of prevailing wind and rain

• Laid either from left to right or from right to left

• Sheets are fixed to purlins from top of corrugations through holes made 3 mm greater than diameter of bolts

• Coach screws are used with wooden purlins and crank bolts with steel purlins

• The laying is commenced from eaves

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• When the laying is done from left to right first sheet us laid uncut while the subsequent sheets in bottom row have top left hand corners mitred

• The sheets in subsequent layers have bothb their right hand bottom corner and left hand top corner mitred

• Every sheet in the top row except the last sheet should have bottom rigt corner mitred

• The process is reversed when sheets are laid from right to left

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Points to be noted while fixing A.C. sheets• it should be laid with smooth side upward and end marked

TOP pointing towards the ridge

• End lap and side lap should be properly maintained

• Purlin spacing and length of sheet should be properly checked

• Holes for fixing accessories should be drilled in the crown of corrugations with a diameter 3 mm more than diameter of the fixing bolt

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• Bitumen washer should be provided under G.I. flat washer.the nuts of the screws or bolts should be tightened when 10 to 12 sheets have been laid

• Ridge cappings should be secured to ridge purlin

• Sheets should be mitred properly as required

• Unsupported overhang of A.C. sheets should not exceed 30 cm

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Galvanised iron corrugated sheets

• Made of iron sheets galvanised with zinc to protect them from rusting action

• Manufactured with corrugations running from one end to another to impart additional strength

• Not used for slopes flatter than 1 in 4

• Stronger than A.C. sheets

• High cost

• Laid similar to A.C. sheets

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Specifications for laying• End lap should not be less than 15 cm and side lap varies

from 1.5 to 2 corrugations

• Holes are drilled in sheet crowns

• Sheets are secured to purlin by G.I. hook bolts,

• Screws and nails with curved washers

• Sheets are fixed to eaves by flat iron wind ties

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Eternit roofing

• Consists of bituminous slates called eternit

• Slates are obtained from quarries as blocks which is reduced to slabs after dividing into sections using diamond or circular saw.Each slab is divided inti thin laminae or slate by uing splitters

• Thickness of slate vary from 600mmx300mm to 400mmx200mm

• Available in three colours-grey,black and red

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Laying:• Laid such that each slate overlaps a slate in the next course below it

• Two holes are made from the bed of the slab at the centre or head so

• Slates are fixed to battens by copper or zinc nails

• The spacing of battens i.e,gauge is determined as gauge=(length of slate-lap)/2

• A layer of felt is used below slate to exclude rqain water and moisture

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Light weight roofing

• Used for wide span industrial structures

• Reduces weight of roof

• Requires cheap structural framing

• A water proof layer of asphalt or roofing felt is required with this roofing

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Aluminium roof sheeting

• Consists of aluminium alloyed with small percentage olf manganese for strength

• Lightest roofing

• Used for sloping roofs

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Asbestos cement sheeting

•Manufactured from asbestos,fibre and portland cement

• Used for sloping roofs

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Wood wool decking

• Made from wood fibre interwoven together and cement bonded under pressure in mould

• The roofing slabs vary in thickness from 50mm to 75 mm and in size of 0.6 m width and upto 3.9 m length

• They are nailed to timber joists at 600 to 900 mm centres using 102 to 105 mm long clout nails

• They can take load upto 0.75 kN/m when unreinforced

• Used for both sloping and flat roofs

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Straw board decking

• Made of compressed straw with thick water proof paper covering

• Thickess is 50 mm,width 1.2mm,and length from 1.8 m to 3.6 m

• The board is supported at 600 mm centres along all edges for roof deckings

• Used for both sloping and flat roofs

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Aluminium alloy and steel decking• Used for sloping and flat roofs

• Suitable upto a superimposed load of 0.75 kN/m2

• The roofing has thickness varying from 0.7 mm to 1.2 mm,depth of corrugation varying from 450 to 900 mm and length upto 10 m

• It I fixed to roof support by hook bolts and cleats,hammer drive screws

• A felt vapour barrier is bonded with bitumen to the top of the deck on which an insulating media is bonded to be covered with felt roofing

• Top layer is finished with a layer of white stone chiplings spread on bitumen to provide for solar reflectivity and reduce heat absorption in summer

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Flat roof is one which is either horizontal or practically horizontal with a slope less than 10⁰. A roof has to have some amount of slope so as to drain the rain water easily and rapidly.

The flat roofs can be constructed similar to upper floors using flag stones, RSJ and flag stones, reinforced cement concrete, reinforced brick work, jack arch roof pr precast concrete units.

The flat roofs 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.

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• Flat roof can be used as terrace for playing, gardening, sleeping and for celebrating functions.

• Construction and maintenance is easier.

• They can be easily made fire-proof, in comparison to pitched roof.

• They avoid the enclosure of the triangular space. Due to this, the architectural appearance of building is improved.

• Flat roofs have better insulating properties.

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• They require lesser area of roofing material than pitched roofs.

• They are more stable against high winds.

• They do not require false ceiling, which is essential for pitched roofs.

• They are economical.

• The construction of upper floors can be done easily over flat roofs, if required.

• In multi- storied buildings, flat roof is the only choice, since over-head water tanks and other services are located on the terrace.

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• The span of flat roof is restricted, unless intermediate columns are introduced. Pitched roofs can be used over large spans without any intermediate columns.

• The self weight of flat roof is very high. Due to this, the size of beams, columns, foundations and other structural members are heavy.

• They are suitable at places of heavy rainfall.

• They are highly unsuitable in hilly areas or other areas where there is heavy snowfall.

• It is difficult to locate and rectify leak in a flat roof.

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• 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 on time.

• The speed of flat roof construction is much slower than the pitched roof.

• The initial cost of flat roof is much more than that of pitched roof.

• The flat roof exposes the entire building to the weather agencies, while the projecting elements of pitched roofs such as eaves provide some protection to the building.

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Mud terrace roofing Brick jelly or Madras terrace roofing Mud-phuska terracing with tile paving Lime concrete terracing Lime concrete terracing with tile paving Bengal terracing roof Light weight terracing roof

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