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Chapter 9 roofs and roofs coverings

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Chapter 9 roofs and roofs coverings

  1. 1. S.S.A.S.I.T, SURAT GTU 1
  2. 2. Roofs The uppermost part of a building which is constructed in the form of framework to give protection to the building against rain, heat, snow, wind, etc. A roof basically consists of structural elements provided at the top of the building for the support of roof coverings The structural elements consists of; - Trusses - Portals - Slabs - Domes - A. C sheets covering - G. I sheets coverings - Shingles - Slates, etc 2
  3. 3. Requirements of good roofs Following are the requirements of a well planned roof; 1. It should be durable against the adverse effects of various agencies such as wind, rain, sun, etc. 2. It should grant the desirable insulation against sound and heat. 3. It should be structurally stable and sound and it should be capable of taking the loads likely to come over it. 4. It should be well-drained. 5. It should have efficient water-proofing arrangements. 6. It should be fire resistance 3
  4. 4. Classification of roofs The roofs are classified into the following three categories: 1. Pitched or slopping roofs 2. Flat or terraced roofs 3. Curved roofs Flat roofs are used in plains where rainfall is less and climate is moderate. Pitched roofs are preferred wherever rainfall is more. Shells and folded plate roofs are used to cover large column free areas required for auditoriums, factories etc. The choice of the type of roof will depend on the climatic conditions, shape of building, availability of materials, importance of building, etc. 4
  5. 5. 5 • 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. ELEMENTS OF PITCHED ROOF
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  7. 7. 7 Ridge
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  9. 9. 9 • 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. • 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 .
  10. 10. 10 • RAFTERS: • COMMON RAFTERS OR SPARS: These are inclined wooden members running from the ridge to the eaves. They are beveled 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. • JACK RAFTERS: These are the rafters shorter in length, which run from hip or valley to the eaves • PRINCIPAL RAFTER: These are inclined member of a truss.
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  12. 12. 12 • 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. • VERGE: The edge of sloping roof at the gable end, running between the eaves and ridge. • 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.
  13. 13. 13 • 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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  15. 15. Pitched Roofs In the areas of heavy rain falls and snow fall sloping roof are used. The slope of roof shall be more than 10°. They may have slopes as much as 45° to 60° also. The sloped roofs are known as pitched roofs. The sloping roofs are preferred in large spanned structures like workshops, factory buildings and ware houses. In all these roofs covering sheets like A.C. sheet, G.I. sheets, tiles, slates etc. are supported on suitable structures. Pitched roofs are those which have the decks or surface with considerable slope for covering the building structure. In pitched roofs, the slope of roof may vary from 1:1 or 1:3. in areas of heavy snowfall steeper slopes of 1:1.5 or 1:1 are provided to reduce the incidence of snow load on the roof. The pitched roofs are classified into; a) Single roofs b) Double or purlin roofs c) Trussed roofs 15
  16. 16. 16 •Advantages : 1.These are the cheapest types of roof. 2.Pitched roofs are ideally suited for hill area. 3.These are suitable for building in coastal areas. 4.Self weight of pitched roof is less as compared to flat roof. •Disadvantages: 1.These roofs cannot be used as terrace. 2.Upper floors cannot be constructed over pitched roof. 3.It is not fire proof. 4.It has no insulating properties. 5.It is not stable against high wind velocity. 6.Construction is difficult. Pitched Roofs
  17. 17. 17 • Pitched roofs may be broadly classified into the following • (а) 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 • (c) Triple-membered or framed or trussed roofs • King-post roof truss. • Queen-post roof truss. • Combination of king-post and queen-post trusses, • Mansard roof truss. • Truncated roof truss. • Bel-fast roof truss or latticed roof truss. • Composite roof trusses. • Steel sloping roof trusses. TYPES OF PITCHED ROOFS
  19. 19. Pitched Roofs Single roofs If the span of roof is less than 5 m the following types of single roofs are used. 1. Lean-to-roof 2. Couple roof 3. Couple-close roof 4. Collar beam roof 5. Collar and scissor roof In all these roofs rafters placed at 600 mm to 800 mm spacing are main members taking load of the roof. Battens run over the rafters to support tiles. 19
  20. 20. Lean-to roof This is the simplest form of a pitched roof. In this type one wall is carried up sufficiently higher than the another one to give the necessary slope to the roof. The rafters are suitably secured on the wall-plates and eaves boards, battens and roof covering is provided as shown. Suitability: It is generally used for sheds, out-houses attached to main buildings, verandahs, etc. 20
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  22. 22. • A king-post truss consists of the following components : • (i) lower tie beam, • (ii) two inclined principal rafters, • (iii) two struts, and • (iv) a king post. 22 King-post Roof Truss
  23. 23. 23 The principal rafters support the purlins. The purlins support the closely-spaced common rafters which have the same slope as the principal rafters. The common rafters support the roof covering as usual. The spacing of the kingpost truss is limited to 3 m centre to centre. The truss is suitable for spans varying from 5 to 8 meters. The principal rafter is jointed to the tie beam by a single abutment and ‘tenon joint’ or by a ‘bridle joint’. The joint is further strengthened by a wrought iron heel strap, would round the joint. The king-post is provided with splayed shoulders and feet, and is tenoned into the upper edge of the tie beam for a sufficient distance.
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  27. 27. • 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, The queen-posts are the tension members. • The straining beams receives the thrust from the principal rafters, and keeps the junction in stable position. • A straining sill is introduced on the tie beam between the queen-posts to counteract the thrust from inclined struts which are in compression. • In absence of the straining sill, the thrust from the strut would tend to force the foot of the queen-post inwards. • These trusses are suitable for spans between 8 to 12 meters. • The joint at the head of queen-post -is formed due to the junction of two compression members (principal rafter and straining beam) and a tension member(queen-post). 27 Queen-post Roof Truss
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  31. 31. • 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. • They are : (i) more economical, (ii) easy to construct or fabricate, (iii)fire-proof, (iv)more rigid, and (v) 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. • Steel trusses may be grouped in the following categories : • (а) Open trusses • (b) North light trusses • (c) Bow string trusses • (d)Arched rib trusses and solid arched ribs. 31 Steel Roof Trusses
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  36. 36. • The sections comprising of a steel truss are readily available in the required dimensions, resulting in minimum wastage of material. • Steel trusses are light in weight, 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. • The members in Steel trusses are equally strong in tension as well as compression. • Steel trusses can be used over any span, while timber trusses are suitable only up to 15 m span. • Steel truss are fire proof. • Steel truss are termite proof. • Steel truss have longer life. 36 Steel Trusses Have The Following Advantages Over Timber Trusses
  37. 37. 37 • 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 agency. • 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, and(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. Galvanized corrugated iron sheets 6.Eternit slates. 7. Light weight roofing. Roof Coverings For Pitched Roofs
  38. 38. 38 • This is the cheapest roof-covering, commonly used in villages. • It s very light, but is highly combustible. • It is unstable against high winds. • It absorbs moisture and is liable to decay. • Thatch roof-covering consists of bundles of reeds or straw . • The frame work to support thatch consists of round bamboo rafters spaced 20 to 30 cm apart and tied with split bamboos laid at right angles to the rafters. • The thatch is tightly secured to the frame work with the help of ropes or twines. • In order to drain the roof effectively, a minimum slope of 45° is kept. • The thickness of thatch covering should at least be 15 cm . Thatch covering
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  40. 40. • Shingles are thin slabs of wood used to cover roofs. • The use of shingles is restricted to hilly areas where local timber is easily available at low cost. • Though shingle roofing is light weight, it is not fire and termite resistant. • Wood shingles are obtained from well seasoned timber, by either sawing or splitting. • They are approximately 10 mm thick at the tail. • They are laid in a similar fashion as tiles and slates. 40 Wood Shingle Roofing
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  42. 42. 42 • Use of tiles for roofing is one of the oldest, and is still preferred for residential buildings and country houses. • This is because country tiles are manufactured from locally available earth. • Tiles are named according to their shape and pattern, and they are manufactured by a process similar to the one used for the manufacture of bricks. • The various types of tiles generally used are : • 1. Plain or flat files. • 2. Curved or pan-tiles. • 3. Pot tiles or half-round country tiles. • 4. Italian or Allahabad tiles. • 5. Inter-locking tiles. Tile Roofing
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  44. 44. • Asbestos cement sheets are now increasingly becoming popular for industrial buildings, factories, sheds, cinema houses, auditorium and even residential buildings, since they are cheap, light weight, tough, durable, water tight, fire-resisting. • The biggest advantage is that they are available in bigger units unlike tiles, and hence supporting frame work (ground work) is also cheaper, easier and lighter. • These sheets do not require any protective paint, and no elaborate maintenance is required. Also, the construction with A.C. sheets is very fast. A.C. sheets are manufactured from asbestos, fiber (about 15%) and Portland cement. Asbestos is a silky fibrous mineral made up of metamorphosed volcanic rocks. • In India, asbestos cement roof coverings are available in following forms: • Everest big-six corrugated A.C. sheets. • Everest standard corrugated A.C. sheets. • Everest Trafford A.C. tiles (or sheets). • These' sheets have length of 1.25 to 3 meters in increments of 15 cm. 44 Asbestos Cement Sheets (A.C. Sheets)
  45. 45. 45 •Procedure for laying A.C. Sheets: 1. The sheets should be laid with smooth side upward and the end marked ‘Top’ pointing upwards the ridge. 2. They are usually laid with an end lap of 15 cm. 3. The side lap for A.C. corrugated sheet should be of half corrugation and that for A.C. Trafford sheet be of one corrugation. 4. The hole for fixing accessories must be drilled and not punched. The hole must be drilled in the crown of the corrugation. 5. The diameter of the hole should be 3 mm greater than the diameter of the screw to be used. 6. 8mm diameter hook bolt, l-bolt, crank bolt or crouch bolt are inserted in the 11mm diameter drilled holes. 7. Miter is necessary to avoid gaps where four sheets meet at a lap.
  46. 46. Asbestos 46
  47. 47. • G.I. sheets are also widely used. • They are stronger than A.C. sheets. However, because of their higher cost, they are now gradually replaced by A.C. sheets. • They are not used for slopes flatter than 1 in 4. • G.I. sheets are made of iron sheets which are galvanized with zinc to protect them from rusting action of water and wet weather. • These sheets are fixed in a manner similar to the A.C. sheets. End lap should not be less than 15 cm and The holes are either drilled or punched in the sheet crowns. • The sheets are secured to purlins by means of G.I. hook bolts, screws and nails etc., with curved washers. • The sheets should be fixed to eaves by means of flat iron wind ties. 47 Galvanized Iron Corrugated Sheets (G.I. Sheets)
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  49. 49. 49 It is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash. Slates
  50. 50. A roof laid at angle of less than 10 degree to the horizontal is known as flat roof. • Advantages: 1.Construction and maintenance is easy. 2.The roof can be used as terrace. 3.It has better insulating properties. 4.It can be made fire proof. 5.Stable against high winds. 6.They do not require false ceiling. 7.Flat roofs are proved to be overall economical. • Disadvantages: 1.It cannot be used for long life span without construction of beams and columns. 2.The self weight of flat roof is high. 3.They are unsuitable for places with heavy rainfall. 4.It is difficult to locate and rectify leaks. 5.The initial cost is more 6.The speed of construction is slow. 50 Flat Roof
  51. 51. Mud terrace roofing 51 Types Of Flat Roof
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  53. 53. MUD TERRACE ROOF • Cheapest form of roofing and is used where rain fall is less. • Made by white earth mud containing large amount of sodium salts. • Commonly constructed in Punjab, M.P.
  54. 54. BRICK JELLY OR CHENNAI TERRACE • Most commonly used in Chennai so it is called Chennai terrace.
  55. 55. MUD PHUSKA TERRACING WITH TILE PAVING • Common in Delhi, Punjab, U.P. and Haryana. • Mostly used over R.C.C. slab or jack arc roofing. • Has very good water proofing and heat insulation properties. • Mud Phuska is prepared by mixing straw and lots of water. • Finished terrace is cured about 10 days.
  56. 56. BENGAL TERRACE ROOF • Mostly used in Kolkata or surrounding areas to cover verandahs.
  57. 57. 57 Thank you