Roofs PDF

ROYAL UNIVERSITY OF BHUTAN COLLEGE OF SCIENCE AND TECHNOLOGY PHUNTSHOLING: BHUTAN Chapter : Roofs and Roof Coverings 10/23/2024 1 Roofs A roof may be defined as the uppermost part of the building provi...

ROYAL UNIVERSITY OF BHUTAN COLLEGE OF SCIENCE AND TECHNOLOGY PHUNTSHOLING: BHUTAN Chapter : Roofs and Roof Coverings 10/23/2024 1 Roofs A roof may be defined as the uppermost part of the building provided as a structural coverings to protect the building from weather. Roof consists of structural elements which support roof coverings. Structural elements can be:  Trusses  Beams  Slabs, etc. Roof Coverings:  C.G.I sheets  PPGL sheets  Shingle  Slate, etc. 10/23/2024 2 Roofs Wooden Truss Steel Truss 10/23/2024 3 Roofs Wooden Shingle Slate C.G.I. Sheets Roofing tiles PPGI/PPGL sheets 10/23/2024 4 Roofs The two primary purposes of roof: i. Insulation: Basic function of a roof is to provide protection and insulation. Proper ventilation must be provided to avoid weather related problems. ii. Drainage: Primary purposes is to protect the occupants from rain and snow. Due to large surface area of the roof, water collected needs to be drained out of the property. 10/23/2024 5 Roofs Requirements of a good roof: It should have adequate strength and stability to carry the superimposed dead and live load. It should effectively protect building against weather action. It should be water proof, and should have effective drainage arrangements. Should provide adequate thermal and sound insulation. Should be fire resistant. 10/23/2024 6 Roofs Types of roof: 1. Pitched or sloping roof, 2. Flat or terraced roofs, and 3. Curved roof. Selection depends upon the shape and plan of the building, weather or climatic conditions and type of construction material available. 10/23/2024 7 Roofs Pitched roof 10/23/2024 8 Roofs Flat roofs or terraced roofs 10/23/2024 9 Roofs Curved roof 10/23/2024 10 Pitched Roofs Different forms of pitched roof: a. Lean-to-roof 10/23/2024 11 Pitched Roofs Different forms of pitched roof: b. Gable Roof 10/23/2024 12 Pitched Roofs Different forms of pitched roof: c. Hip Roof 10/23/2024 13 Pitched Roofs Different forms of pitched roof: d. Gambrel Roof 10/23/2024 14 Pitched Roofs Different forms of pitched roof: e. Mansard Roof 10/23/2024 15 Pitched Roofs Different forms of pitched roof: f. Deck Roof 10/23/2024 16 Pitched Roofs Various elements in pitched roof: i. Span: It is clear distance between the supports of an arch or roof truss. ii. Rise: It is the vertical distance between the top of the ridge and the wall plate. iii. Pitched: It is the inclination of sides of a roof to the horizontal plane. iv. Ridge: Defined as the apex line of the sloping roof. v. Eaves: The lower edge of the inclined roof surface is called eaves. 10/23/2024 17 Pitched Roofs Various elements in pitched roof: vi. Hip: The ridge formed by the intersection of two sloping surfaces, where the exterior angle is greater than 180°. vii. Valley: Ridge formed by the intersection of two rood surfaces, making an exterior angle less than 180°. viii. Hipped End: The sloped triangular surface formed. ix. Ridge piece, ridge beam or ridge board: Horizontal members in the form a beam or board provided at the apex of a roof truss. 10/23/2024 18 Pitched Roofs Various elements in pitched roof: xi. Common rafters or spars: Inclined members running from the ridge to the eaves. xii. Purlins: Horizontal member used to support common rafter or roof coverings. xiii. Hip Rafters: These are the sloping rafters which form the hip of a sloped roof. xiv. Eaves board or fascia board: Are wooden board fixed to the feet of the common rafter at the eaves. xv. Wall plates: Members provided on the top of brick wall for the purpose of fixing the feet of the common rafters. 10/23/2024 19 Pitched Roofs Various elements in pitched roof: xvi. Battens: Thin strips of wood nailed to the rafters for lying roof materials above. xvii. Truss: Is a frame work designed to support the roof coverings. Types of Pitched Roof a. Single roof Consists of rafters which are supported at he ridge and eaves. Limited to span of 5m. There are four types of single roof: 10/23/2024 20 Pitched Roofs i. Lean-to-roof Simplest type and slopes in one direction. One side of the wall is higher than the other side. The slope is usually 30°. Suitable for maximum span of 2.5m. 10/23/2024 21 Pitched Roofs ii. Couple roof Formed by a pair of rafters sloping in both the directions. Upper end of both the rafters are nailed to the ridge piece and lower end is nailed to the wall plates. Span limited to 3.6m. 10/23/2024 22 Pitched Roofs iii. Couple close roof Very similar to couple roof except the lower end is connected by a horizontal member, called tie beam. Suitable for spans upto 4.2m. For larger span, rafter may have a tendency to sag, therefore a vertical rod known as king rod is provided. 10/23/2024 23 Pitched Roofs iv. Collar beam roof When the load is more, the couple close roof have the tendency to bend. Therefore, tie beam is provided at 1/3rd or ½ of the vertical height from wall plate. Suitable for span upto 5m. 10/23/2024 24 Pitched Roofs b. Double or Purlin Roofs When the span exceeds about 2.4m, the necessary size for rafters becomes uneconomical. Hence, to reduce the size of rafters, the intermediate supports called the purlins, are introduced under the rafters. 10/23/2024 25 Pitched Roofs c. Trussed Roofs When span exceeds 5m, framed structures known as trusses are provided at suitable interval along the length. Various types of trusses are: i. King-post truss Consists of a tie beam, two principal rafters, two struts and a king post. Purlins are placed above principal rafters which supports the common rafters. Suitable for span 5 to 8 m 10/23/2024 26 Pitched Roofs ii. Queen-post truss Consists of two vertical post, known as queen post. The vertical posts are connected with the help of horizontal piece known as straining beam. Struts are used to join the queen post to the principal rafters. Straining sill is introduced on the tie beam or connecting the tops of two queen posts to counteract the thrust. Suitable for spans between 8 to 12m.c 10/23/2024 27 Pitched Roofs iii. Combination of King-post and queen-post trusses Suitable up to 18m span. 10/23/2024 28 Pitched Roofs iv. Mansard roof truss Named after its designer Francois Mansard. Is a combination of king-post and queen-post truss. The upper portion consists of king-post and lower portion consists of queen-post. The pitch of upper portion ranges from 30° to 40° and for lower portion, it ranges from 60° to 70°. 10/23/2024 29 Pitched Roofs v. Truncated truss Same as mansard except that its top is formed flat. Used when a room is required to be provided. 10/23/2024 30 Pitched Roofs vi. Steel roof trusses When span exceeds 10m, timber trusses become uneconomical hence steel trusses are preferred. Steel trusses are fabricated from steel members such as channels, angles, T- sections and plates. The type of truss depends on the span. 10/23/2024 31 Pitched Roofs vi. Steel roof trusses 10/23/2024 32 Roof Coverings Is an essential component of pitched roof. The selection of roof covering depends upon: type of building, type of framework, initial cost, maintenance requirements, fabrication facilities, appearance, durability, availability of material and climate of the locality. Following are the types of roof covering materials used: 10/23/2024 33 Roof Coverings i. Thatch Covering Commonly used in villages. The frame work to support the thatch consists of bamboo rafters. The roof covering consists of bundles of reeds or straw. To drain water effectively, minimum slope of 45° is kept. 10/23/2024 34 Roof Coverings ii. Wood Shingle roofing Thin slabs of wood used as covering. Are obtained from well seasoned timber. Length various from 30-40cm, width varies from 6-25 cm and approximately the thickness tappers from 10mm to 3mm. 10/23/2024 35 Roof Coverings iii. Tile Roofing One of the oldest. Tiles are made of clay or concrete but usually clay. These are manufactured from locally available earth. Nibs are projections on the bed or at the head, used for hanging the tiles on the battens. Common rafters are laid and battens after that at a spacing of 4-6 cm. 10/23/2024 36 Roof Coverings H/W Name and draw the various types of tiles which are generally used. 10/23/2024 37 Roof Coverings iv. Asbestos cement sheets (A.C. sheets) They are larger in size compared to tiles, hence lesser frame work. These are manufactured using asbestos, fibre and Portland cement. 150mm is provided as end and head lap and 1½ corrugation or 100mm as side Purlin spacing and length of lap. sheets should be checked They are fixed to purlins with the help of before the commencement of bolts or nails. work. The sheets are fixed to the purlins with Unsupported A. C. sheets the help of bolts or screws. should not exceed 30cm. 10/23/2024 38 Roof Coverings v. Slate roofing The sizes of slates vary from 600mmx300mm to 400mmx200mm. Slate should be hard, durable, not slits when holed and especially non-absorbent. Slates are laid such that each slate overlaps the other slate in the next course below it. 10/23/2024 39 Roof Coverings vi. Corrugated Galvanized Iron sheets (C.G.I sheets) Are stronger than A. C. sheets. G. I. sheets are made of iron which are galvanized with zinc to protect from rusting. G.I. sheets are laid in the same fashion as A.C. sheets. 10/23/2024 40 Roof Coverings vii. PPGI/PPGL sheets (Pre painted galvanized Iron/ Pre painted Galvalume) Are stronger than A. C. sheets. PPGI is coated completely with zinc while PPGL is coated with aluminium, zinc and small amount of silicone. PPGL/PPGI is more corrosion resistant compared to normal CGI sheets. PPGL has superior corrosion resistance and durability over PPGI PPGL/PPGI sheets are laid in the same fashion as CGI sheets. 10/23/2024 41