2 Functional requirements ROOFGENERAL FUNCTION:To give a protective covering to the building, so rain, wind or snow may not damage the buildingFunctional requirementsWeather resistanceStructural stabilityProvide good appearanceProvide thermal insulation
3 Functional Requirements (cont’d) Weather ResistanceRequired to protect a building from the damaging effects of rain & windThe ingress of rain: causes damage to decorations & structural damage to timber due to attacks by fungus & insectsThe roof is covered by a layer of impermeable material which prevents the passage of rain into a buildingThe deflection of building may be cause of wind: accurance of suction on the windward of slope of roof
4 Functional Requirements (cont’d) Structural StabilityProvide a means of support for the roof coveringIn designing a roof structure, the designer must take into consideration the span of structural membersIf span at joist too great – bendTo overcome : introducing a structural element which reduces the span
5 Functional Requirements (cont’d) Provide Good AppearanceMight be a major visual element in the design of a buildingThe detailing of roof can significantly affect the appearance of a building
6 Functional Requirements (cont’d) Provide Thermal InsulationThe roof constitutes a substantial proportion of the external surface area of a building & consequently has considerable potential for heat lossPitched roofs are usually insulated between the ceiling joistsAlternative insulation might be applied between raftersFlat roof insulation is usually laid above the decking, either below or above the roof covering
7 General types of roofs: Sloping Flats Shell Domes
8 SLOPING ROOF / PITCHED ROOF Sloping roof may be categorized into:Medium to high slope – 4:12 to 12:12Low slope - up to 3:12Sloping roofs shed water easily to eave guttersRoof height & area increase with its horizontal dimension
9 SLOPING ROOF (cont’d) Sloping roofs may have a structure of: Rafters & sheathingBeams, purlins & deckingTrusses
10 SLOPING ROOF /PITCHED ROOF Medium & high slope roofs may be covered with shingles, tiles or sheet materialsLow slope roofs require roll or continuous membrane roofing; some shingle & sheet materials may be used on 3:12 pitchesThe roof slope also affects the design loads & the requirements for underlayment & eave flashingSlope roof planes may be combined to form a variety of roof forms
11 FLAT ROOFS Pitch from 0 to 10 degree Slope may be achieved by sloping the structural members or by tapering the deck or rigid insulationSlope usually leads to interior drains; perimeter scuppers can be used as overflow drains
13 Shell RoofA thin shell is defined as a shell with a thickness which is small compared to its other dimensions and in which deformations are not large compared to thicknessThese elements are typically curved and are assembled to large structures. Example: factories and roof structures in some buildings.Advantage in construction : a lot of material is saved as the section needed is very thin.
14 Domes A roof of semi-spherical or semi elliptical shape Constructed of stone or brick or concrete, & supported on circular or regular polygon shaped wallsThe structure is such that within certain height & diameter ratios, very small thickness is needed
15 Common Term Used Term Description Ridge piece A member which runs horizontally at the highest level of the roofHipExternal angle of a sloping roof at which the roof slopes are turned downCommon rafterMembers supporting the battens or boardings under the roof coveringHip rafterMembers which form the hip of the roof & to which the common rafters are attachedRidge
16 Common Term Used (cont’d) RidgeApex line of a sloping roofEavesThe lower edges of the sloping surface of a roofEave-boardWooden board fixed along the eaves connecting the common raftersGableWall which follows the slope of the roof from eave to ridge & covers the end of a roofBarge boardWooden planks used to fix the ends of common rafters projecting beyond the sloping top of a gable wall
17 Common Term Used (cont’d) ValleyA depression formed at the intersection of two sloping roofs at their junctionJack rafterShort lengths of rafters fixed at the hip rafters & eaves boardValley rafterMember at the intersection of 2 inverted slopes to which rafters are fixedDragon beamShort member of wood which carries the foot of a hip rafter which is attached to the tie fixed across the angle formed by the intersection of 2 wall plates at the corner of a buildingPurlinsMembers that laid horizontally to support the common rafters. They transmit the loads to the trusses or walls
18 Common Term Used (cont’d) Purlin cleatShort piece of timber bolted to the rafters of roof truss for fixing the purlinsTrussA framework of triangles
19 ROOF COVERINGSIs a materials which gives a protective surface to the roofing structureThe function is only to prevent ingress or egress of heat & moisture into the buildingVarious types of coverings depending onThe character of the buildingThe type of the roofing structureLocal conditions, cost, etc.
20 Types of Roof Coverings TilesThatchWoodShinglesTilesSlatesAsphaltAsbestos cement sheetsCorrugated iron sheetsBituminous feltGlassSheet metal roof coverings (copper, zink, etc.)ThatchAsbestosWood
21 ROOF DESIGN & CONSTRUCTION FACTORS Roof structure must be designed to carry:Dead loads – roof structure, deck, insulation, roofing & any equipment located on top of or suspended from roofLive loads – accumulated rain, snow & ice & traffic if any.Wind loads – pressure or suction from windType of roofing required against:Surface water from the outsideWater vapor diffusion from the insideMoist air flowHeat flow & solar radiation
Your consent to our cookies if you continue to use this website.