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FBE03: Building Construction & Science. Lecture 4 Roof System. Introduction. main functional requirements of a roof : Strength and stability Weather resistance Thermal insulation Sound insulation Fire resistance Durability Appearance . Strength and stability.
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FBE03: Building Construction & Science Lecture 4 Roof System
Introduction • main functional requirements of a roof : • Strength and stability • Weather resistance • Thermal insulation • Sound insulation • Fire resistance • Durability • Appearance
Strength and stability • Roof design will be affected by it span • materials used in roof construction need to have a high strength • loads carried by roofs are mostly dead loads and superimposed loads • effect of wind pressure is a major consideration in roof design • wind blowing onto a building will create positive and negative pressures at certain points and will also create eddies as the wind is deflected by the shape of the building
Areas on a roof affected by high suction forces from the wind
Thermal insulation • Warm air rises, and therefore heat losses through the roof are quite significant • it is necessary to add materials into the roof construction in order to provide adequate thermal insulation to the roof
Sound insulation • Sound insulation is not as great a problem • very little noise from traffic entering the house through roof • if overhead noise, particularly from aircraft • sound insulation materials will need to be added to the roof construction
Fire resistance • spread of fire from adjacent buildings • Roof contained flammable and may support the surface spread of flame • roof structure must also prevent the early collapse of the roof in a fire
Durability • roof coverings and each material has its own response to frost attack, chemical attack and solar radiation • Appearance • type of roof, • whether it is flat or pitched, • the style of the roof • colour and texture
Flat roof construction • simplest form of roof construction • least expensive to construct • tendency to leak and need much more regular maintenance than pitched roofs
Combating interstitial condensation • The roof is not completely flat, but incorporates a slight gradient for the run off of rainwater. • gradient should be a minimum of 1 in 40 • The rainwater is generally drained to the edge of the roof, called the eaves, and collected by a gutter • longer span roofs, the rainwater may be drained to various collection points on the roof
Combating interstitial condensation • Moisture created by human activity in the house. Eg. Washing, cooking, bathing & etc. • air contains a certain amount of moisture, called humidity • amount of moisture that air can sustain before it becomes saturated • air becomes saturated with a quantity of water vapour is termed the dew point temperature
Types of roof construction to combat interstitial condensation • Cold deck design • Warm deck design • Inverted roof design
Cold deck design • space of at least 50 mm is left between the cold side of the thermal insulation layer and the structural deck • Air is allowed to pass through this space • Dry air will replace high water vapour content air
Cold deck design • vapour check of polythene or aluminium foilto prevent water penetrate into house • vapour checks are impermeable to the passage of moisture vapour, but allow air to percolate through them • advantages of cold deck • waterproof covering easy to maintenance • remove any water vapour
Cold deck design • disadvantages of cold deck design • relies on natural ventilation to remove any water vapour • weather conditions prevent natural ventilation • birds, insects or vermin could enter the roof space
Warm deck design • thermal insulation layer is placed on top of the structural deck • vapour barrier is placed over the deck immediately beneath the insulation • advantages of warm deck design • voidis not required in the roof construction • ventilation is not required within the roof construction • vapour barrier is able to be 100 per cent effective
Warm deck design • disadvantages of warm deck design: • thermal insulation layer must be strong enough to support foot traffic for maintenance purposes • Thermal insulation force to be replace when the roof covering is going to replace.
Inverted roof design • roof covering is impermeable it can also act as a vapour barrier • vapour barrier must be placed on the cold side of the thermal insulation • ballastwill placed on top of thermal insulation to prevent it blow off
Inverted roof design • advantages of inverted roof design: • no need for a separate vapour barrier • waterproof covering fulfills the dual function of covering and vapour barrier. • Vapour barrier being protected by insulation layer
Inverted roof design • disadvantages of inverted roof design: • thermal insulation layer is exposed to the external environment • It must be waterproof • insulation layer must also be strong enough to withstand foot traffic on the roof • Extra dead consideration needed for ballast
Flat roof covering • The materials for flat roof covering must: • impervious • sealed joints • durable • Materials for roof covering: • mastic asphalt, • bituminous felt sheeting or • sheet metal coverings
Mastic asphalt • 20mm thickness for vertical or horizontal • jointless, weatherproof and impermeable material • pliable and never completely rigid • heated on site in boilers to a temperature of 200 Celsius to become fluid • vertical abutments the asphalt is formed into an angle fillet and carried up the wall a minimum height of 150 mm and dressed into a horizontal brickwork joint • metal cover flashing is used to protect the covering
Bituminous felt • Bitumen felt is made from bitumen • Felt may be finishes with a coating of sand or final gravel give a decorative finish • Cost effective and convenient • Gas torch for bituminous felt installation • Bituminous felt on a flat roof should have a long life if properly apply
Sheet metal coverings • produced from malleable metals in sheet form • easily bent to shape without cracking • adapt to various profiles when fitted on the roof • high durability and impermeability • better even than mastic asphalt • metals used are lead, copper, aluminium and zinc
Solar radiation • layer of thermal insulation incorporated into the roof construction prevents solar heat gain • cause a softening of the covering material and a breakdown in its structure • Mastic asphalt will begin to soften as it becomes hotter, and this may cause vertical upstands to slump • Bituminous felt coverings will suffer from the bitumen-bonding compound beginning to liquefy
Solar radiation • Heating up of the roof may also cause entrapped moisture to re-vapourise and attempt to pass through the roof covering to the external air where the vapour pressure is much less
Solution for solar radiation • white or aluminium paints applied to the surface of the covering to reflect the sun’s rays off the roof surface(maintenance needed). • white stone chippings may be embedded in the surface of mastic asphalt • chippings should not have sharp edges or they will puncture the covering when they are walked upon