FBE 03 Building Construction Science Lecture 4 Roof

FBE 03: Building Construction & Science Lecture 4 Roof System

Introduction • main functional requirements of a roof : ü Strength and stability ü Weather resistance ü Thermal insulation ü Sound insulation ü Fire resistance ü Durability ü 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

Wind paths around a structure

Wind paths over a roof

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 ü roof coverings and each material has its own response to frost attack, chemical attack and solar radiation • Appearance ü type of roof, ü whether it is flat or pitched, ü the style of the roof ü 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

Typical temperature gradient through a flat roof

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 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

Cold deck design • vapour check of polythene or aluminium foil to 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 ü waterproof covering easy to maintenance ü remove any water vapour

Cold deck design • disadvantages of cold deck design ü relies on natural ventilation to remove any water vapour ü weather conditions prevent natural ventilation ü 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 ü void is not required in the roof construction ü ventilation is not required within the roof construction ü vapour barrier is able to be 100 per cent effective

Warm deck design • disadvantages of warm deck design: ü thermal insulation layer must be strong enough to support foot traffic for maintenance purposes ü Thermal insulation force to be replace when the roof covering is going to replace.

Warm deck design

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 thermal insulation • ballast will placed on top of thermal insulation to prevent it blow off

Inverted roof design • advantages of inverted roof design: ü no need for a separate vapour barrier ü waterproof covering fulfills the dual function of covering and vapour barrier. ü Vapour barrier being protected by insulation layer

Inverted roof design • disadvantages of inverted roof design: ü thermal insulation layer is exposed to the external environment ü It must be waterproof ü insulation layer must also be strong enough to withstand foot traffic on the roof ü Extra dead consideration needed for ballast

Inverted roof design

Flat roof covering • The materials for flat roof covering must: ü impervious ü sealed joints ü durable • Materials for roof covering: ü mastic asphalt, ü bituminous felt sheeting or ü sheet metal coverings

Mastic asphalt 20 mm 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 • •

Mastic asphalt

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

Bituminous felt

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

Welted joint to copper sheet roofing

Drip joint to lead sheet roofing

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

Mastic asphalt vertical upstands to slump

blistering

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