Ceiling/Roof System

Ceiling System

The ceiling is the top surface of an internal space. They give a sense of enclosure and are also used to decorate the room. Ceilings also provide some degree of thermal and sound insulation depending on their thickness.

There are two types of ceiling, directly attached ceiling and suspended ceiling. Directly attached ceiling are incorporated with the Roof / ceiling system and the lining is nailed onto the underside of the ceiling joist/bottom chord. Fixing can be reinforced by elastomeric adhesive.

Suspended ceiling needs independent ceiling joist to support ceiling lining. This type is commonly used at lower floors of a building with rise at least two levels where a void is needed between the underside of the upper floor and the intended ceiling line. The void is for the plumbing pipes and electrical wiring to pass through without creating visual pollution.

Installing plasterboard to ceiling takes about 6 days to do and the time it takes depends on the area of the ceiling.

Roof / Ceiling System

The roof is the top part of a building. It provides shelter for the occupants. Its design is the most complex compared with other system because most environmental factors have major impact on the roof than other building system.

For conventional roof with complexity to some degree (include eaves), it require about 9 days to build. Other timber roof system may require more or less depends on their complexity. Roof covering (include sarking and battens) need about 5 days to do but the time varies depend on the overall roof area.

Skillion, conventional and truss roof are very common in use for small scale building. These system will be analysed below.

Conventional Roof (Pitched Roof)

The construction of conventional roof is very similar to the construction of timber floor system. Its advantage is that the internal roof area are sufficiently free from the obstruction of structural members and can be used as an attic for storage. It can also be built on virtually every building because it must be built on site. Due to this, it is also more expensive to built due to the degree of skill and labour involved in building them. Conventional frame roof also consume more time and material to built than other forms of roof. From these disadvantages, it is no longer the common roof system used.

The conventional roof system consist of several members which help the roof to distribute its load to nearby load bearing wall both externally and internally. Each member is analysed below:

  • Rafter - The sloping member forming the pitch of the roof. They are used to support roof cladding materials and other live loads and transfer them to the external supporting wall. Rafters mostly come in opposing pairs so that they meet at the apex directly without applying more pressure to the ridge board. The bottom side of the rafter is cut like a "bird's mouth cut" to allow the rafter to sit directly on top of the wall's top plate. Rafters are usually placed at 450 to 600 centers with cross sectional area of 100*50.

  • Underpurlin - Horizontal running members fixed under the rafters. Usually fixed within the middle third of the rafters single span length, they are used to prevent the sagging of the rafters by shortening the distance of the rafter's unsupported length. Hence rafters need not to be in big cross sectional dimension to resist sagging. Underpurlins usually have a cross sectional area of 100*75 and spacing of 2100 centers for tile roof.

  • Strut and prop - Angled members to support the underpurlins and transfer the load to a local internal wall. The placement of this member must be greater than 45 degrees to the horizontal to be effective. Sizes used are usually 100*75.

  • Collar tie - A horizontal member fixed around the middle third of two opposing rafters to prevent then spreading apart due to the load they are supporting. Collar ties usually used have cross sectional area of 100*38 and spacing of 1200 centers for tiled roof.

  • Ceiling joist - The member running horizontally across the top surface of the wall. Their function is to provide fixture for ceiling lining and to prevent the spreading of the rafters. Ceiling joist mostly used at 100*38, 600 centers.

  • Anti-slip Board - A member that is fixed at the side of the ceiling joist wherever there is a prop fixed to it. Its function is to stop the prop from spreading if the fixing failed hence it may reduce the amount of shear force acting on the fixing.

    Pitched roof can also be constructed from reinforced concrete but this is not so common because they are more expensive to build. But they are very durable.

    Section of a typical conventional roof

    Truss Roof

    The construction of the truss roof is quite different from conventional roof. It is designed so that the roof load is transfer to the external wall only. Since the internal wall are non load bearing, the advantage of this is that it allow flexibility in initial planning as well as any future renovation involving re-partitioning the house.

    Trusses are designed to be very efficient in span to weight ratio by incorporating a series of triangular frames. Most of the trusses used in Australia are factory pre-fabricated and transport to site. The advantage of this is that it doesn't require much skill labour to build them. It also save time and money since the efficiency of the truss cut the amount of materials used and that the only work to do is to put them into place. Trusses can also be made from steel which give better strength due to the properties of steel though it may require more skills to put it into place. The only disadvantage is that it does not give much flexibility for initial planning as they need to be redesigned whenever the external wall line is altered. But still, truss roof is very popular in small-scale buildings.

    General view of trussed roof (gable)

    The truss roof system consists of several members which help the roof to distribute its load to the external wall. Other that the functions of each member described below, the members also form a static mechanical system to achieve optimum span to weight ratio. The members are analyzed as follow:

  • Top Chord - The sloping members (mostly, but can also be flat) consist of two pieces of timber which are joined at the apex forming the pitch of the roof. Its function is the same as that of a rafter in conventional roof.

  • Bottom Chord - The horizontal member (mostly but can also be sloped) usually running across the top surface of the wall. Its function is very similar to the ceiling joist in conventional roof.

  • Web - The angled members between top and bottom chords. They are used to resist compression and tension applied from the roof and transfers some of the load via the bottom chord.

  • Joint Fixture - Galvanized steel nailing plates are used to secure the members together to form the system. This nailing system provides a cheap, efficient and effective solution.

    In most cases, trusses are spaced at 600 centers for tiled roof. This spacing provide economic sizes of battens and easy fixture of plasterboard.

    Skillion Roof

    Often refer to as a mono-pitched roof. Skillion roof is of similar construction principles to conventional roof but relies directly on gravity to maintain its structural integrity. Its advantage is that the construction is quick and simple with fewer members involved than other pitched roof system and hence cheaper to build. It is the ideal roof system for building works of minor importance such as garages. However, due to fewer members involved. If any member fail, it may cause the whole roof to collapse - a case very unlikely in other pitch roof system.

    The skillion roof system consist of few members which help the roof to distribute its load to nearby load bearing wall both externally and internally. Each member is analysed below:

  • Rafter - The sloping member forming the pitch of the roof. They are used to support roof cladding materials as well as ceiling lining materials and transfer the loads to the outermost supporting walls. If the rafter length is long, a beam is needed to support the rafters from unacceptable deflection. "Bird's mouth cut" is used in the rafter to connect to supporting walls or beams. Rafters are usually placed at 450 to 600 centers with cross sectional area of 100*50.

  • Beams - Members going horizontally at right angle to the rafters. They are only needed if the cross sectional area of the rafter exceeded their recommended unsupported length.

    Flat roof

    Flat roof are another variety of the skillion roof but its pitch is flat. Its advantage is that the construction is quick, simple and flexible and does not require much skill to build. It can fit into all sort of shape including irrectangular shape where pitched roof may require complicated structure to perform. It uses the least materials to construct as they are used at its maximum plan area. The only disadvantage is that any failure of one member would cause the whole structure to fail. While it is called a flat roof, it is pitched at a small degree to allow drainage of water.

    Flat roof if not constructed properly will leak. This roof system is often responsible for various thermal uncomfort experience by its users. In general, this roof system is more suitable in a dry climate free from hale as their weight can cause the structure to fail.

    Flat roof can be constructed from reinforced concrete like a suspended floor and is very common in large scale buildings because they allow a space for rooftop recreational areas. Reinforced concrete flat roofs can be build from pre-fabricated panels or build on site.

    Eaves

    Eaves are the projecting overhang at the edge of a roof. Their function is to protect the exterior wall from rapid weathering by rain. They are also used to reduce sunlight penetration into a wall opening. There are two common types of eave - boxed and open eave. They serve the same purpose but their physical appearance is different. It needs more skills to build an open eave than the boxed eave.

    The members used around the eaves are analysed as follow:

  • Fascia - A horizontal member running along the roof edge fixed at the lower ends of the rafter/top chord. Usually 175*25 in size, it is used to support the gutter and provide enclosure for the box eave.

  • Soffit Bearer - A horizontal member running the same direction as the rafter. It provide a place for the fixture of the soffit lining. Soffit bearer usually at sizes 50*38 at span less than 600 and at 600 centers.

  • Soffit hangers - A vertical member sitting between the outer leaf of a cavity wall and the rafter. It provide additional support for the roof load to minimize the load pressure at the eaves. Usually at size 50*25 at 600 centers.

  • Eaves Gutter - The horizontal member running along the edge of the roof. They are used to collect stormwater and direct them to a local downpipe. Gutters are usually of 100*50 in rectangular size and of aluminium alloy or PVC.

  • Box Gutter - The horizontal member running through the internal of the house. This type of gutter is rarely used in small scale buildings. They have much bigger section than eave gutter to give more safety margin. Their disadvantage is that if it fails, it will cause local timber members to rot. Therefore, the use of this gutter is only when it is absolutely necessary. Usually of aluminium alloy.

  • Valley Gutter - A secondary gutter running at the valleys of the roof which discharge water to the local primary gutter. Usually of aluminium alloy.

  • Downpipe - A vertical member which discharge water from the gutter to the stormwater drain. Usually at 100*50 in size, circular section may also be used as they are more efficient in discharging at the same cross-sectional area as the rectangular downpipes.

    In all cases, it is sensible to avoid as much cornering as possible in the gutter and downpipe so chances of failure would reduce. Gutters should be designed to allow safe overflow without causing damage to the fascia and other timber members.