What is the Roof Truss?
The roof truss is a the top level of the building which is considered to give protection to the building against rain heat snow wind etc. A roof basically consists of structural elements provided at the top of the building for the support of roof covering.
Hello, guys what’s up? Today in this topic I will describe the building roof in engineering construction. If you are an engineer this is an important topic for you. So friends in this article you can get more information about the building roof truss in engineering construction technology.
I will discuss the following topics on this page:
• Definition of the roof.
• Requirements of the roof.
• Type of roof truss.
• Timber roof truss.
• Steel roof truss.
• Tiles roof truss.
So, friends let’s get started on today’s topic. I hope this article is very helpful for you about the roof.
What are the Requirements for roofs?
The following are the common requirements for installing roof trusses in the building:
- It should have adequate strength and stability to carry the superimposed dead and live load.
- It should be provided for the protection of the building against rain, sun, wind, etc. and it should be durable against the effects of these agencies.
- It should have an efficient drainage arrangement.
- It should have adequate thermal insulation.
- It should be fire resistance.
How to classify Roofs?
The roofs are classified into the following three categories:
• Pitched roofs
• Flat roofs
• Curved roofs
A sloping roof is known as a pitched roof. In the pitched roof following components are included:
• Hip end
• Gable end
• Lean-to roof
• Hipped end
• Ridge piece
• Eaves board
Type of pitch on the roof:
Pitched roofs are classified into the following three categories:
• Single roof
• Double or purlin roofs
• Trussed roofs.
Single roofs are those which consist of only the rafters which are supported at the ridge and the edge. Such roofs are used only when the span is limited to 5 meters span. The maximum span of the rafters is taken as 2.5 m.
The types of a single roof
• Lean-to roof
• Couple roof
• Couple close roof
• Collar beam roof
This is the simplest type of sloping roof, in which rafters slope to one side only. This type of roof rafter is suitably secured on the wall plates.
A wooden wall plate is supported either on a steel corbel or a stone corbel, which is provided at 1m center to center. A lean-to roof is generally used for outhouses attached to the main buildings and veranda.
This is the type of roof that is formed by a couple or pair of rafters that slope to both sides of the ridge of the roof. The upper ends of each pair of rafters are nailed to a common ridge piece and their lower ends are notched and nailed to the wooden wall plate embedded in the masonry on the top of the outer wall.
A couple of roofs are suitable for spans up to about 3.6 meters.
Such a roof is not very much formed because it has the tendency to spread out at the feet (wall plate level) and thrust out the walls supporting the wall plate.
Couple close roof
A couple of close roofs are similar to the couple roof, except that the end of a couple of common rafters is connected by a horizontal member called a tie beam, to prevent the rafter from spreading and thrusting out of the wall.
The tie beam may be a wooden member or steel rod. The connection between wooden ties and the feet of rafters is obtained by dovetails halved joint.
A couple of close roofs are suitable for span lengths up to 4.2 m. For increased span or for greater loads, the rafters may have a tendency to sag in the middle. This can be checked by providing a central vertical rod, called a king rod or kingbolt.
Collar beam roof
When the span increases or when the load is more the rafters of the couple close roof have the tendency to bend.
This is avoided by raising the tie beam and fixing it at one-third to one-half of the vertical height from the wall plate to the ridge.
This raised beam is known as the collar beam. This is suitable for a span of up to 6 meters. A lower collar position gives a stronger roof.
Double and triple roof situation for their use, their elements, and construction details
Double or purlin roof
These roofs have two basic elements
The purlins give intermediate support to the rafters and are supported on end walls. The intermediate supports provided in the form of purlin, reduce the size of rafters to the economical range. The rafters are provided fairly close (40-60cm c/c ).
Each rafter is thus supported at the three points.
At the bottom of the wall through the wall plate
At the top by the ridge
At the center by a purlin
By supporting the rafters at their mid-span or point in this manner with a purlin, the span is halved, thus enabling the rafters to be made considerably lighter than they would need to be if they spanned the whole distance from the eaves to the ridge.
When the span of the roof is more than 5 meters then this type of roof is provided. Where there are no constructed sidewalls to support the purlins, framed structures known as trusses are provided at suitable intervals along the length of the room. Spacing is generally limited to 3 meters for wooden trusses.
In this system the roof consists of three elements:
Rafter to support the roofing material
Purlin to provide intermediate support to the rafter
Trusses to provide support to the ends of purlins.
What are The Types of trusses in use for Construction
King Post roof Truss
The truss is suitable for spans varying from 5 to 9 meters. The spacing of the king post roof truss is limited to 3m from center to center.
The truss consists of
• Lower tie beam
• Two inclined principal rafters
• Two struts
• King post
The purlin supports the closely spaced common rafters which have the same slope as the principal rafters.
The common rafters support the roof covering. The lower horizontal tie beam receives the ends of the principal rafters and prevents the wall from spreading out to trust. The stub connected to the tie beam and the principal rafters are inclined in a direction, preventing the sagging of the principal rafters. A ridge beam is provided at the apex of the roof to provide end support to the common rafters.
Queen Post Truss
For Spans greater than 8 m and less than 12 m queen post truss is commonly used in timber roof connection. It may be defined as a framework consisting of two principal rafters, two queen posts one straining beam, two inclined struts, and a straining sill.
The heads of the queen part are strutted apart by a horizontal member termed a straining beam.
The straining beam receives the thrust from the head of the principal rafters and keeps the junction stable. The queen post has single-splayed shoulders at their feet to receive the scrubs. The thrust from the two struts tends to force the feet of the queen posts inwards which is combinedly resisted by the tension joint between the tie beam and the queen post feet and the straining sill fixed between the feet of the queen post.
Mausard Roof Truss (Francois mausard)
It is a two-story truss with the upper portion consisting of the king post truss and the lower portion of the queen post truss.
The entire truss has two pitches. The upper pitch varies from 30° to 40° while two lower pitch varies from 60° to 70°.
A truncated truss is similar to a mansard truss except that its top is formed flat with a gentle slope to one side. This type of truss is used when it is required to provide a room on the roof between the two queen posts.
Composite Roof Truss
This type of truss is made of two materials, such as timber and steel, which are known as composite roof trusses. In a composite truss, the tension member is made of steel, while the compression member is made of timber. If tension member one is made of timber then the section becomes very heavy because of the reduction of section at the joints.
The joints in the composite truss should be such that cart or forged fitting can be easily used. Generally, feting used of CI head, CI shoe, steel angle bolts, straps, etc.
Bel fast roof truss (Bowstring truss)
This truss in the form of a bow consists of thin sections of timber with its top chord curved. If the roof covering is light this roof truss can be used up to a 40-meter span.
Steel Roof Truss
If the span exceeds 10 m, the timber truss becomes heavy and uneconomical.
- Steel trusses are more economical for longer spans.
- Easy to construct or fabricate.
- Fireproof or fire resistance
- More rigid
- Termite proof
Steel trusses have the following advantages over timber trusses:
- The section comprising a steel truss is readily available in the required dimension resulting in minimum wastage of materials.
- Steel trusses are light in weight compared to timber and can be fabricated in any shape depending on structural and architectural requirements.
- Steel trusses are stronger and more rigid in comparison to timber.
- Steel trusses can be used over any span while timber trusses are suitable only up to a 15-meter span.
- Steel trusses are fireproof.
- Steel trusses are termite-proof.
- Steel truss has a longer life and more resistance to environmental agencies.
- The fabrication of steel trusses is easier and quicker than a wooden truss.
The Roof Truss of steel
This form of roof structure is used mainly for short and medium-span single-story buildings intended for industrial or recreational use.
A steel roof truss is a plane frame consisting of a series of rigid triangles composed of compression and tension members.
The compression member is called rafters and struts, whereas the tension members are termed ties.
The structural members are connected together, where the center lines coverage with flat-shaped plates called gussets.
They can be riveted, bolted, or welded together to form a rigid triangulated truss. The internal arrangement of the struts and ties will be governed by the span. The principal or rafter is divided into several equal divisions that locate the intersection points for the centerline of the internal strut or tie.
Angle purlins are used longitudinally to connect the trusses together and provide the fixing medium for the roof covering. It is the type of covering chosen that will determine the purlin spacing and the pitch of the truss; ideally, the purlins should be positioned over the struts or tie intersection points to avoid setting up local bending stresses in the rafters. Purlins are connected to cleats attached to the beam of the rafters. Steel roof trusses are positioned at 3 to 7 m center and supported by capped universal columns or bolted-to-pad stones bedded onto brick walls or piers.
The main disadvantages are the frequent necessity of pasting the members to inhibit rust and that the flanges of the angles provide an ideal ledge on which dust can be fabricated from welded. Steel tubes that are lighter in weight and cleaner appearance have less surface area on which to collect dust.
However, steel trusses are more commonly used nowadays for all spans small or large. Specification of the steel trusses
- More economical
- Easy to consisted or fabricate
- More rigid
A roof laid at an angle less than 10° to the horizontal is known as a flat roof. A flat roof may be of reinforced cement concrete, reinforced brickwork, precast concrete with ( channel unit, waffle unit, cored unit), flagstones supported on rolled steel joint, and jack arch type.
Sufficient waterproofing and roof drainage is an important requirement of a flat roof.
A larger of materials ( mud, lime, concrete) is laid over the roof surface to provide adequate thermal insulation.
The slope should be 1 in 40 to 1 in 60.
Advantages of flat roofs.
- The roof can be conveniently used as a terrace for playing gardening or sleeping purposes.
- Construction and maintenance are easier.
- They can be easily made fireproof, in comparison to a pitched roof.
- It avoids the enclosure of the triangular shape.
- They require a lesser area of roofing material than a pitched roof.
- They are more stable against high winds.
- In a multi-story building flat roof is the only choice since overhead water tanks and other services are located on the terrace.
- They do not require a false ceiling, which is essential in a pitched roof.
Disadvantages of flat roofs.
- Their span is restricted and as such, they cannot be used for large spans without the introduction of intermediate columns and beams.
- The self-weight of a flat roof is very high. Due to this, the sizes of beams, columns, foundations, and other structural members are heavy.
- They are unsuitable at places of heavy rainfall and snowfall.
- It is sufficient to locate and rectify the lease on a flat roof.
Types of flat terraced roofing
- Mud terrace roofing
- Brick jelly or median terrace roofing
- Mud paska terracing with tile paving.
- Lime concrete terracing with tile paving.
- Bengal terrace roofing
- Lightweight flat roofing
Mud terrace roofing
- It can be provided either on tiles or on a wooden board.
- Terracing is made with white earth mud containing a large percentage of sodium salt.
- The wooden joists are resting on RSJ with a furring piece to adjust the required slope.
- Special terrace brick (15cm×5cm×12cm) is placed on the edge in a lime mortar (1:1.5) and laid diagonally across the joist
- 10 cm thick brickbat concrete ( 1 part gravel and sand and 50 percent of lime mortar)
- Flat tile (15cm×10cm×12mm) in lime mortar (1:1×1/2).
- Three coats of lime mortar. The surface is rubbed or polished.
Mud Phuska Terracing with tile paving.
- The RCC slab is cleaned of dust and lost materials.
- Hot bitumen paint at the rate of 1.7 kg /m^2.
- Coarse sand is immediately spread out over the hot bitumen at the rate of 0.6 m^3 of sand per 100 sqm.
- Mud phuska 10 cm layer and slope 1: 40
- Plastered with a 13 mm coat of mud cow during the motor.
- Tile bricks are laid flat on a plastered surface. The joints are grouted in (1:3) cement mortar.
Bengal terrace roofing
This type of roofing is adopted for timber roofs of the verandah. The construction of this type of roof is given below:
It is mostly used to cover verandahs. In this type of terrace, rafters are placed at 300 mm to 500 mm c/c giving a slight inclination towards the free end of the verandah.
One end of the rafters is inserted into the main wall for a depth of nearly 200mm, and another end is supported on a verandah wall constructed over lintels provided by verandah openings. Timber battens of small size are nailed to rafters at a c/c distance of about 150 mm. A layer of flat, well-burnt tiles is laid over battens in cement mortar, and the surface of the roof is finished.
Lime concrete terracing with tiles
This type of terracing is commonly adopted over RCC roofing. The process of lime concrete terracing with tiles is given below:
- The RCC slab is cleaned off dust etc., and a land layer of hot bitumen is applied at the rate of 1.7 kg per sqm of roof surface.
- A layer of course sand is immediately spread over the hot layer of bitumen at the rate of 0.6 cubic meters of sand per 100 sqm of the roof on the surface of the building.
- A 10 cm thick layer of lime concrete is laid on a proper slope. The lime concrete may consist of 2 parts of lime 2 parts of the skin and 7 parts of brick ballast of 25 mm gauge.
- Two courses of flat brick tiles are laid in 1:3 cement mortar. The joists of the top course are pointed with 1:3 cement mortar. The vertical joints in the two courses are broken.
Roof covering tiles, slates, cut sheets, etc.
Roof coverings are an essential component of a pitched roof to be placed over the roof framework, to protect it from rain, snow, sun, wind, and other atmosphere agencies.
The selection of roofing material depends on the
• Type of building
• Type of roof framework
• Initial cost
• Maintenance requirement
• Fabrication facilities
• Architectural appearance
• Availability of material
• Climate of locality
Commonly used materials for pitched roofs are as follows:
• Asbestos cement sheet
• Corrugated galvanized iron sheet
This is the cheapest type of roof covering, commonly used in village areas. It is very light but is highly combustible. It is unstable against high winds. It absorbs moisture and is liable to decay.
The framework to support thatch consists of a rounded bamboo rafter spaced 20 to 30 cm apart and tied with split bamboo laid at a right angle to the rafters. The thatch is tightly secured to the framework with the help of ropes or twines dipped in the tax.
To drain the roof effectively a minimum slope of 45° is kept. The thickness of thatch covering should at least be 15 cm normal thickness varies from 20 to 30 cm according to the quality and pitch of the roof.
What are Tiles? Their Types.
Tiles for roofing are one of the oldest and are still preferred for residential buildings. The tiles are manufactured from locally available earth. The various types of tiles generally used are
• Plain or flat tiles
• Curved or pantiles
• Pot tiles or half-round country tiles
• Italian or Allahabad tiles
• Interlocking tiles
Plain or flat tiles
Plain tiles are made of clay and size varying from 25 cm × 15 cm to 28 cm ×18 cm with a thickness from 9 mm to 15 mm. The tiles are not perfectly flat but have a slight chamber of 5 to 10 mm in their length which ensures that the tails will bed and not ridge on the backs of those in the course below. Each tile has two holes formed and the tiles can be nailed through their holes. Plain tiles are laid in regular bonds.
Curve or pantiles
This type of tile is flat longitudinal but is curved transversely to a flat wave or s curve. One nib is provided at the head on the underside of the trough of the wave, a nail hole is formed below the nib and two of the opposite diagonal corners are splayed or rounded. Pantiles are unbounded, having continuous side joists from eaves to the rigid. Plain tiles are also nailed.
Half-rounded country titles
- This type of tile is commonly used in villages.
- These tiles are laid in pairs of under tiles and over tiles are laid with convex surface upwards.
- These tiles are semi-circular in sections at each end, but the diameter tapers longitudinally.
- In one variety of tiles, the under tiles are flat with a broader head tapering toward the tail, while the over tile is segmental in section with an under tail and narrower head. In another variation, both the under tiles as well as over tiles are semi-circular and taper from head to tail.
- The country tiles are similar to the Spanish tiles. The over tiles taper down from the tail to the head while the under tiles taper down from the head to the tail.
Italian or Allahabad tiles
These tiles are also used in pairs with a flat broad bottom which alternates with a convex curve over the tile.
The under-flat tapered with upturned edges or flanges at the sides is 23 cm at the interior end, 26 cm at the end with a length of 37 cm, and a flange height of 4 cm.
The over tile is half-round in and tapered in the plan. The diameter tapers from 16 cm at the end to 12 cm at the head.
The tile may be slightly shouldered allow it to clear the under tile in the course above at the headlap.
The headlap varies from 6.5 to 7.5 cm depending on the pitch while the side lap is 5 cm. The taper in over tile allows the tile in the next course to fit in.
Asbestos cement sheets (AC sheets)
Generally, asbestos cement sheets are used in industrial buildings, factories, sheds, cinema halls, auditoriums, and residential buildings.
They are cheap, light in weight, tough, durable, watertight, and fire-resistant. These sheets do not require any protective paint and no elaborate maintenance is required. Also, the construction with
AC sheets are very fast.
AC sheets are manufactured from asbestos, fiber and Portland cement. It is found in several varieties but white asbestos which is a compound of magnesia and silica is principally used. Asbestos cement is now used for the manufacture and roofing of slates, tiles, and corrugated sheets.
The following points should be noted while fixing A.C sheets:
The A.C. sheets should be laid with smooth sides upward, and the end marked Top pointing toward the ridge.
The end lap and side lap should be properly maintained. Generally, the end lap is 15 cm.
These are the types of tiles which is used in the roof truss in the building construction.
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