Roofs and Roof trusses
Roofs and Roofs Trusses
A roof is the uppermost part of a building which is supported on structural members and converted with roofing materials to give protection to the building against rain, heat, snow etc. A good roof is just as essential as a safe foundation. A roof is just as essential as a safe foundation. A roof must be designed and constructed to meet the requirement of different climates and the covering materials available.
A roof should be durable and stable enough to take the loads coming on it, be well drained and water proofs.
(a) Span: - It is clear distance between the supports of beams, roofs, truss or arch.
(b) Pitch: - It is the inclination of sides of a roof to the horizontal and is expressed either in degrees or as ratio of rise to span.
(c) Ridge Pieces: - It is a horizontal member provided at the ridge line of a sloping roof.
(d) Wall Pieces: - These are the members placed at the top of the walls to receive the common rafters.
(e) Purlins: - These are horizontal members placed over principal rafters to support common rafters.
(f) Common rafters: - These are inclined wooden members running from the ridge to the eves. They support battens to support roof covering. The rafters are spaced 30 to 45 centre to centre.
(g) Cleats: - They are small sections of steel or timber, fixed to the rafters of the truss to support purlins.
(h) Battens: - These are small sections of steel or timber which are fixed to the rafter for placing tiles on the sloping roof.
(f) Trusses: - It is a framework of triangle and used to carry the load of roof covering materials and other members of the roof.
Requirements of a Roof
>A roof should be water proof, exclude sun’s heat and dust proof.
>The supporting roof covering should be strong enough to withstand its own weight and of a workman who may occasionally climb up for maintenance and wind loads.
>The roofs should have provisions to drains rainwater in the simplest and direct manner.
>A roofs should have provisions to drain rain water in the simplest and direct manner.
>A roof should be made of durable materials.
>It should have good sound insulation properties.
>When a large area is covered natural light from windows will be inadequate.
>An industrial roof may require ventilation through roofs.
>A roof should present pleasing appearances.
Types of Roofs
Roofs are classified in to
(a) Flat roofs (or) Terraced roofs
(b) Sloped (or) Pitched roofs
(a) Flat roofs
Flat roofs a he top of buildings may be given a gentle slope for drainage purposes. Very commonly used flat roof are Madras terrace roof and reinforced cement concrete roof.
(i) Madras terrace roof
Madras terrace is a type of flat roof popularly used in the southern states of our country. It is quite durable, strong and water proof. It can be constructed quickly but requires skilled workmanship. There is no problem of temperature and shrinkage cracks with this roof.
Timber, rolled steel, precast or pre stressed RCC joists are placed over the walls or beams along the short span at a maximum spacing of 450 mm centre to centre. A slope of not less than 1:48 is provided to the joist arrangement by proper weighing. Well burnt bricks 150*75*25 mm soaked in water for at least 4 hours before used as laid on edge diagonally across the joists in lime mortar of mix 1:1.5 (lime putty :sand). The mortar joint thickness should not exceed 10 mm. the brickwork is cured for at least 10 days keeping it damp. After the bricks on edge course has set, a layer of lime, broken bricks aggregates concrete of mix1:2.5 (slaked lime: Broken brick aggregates) is spread to an average thickness of 100 mm. this layer is compacted to 75 mm thickness. Compaction is continued with hand beater for at least 7 days until the wooden hand beater stops making impressions. During these compactions the surface is kept wet by sparingly lime water and sugar solution. The concrete is cared for six days by sprinkling water. After six days, the roof or floor finish is laid. Roof or floor finish consists of two courses of flat tiles laid in both the directions in cement mortar 1:3 mixed with crude oil, the joints between the tiles being 6 mm. the tiles are laid diagonally breaking joints. The joints are raked out and the whole surfaces are rubbed with thin bar trowel. These joints are then pointed flush with cement mortar 1:3 mixed with crude oil. Curing is done for at least 3 weeks after construction. The bottoms ceiling between the joists is plastered with cement mortar 1:3, 12 mm thick. Wooden and steel joists are painted with three coats of approved paint, the primer coat being applied before placing the joists in position.
The brickwork is self supporting due to the arch action developed in the brickwork.
(ii) Reinforced cement concrete roofs
Now a-days, most of the roofs constructed are of reinforced cement concrete. R.C.C roof is durable, fire resistant, economical and also easy to construct.
The thickness and reinforcement required for the roof slab are determined for the given loading conditions. If the slab is supported by R.C.C beams, the dimensions of the beams and reinforcement to be provided are also determined. At the top level f walls, centering sheets or planks are arranges horizontally over wooden or steel props. Steel rods are placed in the form of grid on the top of horizontally over wooden or steel props. Steel rods are placed in the form of grid on the top of centering sheets as per design. A clear cover of 15 mm between the steel reinforcement and centering sheets is given by using pre-cast cover blocks of 15 mm thickness made of CM of mix at least 1:2. Cement concrete of mix usually 1:2:4 mixed thoroughly with sufficient water is placed on the centering sheets. It is well compacted preferably with mechanical vibrator to the required thickness and is leveled. After about 12 hours of air drying, the exposed surface of slab is cured properly for at least 7 days.
When long span are to be covered, they are provided in to bays and the slabs are stiffened by means of ribs. These ribs act monolithically with part portion of the slab and form beams in the shape of “T”. Such beams are called “T” beams. The width of the rib must be sufficient enough to accommodate the tensile reinforcement at the bottom.
T-beam slab is the most popularly adopted floor or roof slab for residential, office and public buildings.
Weathering course is a layer provided on the top of R.C.C or madras terrace roof to protect the roof from the weathering agencies like rain, wind, sun, and snow. Weathering course prevents entry of rainwater in to the roof slab or terrace. It also arrests the penetration of heat in to the room below the roof. Weathering course consists of lime concrete with broken bricks aggregate and two course of flat tiles set in CM 1:3 mixed with crude oil. Lime concrete having proportions of 1:2:5 is laid to a thickness of 100 mm and compacted to a thickness of 75 mm. a minimum slope of 1 in 0 is given to the lime concrete layer towards the rain water drainage pipes. After the concrete is cures for six days by sprinkling water, two courses of flat tiles are laid.
(b) Sloping Roofs
The different types of roofs made of timber are
(ii) Coupled roof
(iii) Coupled-close roof
(iv) collar roof
(v) King post truss
(vi) Queen post truss
(i) Lean-to roof
It slopes on one side only. It is used for spans up to 2.5 m. In the lean – to- roof, rafters are kept sloping at uniform intervals. The rafters are supported by wall plates at both ends. The upper wall plate rests on stone or steel corbel. Wooden battens are fixed on to the rafters at about 150 mm centre to centre. The roofs covering materials like Mangalore tile, is fixed to these battens.
Lean-to-roof is used for covering sheds and verandahs.
(ii) Coupled roof
Ion coupled roof, two rafters are arranged with necessary slope as shown. At the upper end, the rafters are connected to a ridge piece running along the length of the roof. They are fixed to wall plates at the lower end. The walls plate on the wall plates rest on wall. The rafters are placed at uniform intervals. Battens are fixed to the rafters. The roofing materials are fixed to these battens. Coupled roof can be used for span up to 3.5 m.
(iii) Couple –close roof
When the span and the intensity of loading are increased, the horizontal thrust developed is likely to push the rafters away. To prevent this tendency the rafters are connected by a horizontal member called tie at the bottom. This type of roof is known as couple – close roof. It can be used for span up to 5 m. for still greater spans, the ridge piece and the centre of the i.e. are connected by a vertical rod called king rod.
This roof is similar to couple – close roof. The tie fixed near the middle of the rafters instead of the bottom end. Here also, the tie serves the purpose of preventing the raft from being pushed out by the horizontal thrust developed. Collar roof can be used for spans of 4m to 5.5 m. here the tie is called beam.
King post truss
Here the common rafters are supported by wooden frame work called truss at required intervals. The frame work consists of king post, two struts, two principal rafters and a tie beam. The truss rests on stone bed blocks at either end. The bed blocks are kept on walls. The common rafters rest on the wooden purlins which in turn are fixed to the principal rafters of the truss. The king post connects the ridges piece and the middle of the tie beam. The struts are connected to the king at the bottom and the principal rafters at the top as shown. Purlin and ridge connect the trusses. The roofing material is fixed to the common rafters. The king post is used for spans of 5m to 9 m.
Queen post truss
The frame work consists of two principal rafters, two queen posts one straining sill, two struts, one tie beam and one straining beam. The straining beam rest on wooden purlins. The straining beam resists the horizontal thrust developed. The struts are connected to the queen posts at the bottom and the principal rafters at the top. Purlin and ridges pieces connect the trusses, and he roofing materials is fixed to the common rafters. Queen post truss is used for spans of 9m to 14 m.
Roof trusses are formed by a number of straight members connected in the shape of triangles. The members are made of steel angles. The members are joined by rivets or wielding. The joints are called nodes.
It is assumed that the external loads act at the nodes only and hence, the members are assumed to be subjected to only tension or compression. The members are not designed to only tension or compression. The members are not designed to take bending stresses. The compression members are called struts and the tension members are known as ties.
Roof trusses are used where>Large spans are to be covered.
>Intermediate columns are to be avoided to have an unobstructed working area inside.
>There is heavy rain, snow fall.
>Steel roof trusses are used in factories, workshops, godowns, theatres, warehouses, schools and colleges.
Types of Roof Trusses
The types of truss to be selected depend upon the span, loading condition and locations. King post truss is used for spans up to 6 m. simple flink truss is used for spans of about 7.5. Fan fink truss is for span of about 10.5. French truss is used for span of about 13.5 m span. Pratt truss, however truss and quadrangular truss are used for still longer spans ranging from 15 m to 18 m.
North light roof truss
North light or saw toothed truss is a special and modified type of roof truss suitable for factories engaged in manufacturing process. North light truss is saw toothed and used in the regions north of tropic of cancer. Natural lighting truss is taken advantages of during day time by using north light roof trusses and vertical drops are provided. These drops are covered with glasses, so that to permit light in to the interior.
Precast concrete trusses
R.C.C or pre stressed concrete are cast at the site with high tensile tendons. After sufficient curing, they are erected in position. Precast concrete trusses are cheaper than steel trusses. Their trusses are provided with concrete cleats to support pre stressed concrete purlins. Asbestos sheets, that serve as roof covering materials are fastened to the purlins.
Like it on Facebook, Tweet it or share this article on other bookmarking websites.