Roof may be defined as a covering provided over the top of a building with a view to keep out rain, snow and wind and to protect the building from the adverse effects of these elements. A roof basically consists of roof covering materials supported on structural elements installed on the building top. The structural elements may be trusses, portals, flat slab, shell, dome or space frame whereas the roof covering materials may be thatch, wooden single, tiles, slates, A.C. sheets, G.I. sheets etc. Roof is considered.
As it is important to
make safe foundations for a building, it is equally important to provide good
roof above it. Good foundations protect the building against damage from below
the ground and in a similar way good roof prevents deterioration of the
building starting from the top.
The choice of the type
of roof for a building should be made after paying due consideration to the
climatic conditions as well as the availability of selected materials at site.
1.It should be structurally sound and strong enough to carry
the anticipated dead and live loads safely.
2. The roof should be durable against adverse effect of
elements like rain, snow, sun, wind etc.
3. It should have effect water proofing and drainage
provisions.
4. The roof should provide desired insulation against heat.
5. It should provide desired insulation against sound.
CLASSIFICATION OF ROOFS
Roofs can be broadly
classified in the following two main types:
1.Pitched roofs
2. Flat or terraced roofs
Pitched or sloped
roofs are considered suitable for building in coastal regions or in areas where
rainfall is very heavy. On the other hand, flat roofs are considered suitable
for buildings in plains or in hot regions where rainfall is scanty. Sometimes however
it becomes necessary to provide roof having shapes other than pitched or flat
type to meet the functional as well as aesthetical requirements of the
building.
PITCHED ROOF
It is the most common
form of roof and is generally regarded s the cheapest alternative for covering
a structure. Pitched roofs are almost always constructed in wood or steel. Wooden
pitched roof consists of a system of joists, rafters and purlins arranged in
the form of a triangular support known as truss. The lower ends of the rafter
rest upon the wall plates and their upper end they are connected to a common
ridge piece.
In pitched roof a
slope of less than 1 in 3 is generally not considered satisfactory from
drainage point of view. In areas of heavy snow fall steeper slopes of say 1 to
1 ½ or 1:1 are provided to reduce the incidence of snow load on the snow.
When sloped roofs are
to be constructed in hot regions, it is desirable to provide false ceiling from
consideration of thermal comfort. The false ceiling should be of insulating
material like plywood, fibre board, gypsum board, asbestos cement sheet etc.
TYPES OF PITCHED ROOFS
Lean to Roof: This is the simplest type of pitched roof consisting of
rafters sloping on one side only. At their upper ends the rafters are nailed to
the wooden wall plate, placed on corbel which may be stone, brick or steel. At the
lower end the rafter is notched and nailed to the wooden post plate. The post
plate consists of a timber section running parallel to the wall and supported
by posts at intermediate points. Battens spaced at 15 cm. centre to centre are
nailed to the inclined rafter and upon this the covering material is laid.
Coupled Roof: Coupled roof is formed by a pair of inclined rafters with
their upper ends nailed to a common ridge piece and their lower ends, notched
and nailed to the wooden wall plates embedded in masonry on the top of the wall
on either end. The rafters are spaced at suitable intervals and the battens are
nailed on the top. Upon the frame work thus prepared, roof covering is laid. The
type of construction is considered suitable for maximum span of 3.7 meter.
Couple close Roof: In a coupled roof the notches provided at the feet of the
rafters prevent them from spreading. If the span of the roof is increased or
the loading on the truss in increased gradually, it will be found that at a
certain stage the notch will be incapable of resting the thrust at the end of thereafter
and the roof will collapse. Thus, to prevent this the feet of the rafters are
joined by a tie, which by holding them in position, prevents them from
spreading and thrusting out of the wall. This is called couple close roof. With
ordinary loading conditions, this type of roof can be used for spans up to 5 m.
Collar Roof: With excessive loading conditions or increased spans the
rafter of the couple closed roof tends to bend in the middle. This tendency is
overcome by fixing the tie near the middle of the rafters. The truss then
formed is called collar roof. Thus collar roof is similar to couple, closed
roof with the only difference that the tie instead of being placed at feet of
the rafters is raised up. The tie in collar roof is temed as collar beam. In this
type of roof, space can be utilised, as the ceiling can be attached to the
collar beams and the exposed faces of the rafters.
King Post Roof Truss: It is a form of roof truss which is commonly used for spans
varying from 5 m. to 9 m. It consists of a frame work, comprising of two
principal rafters, one tie beam, two struts and a king post. The trusses are
spaced not more than 3m. centre to centre.
The trusses are
connected to each other through purlins which are placed at right angles to the
sloped rafters and are secured to them through cogged joints and cleats. The
purlins also support the common rafters upon which roof covering is laid. In
order to distribute the concentrated load due to the truss at its end,
templates or bed blocks of stone or concrete are provided under the truss on
the supporting walls.
Queen Post Roof Truss: For spans
greater than 9 m. and less than 14 m. queen post truss is commonly used in
timber roof construction. It may be defined as frame work consisting of two
principal rafters, two queen posts one straining beam, two inclined struts and
a straining sill. It differs from the king post truss in having two vertical
tension members called queen post. The heads of the queen post are strutted
apart by a horizontal member termed as straining beam. The straining beam
receives the thrust from the head of the principal rafters and keeps the
junction stable. The queen posts have single splay shoulders at their feet to
receive the struts. The thrust from the two struts tend to force the feet of
queen posts inwards which is combinedly resisted by the tension join between
the tie beam and the queen post feet and the straining sill fixed between feet
of the queen posts.
Composite Roof truss: Trusses built with the combination of wood and steel are termed as composite trusses. In this combination, advantage is taken of the high tensile strength of steel. In the truss frame work the members in tension are usually replaced by the rods.
FLAT ROOFS
Flat roof may be of
reinforced cement concrete, reinforced brick work, precast concrete with flag
stones supported on rolled steel joists or jack arch type. Flat roofs are
constructed in a similar manner as the upper floors except that the roof
surface is properly treated to protect it from the adverse effect rain, snow,
heat etc. Efficient water proofing and roof drainage is an important
requirement of flat roof. The roof surface has therefore to be provided with
proper water proofing treatment and necessary slopes to satisfy these
requirements. In addition, a layer of insulating material is laid over the roof
surface to provide adequate thermal insulation to the space below. This layer
is known as terracing or grading. In case of R.C.C. or R.B.W. roof slab,
provided with terracing, a slope of 1 in 40 to 1 in 60 is considered desirable
for proper drainage of rain water in areas of moderate rain fall. The slope may
be increased, in case of flat roofs in heavy rain fall regions. The slope may
be increased, in case of flat roofs in heavy rain fall regions. The slope to
the roof can be given either by varying the thickness of the terracing material
or by constructing roof slab in slab itself and the roof surface is given
adequate water proofing treatment by using bitumen felts etc.
Although flat roofs
are comparatively expensive yet keeping in view their numerous advantages, they
are commonly constructed these days. The advantages and disadvantages of flat
roof are given below:
Advantages of flat roof:
1.The roof can be
conveniently used as terrace for playing, gardening or sleeping purposes.
2. It is easy to
tender flat roof fire resistant.
3. It avoids the
enclosure of the triangular space.
4. It has good
insulating properties.
Disadvantages of flat roof:
1.Their span is
restricted and as such they cannot be used for large spans without the
introduction of intermediate columns and beams.
2. Self-weight of the
flat roof is considerable which makes the construction more expensive.
3. Flat roofs are
exposed to sun and are subjected to violent temperature changes, which may lead
to cracks in the surface of the roof.
TYPES OF FLAT TERRACED ROOFING
Mud Terrace Roof: This type of roof is extensively used in places where
rainfall is less. This is the cheapest form of roof terracing and is fairly
water tight. The terracing is made with white earth mud containing large
percentage of sodium salts.
The roof in this case
consists of T-sections 50 mm. x 50 mm. x 6 mm. spaced at 32 cm. centre to
centre and supported on R.S.J. beams of the designed sections. The rolled steel
joists, span along the width of the room and are spaced at suitable distance
apart. Well burnt tiles measuring 30 cm. x30 cm. x5 cm. or 30 cm. x 15 cm. x 5
cm. are placed between the flanges of the T-sections, and are set in lime
mortar. A 15 cm. thick layer of stiff mud made from good white earth containing
large percentage of sodium salts is laid over tiles.
Madras Terrace Roofing: It consists of a course of thoroughly brunt terrace
bricks 15 cm. x 8 cm. x 2 cm. laid on edge in lime mortar 1:1 ½ diagonally
across the joists of the timber roof. The bricks are immersed in water 24 hours
before use and the thickness of joist is kept 10 mm. After the bricks are
completely set, a 10 cm. thick layer of brick concrete 2:1 ½ is laid over brick
on edge course and beaten down to 7.6 cm. thickness by wooden hand beaters. The
beating is continued till the beater makes no impression on the concrete and
gets readily rebound by the surface when stuck upon it. The whole surface when
struck upon it. The whole surface is then cured for about a week by sprinkling
lime water.
ROOF COVERING FOR PITCHED ROOF
Roof covering is a
material covering provided over the form work of roof structure to act as a
barrier for the rain, sun, wind or other such elements. The selection of
covering material depends upon factors such as availability of material, its
initial cost and cost of maintenance, appearance and durability etc. There are
various type of roof coverings and the one’s which are commonly adopted in
India are given below:
1.Thatch Covering.
This form of roof covering is largely used in low cost houses and villages. It is suitable for rural areas mainly because it is the cheapest form of covering and is simple in construction. The frame work for supporting the thatch consists of round bamboo rafters spaced at 30 cm. apart and tied with split bamboos laid at right angles to the rafter. The thatch is lightly secured to the frame work with the help of ropes or twines dipped in tar. For proper drainage of rain water, the thatch covering should be at least 15 cm. it harbors rates and the bad smell of rotting materials makes it difficult to stay under the roof. On account of this defect, thatch covering is never allowed in good work.
The use of shingles is generally restricted to hilly areas where wood is easily available at low cost. Wooden shingles cut from well-seasoned timber are lid in a similar manner as slates or tiles. Shingles are obtained in lengths varying from 30 cm. to 38 cm. and widths varying from 6 cm. to 25 cm. Generally, two courses of shingles are laid at the eaves.
The tiles are named according to their shape and pattern and are manufactured in somewhat similar manner as bricks. The commonly used tiles are; Flat pan tiles, Half round country tiles, Mangalore tiles, Quilon and other similar tiles. Tiling is one of the oldest methods of roof covering but with the introduction of asbestos cement sheets, its use is gradually day by day.
Asbestos cement is material obtained by the combination of cement with about 15 percent of asbestos fibre. The sheets made out of this material are light, cheap, impervious, durable and fire resisting. It does not require any protective paint and cannot be eaten away by vermins. On account of these properties, asbestos cement covering is commonly adopted as a roofing material for factories, workshops, offices, garages and residential buildings.
These sheets are the
least preferred option for roofing applications in most places in the country. As
they are less durable than other types of roofing sheets, they are mainly ued
as garden sheds or for covering temporary structures. The better the quality of
the plastic, the higher will be the price of the roofing sheet. This elevants
the overall cost of installation of plastic sheets.
The prices of plastic roofing sheets generally vary in the range of Rs. 500/square meter to Rs. 1000/square meter.
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