Building materials and construction Technology

2. ROOF
• A roof may be defined as the uppermost part of the building
provided as structural covering to protect the building from
weather.
• Roof consists of structural elements which support roof
coverings.
• The structural element may be trusses, portals, beams, slab,
shells or domes.
• Roof coverings receive rain and snow more directly and in
much grater quantity than walls.

3. Main function:
• To provide protection from the weather
• To enclose space and to protect the space it
covers (against rain, wind and snow)
• To protect interior of the building from
moisture and excessive heat loss
• An integral part of the structural frame –
sustain load

4. Strength and stability
• Roof structure –A major factor for consideration in the
design and choice of roof structure are span, materials
used and its load over spans of varying degrees
• The economic consideration of roof structure is to keep
its dead weight to a minimum
• Minimum dead weight becomes critical with wide span
building, as such the structural problem in design is
trying to achieve a low dead and live load ratio
• The solutions are: Characteristics of materials to be
used must be stiff and do not easily bent
• Form and shape of the roof

5. Wind pressure
• Varies according to its velocity, the height of the building
and the locality of the building
• Wind exert pressure and suction on roof
• Excessive suction can cause uplift to roof (stripped off)
Weather resistance
• Adequate weather resistance should be provided by the
roof covering
• The nature or type of roof covering will affect the form or
shape and some details of the roof structure

6. Flat Roof (R.C.C)
• Roofs of modern buildings are invariably made of Reinforced
Cement Concrete (RCC).
• Concrete is strong in compression and weak in tension, however it is
suitably reinforced with the help of steel bars which take the entire
bending tension.
• Due to this the overall thickness of the RCC roofs is comparatively
small thereby reducing the self weight of floor.
• RCC roofs can be classified into the following types
a. Simple slab roofing
b. Reinforce brick roofing
c. Beam-slab roofing
d. Flat slab roofing
e. Ribbed roofing or hollow tile roofing

7. Classification of Roofs
1. Flat roofs or terraced roofs
2. Pitched or sloping roofs
3. Curved roofs

12. Simple RCC slab roofing
• In RCC roofing the RCC slab bends
downwards, causing tension at the bottom
fibers of the slab keeping a minimum clear
cover of 15 mm.
• Half these bars are bent up near ends to
take up negative bending moment caused
due to partial fixity at the ends.
• This main reinforcement is placed in the
direction of the span of the slab, which is
equal to the width of the room, specially
when the length of the room is more than
1.5 times the width of the room. Such a
slab is know as one way reinforced slab.
• Nominal reinforcement is placed in the
perpendicular direction.
• Hooks are placed at the end of each plain
bar, though these are not required in ribbed
bars (tor-reinforcement).
• The bearing of the slab in the wall should
neither be less than its thickness, nor less
than half the width of the wall.
• Figure shows one way reinforced slab.
Such slabs are quite suitable and
economical for spans up to 5 m.

13. Simple RCC slab roofing
• The slab is cast on timber or steel shuttering.
• After erecting the centering, properly bent
reinforcement is place in position.
• Distance pieces of stone or concrete are placed
between the reinforcement and the shuttering plate
so that proper cover maintained.
• Cement concrete of appropriate mix (1:1.5:3) is
then poured and well-compacted.
• The slab is then properly cured. Shuttering is
removed only when the concrete has fully set.
• When the length of the room is less than 1.5 times
the width of the room, the slab spans in both the
directions
• It is essential to provide reinforcement on both the
directions.
• Such a slab is known as a two-way reinforced
slab.
• At the corner suitable mesh reinforcement is
provided at the top and bottom to prevent their
lifting.

14. Sloped Roof or Pitched roof
• A roof with sloping surface is known as a pitched or
sloped roof.
• These roofs suitable for buildings in coastal regions
or in areas where in rainfall are very heavy.
• Buildings with limited width and simple shape can
generally be covered satisfactorily by pitched roofs.
• The slope varies according to span, climatic
condition, and nature of covering materials.

15. Elements of a Pitched roof
1. Single roofs
• Single roofs consist of only common rafters which are secured
at the ridge (to ridge beam) and wall plate.
• These are used when span is less so that no intermediate
support is required for the rafters.
Lean-to-roof

16. Single roofs
Lean-to-roof
• This is the simplest type of sloping roofing in which rafters slope to
one side only it is also known as the pent roof or Aisle roof.
• In this type one wall is carried up sufficiently higher than the other
one to give necessary slope to roof.
• A wooden wall plate is supported either on a steel corbel or a stone
corbel which may be of stone, brick or steel.
• The difference in elevation between the two wall plates is so kept
that the desired slope is obtained.
• The common rafters are nailed to wooden wall plate at their upper
end and notched and nailed to the wooden post plate at their lower
end.
• A lean to roof is generally used for sheds, out-houses attached to
main buildings, verandahs etc. it is suitable for a maximum span of
2.40m.

17. Lean-to-roof

18. • This type of roof is suitable for maximum span of
2.5 m.
• These are provided for sheds, out-house attached
to main building, verandahs, etc.
• Due to the slope of the roof, water and snow
easily run off and reduces, otherwise necessary
extra waterproofing.
• Hence this roof is best for constructing in high
rainfall and snowfall regions. This may be done
either as a part of the original construction or
even after execution after some year
Lean-to-roof

19. Couple Roof
• This type of roof is formed by couple or pair of
rafters which slope to both the sides of the ridge of
the roof.
• In this type of roof the common rafters slope upwards
from the opposite walls and they meet on a ridge
piece in the middle as shown in the fig.
• Such a roof is not very much favoured because it has
the tendency to spread at the feet and thrust out the
walls supporting the wall plates.
• A couple roof is suitable for spans up to about 3.6 m.

21. COUPLE CLOSE ROOF
• This roof is just similar to couple roof except that
the ends of the couple of the common rafters is
connected by horizontal member, called tie beam.
• The tie beam prevents the tendency of rafters to
spread out and thus danger of overturning of the
walls is avoided.
• The tie beam may be a wooden member or a steel
rod.
• This roof can be adopted economically up to the
span of 4.2m.

22. COUPLE CLOSE ROOF

23. 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 collar beam.
• This beam roof is adopted to economise the space
and to increase the height of a room.
• This roof can be adopted up to a maximum span
of 4.8m.

24. COLLAR BEAM ROOF

25. DOUBLE OR PURLIN ROOFS
• Two basic element i) rafters and ii) purlins
• When the span exceeds 2.4m, the necessary size for
the rafters becomes uneconomical
• Hence in order to reduce the size of rafters,
intermediate supports called purlins are introduced
under the rafters as shown in fig.
• The intermediate support so provided in the form of
purlins, reduce the size of the rafters to the
economical range.
• This roof can be adopted economically up to 4.8m.

27. FRAMED OR TRUSSED ROOF
• When the span exceeds 5 m and when there are no inside
supporting walls or partitions for purlins, framed structure
known as trusses are provided at suitable interval along the
length of the room.
• The spacing is 3m for wooden trusses.
• Here roof consists of 3 elements
a. Rafters to support the roofing material (tiles etc.)
b. Purlins to provide intermediate support rafters.
c. Trusses to provide support to the ends of purlins.
• Trusses carry the ridge piece and purlins on which the
common rafters rest.

28. Trussed Roof
• Various type of trusses are
• King-post truss.
• Queen-post truss
• Mansard truss
• Combination of King and Queen post trusses.
• Bel-fast truss
• Truncated truss
• Steel truss
• Composite truss

29. KING POST TRUSS
• King-post consists of i)lower tie beam, ii) Two inclined
principal rafters, iii) two struts and iv) a king post.
• The inclined members, known as struts, prevents the
principal rafters from bending in the middle.
• The principal rafters support the purlins.
• The purlins support the closely-space common rafters
which have the same slope as the principal rafters.
• A king-post truss suitable for roofs of span varying from
5 to 8 m as shown in fig.

31. • The king-post prevents the tie-beam from sagging at its center
of span.
• The struts connected to the tie beams and the principal rafters
in inclined direction, prevent the sagging of principal rafters.
• The trusses are supported on the bed blocks of stone or
concrete, embedded in the supporting wall so that load is
distributed to a greater area.
• The common rafters may be connected to eaves board or to
pole plate at the other end.
• Pole plates are horizontal timber sections which run across the
tops of the tie beams at their ends or on principal rafters near
their feet.

32. QUEEN POST TRUSS
• This truss is differ from a king-post truss in
having two vertical members known as queen
posts.
• The upper ends of the queen posts are kept in
position by means of a horizontal member known
as straining beam.
• Additional purlins are supported on the queen
posts.
• A queen post truss is suitable for roof spans
varying 8 to 12 m as shown in the fig below