1.
Super-structure : Roofs
Mohd. Zameeruddin
Mohd. Saleemuddin
Assistant professor,
Department of Civil Engineering,
MGM’s College of Engineering, Nanded (m.s)
© mzsengineeringtechnologies
email@ zameerstd1@hotmail.com;
md_zameeruddin@mgmcen.ac.in

2.
Roofs
The uppermost part of a building which is constructed in the form
of framework 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 coverings
The structural elements consists of;
- Trusses
- Portals
- Slabs
- Domes
- A. C sheets covering
- G. I sheets coverings
- Shingles
- Slates, etc
© MZS Engineering Technologies

3.
Requirements of good roofs
Following are the requirements of a well planned roof;
1. It should be durable against the adverse effects of various
agencies such as wind, rain, sun, etc.
2. It should grant the desirable insulation against sound and heat.
3. It should be structurally stable and sound and it should be
capable of taking the loads likely to come over it.
4. It should be well-drained.
5. It should have efficient water-proofing arrangements.
6. It should be fire resistance

4.
Classification of roofs
The roofs are classified into the following three categories:
1. Pitched or slopping roofs
2. Flat or terraced roofs
3. Curved roofs
Flat roofs are used in plains where rainfall is less and climate is
moderate.
Pitched roofs are preferred wherever rainfall is more.
Shells and folded plate roofs are used to cover large column free
areas required for auditoriums, factories etc.
The choice of the type of roof will depend on the climatic conditions,
shape of building, availability of materials, importance of building,
etc.

5.
Flat Roofs
These roofs are nearly flat. However slight slope (not more than 10°)
is given to drain out the rain water.
The advantages of flat roofs are:
a) The roof can be used as a terrace for playing and celebrating
functions.
b) At any latter stage the roof can be converted as a floor by adding
another storey.
c) They can suit to any shape of the building.
d) Over-head water tanks and other services can be located easily.
e) They can be made fire proof easily compared to pitched roof.

6.
The disadvantages of flat roofs are:
a) They cannot cover large column free areas.
b) Leakage problem may occur at latter date also due to
development of cracks. Once leakage problem starts, it needs
costly treatments.
c) The dead weight of flat roofs is more.
d) In places of snow fall flat roofs are to be avoided to reduce
snow load.
e) The initial cost of construction is more.
f) Speed of construction of flat roofs is less.
Flat Roofs

7.
Pitched Roofs
In the areas of heavy rain falls and snow fall sloping roof are used.
The slope of roof shall be more than 10°. They may have slopes as
much as 45° to 60° also. The sloped roofs are known as pitched
roofs. The sloping roofs are preferred in large spanned structures like
workshops, factory buildings and ware houses. In all these roofs
covering sheets like A.C. sheet, G.I. sheets, tiles, slates etc. are
supported on suitable structures.
The pitched roofs are classified into;
a) Single roofs
b) Double or purlin roofs
c) Trussed roofs

8.
Pitched Roofs
Single roofs
If the span of roof is less than 5 m the following types of single roofs
are used.
1. Lean-to-roof
2. Couple roof
3. Couple-close roof
4. Collar beam roof
5. Collar and scissor roof
In all these roofs rafters placed at 600 mm to 800 mm spacing are
main members taking load of the roof. Battens run over the rafters to
support tiles.

9.
Lean-to roof
This is the simplest form
of a pitched roof. In this
type one wall is carried up
sufficiently higher than the
another one to give the
necessary slope to the
roof. The rafters are
suitably secured on the
wall-plates and eaves
boards, battens and roof
covering is provided as
shown.
Suitability:
It is generally used for sheds, out-houses attached to main buildings,
verandahs, etc.

10.
Couple roofs
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 figure
The common rafters are
firmly secured in position at
both the ends, one end being
on the ridge piece and the
other on the wall plate.
Suitability:
This type of roof is used for span up to about 3.60 m

11.
Coupled closed roof
This roof is similar to
couple roof except that the
legs of common rafter is
connected by a tie,
preventing the spread out
and overturning of walls
Suitability:
This type of roofs are adopted economically up to a span of 4.20 m

12.
Collar beam roof
This roof is variation of
couple close roof. The tie
beam is raised and placed
at a higher level. The tie
beam is the known as a
collar or a collar beam.
A collar beam is adopted
to economise the space
and to increase the height
of room. The collar beam
is usually fixed at one-
third to one-half the
vertical height from the
ridge
Suitability:
The roof can be adopted up to a
maximum span of 4.80m

13.
Collar and scissors roof
This roof is similar to
the collar beam roof
expect that two collar
beams which are
crossing each other to
present an appearance of
scissors are provided
(e) Collar and scissors roof

14.
Double or Purlin Roofs
If span exceeds, the cost of rafters increase and single roof becomes
uneconomical. For spans more than 5 m double purlin roofs are
preferred. The intermediate support is given to rafters by purlins
supported over collar beams. Figure shows a typical double or purlin
roof.

15.
Trussed Roof
If span is more, a frame work of slender members are used to support
sloping roofs. These frames are known as trusses. A number of
trusses may be placed lengthwise to get wall free longer halls. Purlins
are provided over the trusses which in turn support roof sheets.
For spans up to 9 m wooden trusses may be used but for larger spans
steel trusses are a must. In case of wooden trusses suitable carpentry
joints are made to connect various members at a joint. Bolts and
straps are also used. In case of steel trusses joints are made using
gusset plates and by providing bolts or rivets or welding.

16.
Depending upon the span, trusses of different shapes are used.
Trussed Roof

17.
Trussed Roof

18.
Shells and Folded Plate Roofs
Shell roof may be defined as a
curved surface, the thickness of
which is small compared to the
other dimensions. In these roofs
lot of load is transferred by
membrane compression instead of
by bending as in the case of
conventional slab and beam
constructions. Caves are having
natural shell roofs.

19.
Advantages of shell roofs are:
a) Good from aesthetic point of view
b) Material consumption is quite less
c) Form work can be removed early
d) Large column free areas can be covered.
Disadvantages are:
a) Top surface is curved and hence advantage of terrace is lost.
b) Form work is costly.
Advantages and Disadvantages of Shell Roofs