ROOFS
Roof is one of the most important elements in a building. The types of roof you build for a
particular structure depends on many conditions such as: types of structure span of the
building, cost and materials. Some of the most primary needs that a structure demands from a
roof are protection from the weather, overall design compatibility with the existing structure,
and housing of internal elements such as piping, electrical wiring, ventilation, insulation.
Throughout the past couple hundred years the styles of roofs vary from area to area. The
varying designs, styles, and shapes of roofs exist and have been created to accommodate the
needs of the structure they are covering. Below is a typical timber roof truss (Figure 9) and
common types of roofs (Figure 9a to 9i)
Figure 9 A typical timber roof truss
ROOFS
Roofs are coverings of the tops of buildings designed and constructed to protect against rain, snow,
sunlight, wind and extreme temperature. Major determinants of roof types are: structure span of the
building, cost and materials. Some of the most primary needs that a structure demands from
a roof are protection from the weather, overall design compatibility with the existing
structure, and housing of internal elements such as piping, electrical wiring, ventilation,
insulation. Materials used for roof covering are: thatch; galvanised corrugated iron sheets;
corrugated asbestos cement sheet; corrugated aluminium sheet; files (concrete or clay); reinforced
concrete; timber shingles; slates; bituminous felt/roofing felt; bamboo; and earth decking. For roof
to meet these and other needs, there are essential functional requirements.
Functional Requirements
Strength and Stability - The strength and stability of a roof depend on the characteristics of the
material from which it is constructed and the way in which the material are formed as a horizontal
platform or a triangular framework. The strength and stability of a flat roof depend on adequate
support from walls or beams and sufficient depth or thickness of timber joist or concrete relative to
span to avoid gross deflection under the dead load of the roof itself and the load of snow and wind
pressure or uplift that it may suffer. There is an inherent instability across the slopes of this roof,
parallel to the ridge, to the extent that wind pressure may cause the frames to rack or fall over of
books on a shelf. To resist racking, the frames are braced by gable end walls, hipped ends or by
cross-bracing of diagonal roof boarding or braces across slopes.
Weather resistance or exclusion of wind, sun or rain - In general, the smaller the unit of roof
covering, such as tile or slate, the greater must be the pitch or slope of the roof to exclude rain that
runs down in the joint between the tiles or slates on the back of another tile or slate lapped under and
so on down the roof. Larger unit such as profiled sheets can be laid at lower pitch that that required
for tiles. Impermeable material such as asphalt and bitumen that are laid without joint can be laid flat
and sheet metals such as aluminum and copper that are overlapped or joined with welts can be laid
with a very shallow fall. The small open jointed units of tile and sates which provide little resistance
to the penetration of wind into the roof require a continuous layer of felt or paper to exclude wind.
A roof structure will be subject to movement due to variations in loading by wind pressure or
suction, snow loads and movement due to temperature and moisture changes. The great advantage
of the traditional roofing material slate and tile, is that the small units are hung, overlapping down
the slope of the roof, the very many open joints between the tiles or slates can accommodate
movement s in the roof structure without braking slates or tiles or letting in rainwater, whereas large
unit size material and continuous roof coverings may fail if there is inadequate provision of
movement joints.
Durability and freedom from maintenance – the durability of a roof depend on the ability of the
roof covering to exclude rain, snow and the destructive action of frost and temperature fluctuations.
Persistent penetration of water into the roof structure may cause or encourage decay of timber,
corrosion of steel or disintegration of concrete. Slates and tiles when laid on adequate slope and
properly double lapped to exclude rain, will if undisturbed, have a useful life of very many years. It
will require little if any maintenance and may survive the anticipated life.
The non ferrous sheet metal coverings, lead, copper, zinc and aluminium, overlapped and jointed and
fixed to accommodate wind movement and with a slope adequate for rainwater to run off, will have a
useful life of many years and should require little or no maintenance during the life time of most
buildings.
The flat roof materials asphalt and bitumen felt are by nature , short to medium term life material due
to gradual oxidation and hardening of the material which has a useful life of some 20 to 30 years at
most.
Resistance to the passage of heat – the material of roof structure and roof covering are generally
poor insulators against the transfer of heat and it is usually necessary to use some material which is
good insulator, such as light weight boards, mat or loose fill to provide insulation against excessive
loss or gain of heat. The requirement of the building regulations for the insulation of roofs of
dwellings is a maximum U value of 0.25W/m2K.
The most economic method of insulating a pitched roof is to lay or fix some insulating material
between or across the ceiling joist, the area of which is less than that of the roof slop or slopes. These
insulating layers will at once act to reduce loss of heat from the building to the roof space, and
reduce gain of heat from the roof space to the building. As the roof space is not insulated against loss
or gain of heat, it is necessary to insulate water storage cisterns and pipes in the roof against possible
damage by freezing. where the space inside a pitched roof is used for storage or as part of the
building, it is usual to insulate the roof slopes.
Resistance to the passage of sound – The resistance of a roof to the penetration of airborne sound is
not generally considered unless the building is close to a busy airport. The mass of the material of a
roof covering and ceiling is the main consideration in the reduction of airborne sound. A solid
concrete roof will more effectively reduce airborne sound than a similar timber roof. The
introduction of mineral fiber slabs, batts or boards to a timber roof will have some effect in reducing
intrusive, airborne sound.
ROOF TYPES
Flat roof as the name implies, this roof appears flat in structure, although it is always pitched
at a wider angle to allow water runoff. Typically, flat roofs are ideal for drier areas, not those
that receive large amounts of rainfall. They are easier to construct, easier to access (especially
for those that love watching the night sky) and accord the house a modern look. Their main
downside is the accumulation of materials on the roof, which requires regular maintenance.
They also require waterproofing to prevent infiltration of water. Flat roofs are common
especially with commercial buildings. Flat roofs are definitely the most simple roof to build
because they have little to no pitch. The most common types of roofing systems used with
flat roofs are rubber roofing systems.
Figure 9a Flat roof
The gable roof is typically triangular, forming an inverted/upside ‘V’ shape. It is basically
made of two sides with a middle gap called a gable. While they are some of the simplest
when it comes to roof construction and provide room for a spacious attic, they are not ideal
for places with high, strong winds. When not well fastened, the roof can easily be blown off
by the wind. Gable roofs are not ideal for areas with high wind because they easily can catch
the wind much like a sail would.
Figure 9b Gable roof
Hip roof is made of four or more sloping sides/planes and can be square or rectangular. The
planes are designed to slope downwards from the ridge at the top, at an angle determined by
the size of the house. Function wise, they are good for areas of high wind and strong storms.
They are noted for their strength, stability and shade-giving eaves but are also more complex
and expensive to construct, courtesy of their complicated truss and gable structure. Variations
include the cross and open hip roofs.
Hip roofs are a common residential style roof. This type of roof is more difficult to construct
when compared to flat roofs and gable roofs because they have a more complicated truss and
rafter structure.
Figure 9c (I and ii) Hip roof
Similar to both the gable and mansard roofs in design, the gambrel roof is made of two
planes sloping downward from a top ridge. Much like the mansard, one plane of the roof has
a distinct lower pitch than the other (Figure 9d). More living space and lower cost of
construction are the main benefits, while their limited strength is their main downside. They
are not recommended in areas of high winds and heavy snowfall.
The best way to describe a gambrel roof is by saying barn roof. The gambrel style roof is
most commonly used on barns. However, it is also used in residential construction. This type
of roof has the benefit of providing a good amount of space in the attic. In fact, it provides so
much extra space that it is often turned into bedrooms or other living areas.
Figure 9d Gambrel roof
In contrast with the gabled roof, this roof shape is made of two planes slanting downwards
toward each other to form a butterfly wing shape (Figure 9e). Their middle point of contact
is fitted with a trough for rain water, eliminating the need for gutters like on normal roofs. A
modern exterior and feel is one of the advantages of this roof shape, while its disadvantages
include expensive construction and inefficient water collection. The butterfly roof is not a
roof style that is widely used. The style provides plenty of light and ventilation but is not the
effective when it comes to water drainage.
Figure 9e Butterfly roof
The mansard roof is a four-sided roof with a double slope on each side. Originally used in
France, the lower slope on each side is steeper than the upper, with the sides being flat or
curved. Its advantages include added living space in the attic and beauty. On the downside,
they are not ideal for areas that receive heavy snowfall, especially when lowly pitched, and
can be expensive to construct. The mansard roof is a french design and is more difficult to
construct than the hip or gable roof.
Figure 9f Mansard roof
Lean-to/Skillion/Shed roof is similar to the flat roof; it only has a steeper slope to allow for
better drainage. Also called the shed roof, it is usually not attached to another roof. Their
advantage lies in the various design options that can be incorporated in them while their
disadvantage is their cost of construction and low ceilings in some designs
Figure 9g Lean to/Skillion/shed roof
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