is the term generally used to describe the installation on an existing
building of a new external roof (normally a pitched roof), comprising
a cladding system together with a primary structure to support the
new cladding and to facilitate any alteration to services etc. which
result from the new roof form. Roof-top extensions can combine over-roofing
(whether as part of a major renovation or not) with the creation
of new habitable space, provided that the existing structure can
support the additional loads.
Over-roofing is the main alternative
to total replacement of the existing roof and is carried out for
a variety of reasons (see below). Total replacement of the roof
is almost always more expensive than over-roofing, and often leads
to the building being out of use for an extended period. As the
particular requirements of projects involving total replacement
of the roof may vary significantly, this option is not considered
in the publication.
Light steel construction is ideal
for over-roofing schemes, allowing for rapid construction and usually
avoiding the need to strengthen or alter the existing building structure.
Light steel components are ideally suited to use in both over-roofing
and over cladding projects, as they are strong, light in weight,
durable, available in standard forms, easily adaptable and recyclable.
A variety of cladding materials and other finishes may be used in
the construction of the new roof.
Over-roofing is often undertaken
as part of a comprehensive renovation package that includes over-
Reason for Over roofing of existing
- creation of additional habitable space
- over come leakage problems in flat roofs
- reduce heating costs
- improve appearance of building
- avoid disruption that is inevitable in the
replacement of an existing roof.
By converting existing flat roof
structures to a pitched roof system, both the visual appearance
and value of the existing property is enhanced - whilst at the same
time eliminating the inherent repair and maintenance problems associated
with traditional flat roof systems.
The system comprises lightweight truss
member constructed from cold formed galvanized channel sections. These
are available in a comprehensive range of depth, flange widths and gauges,
allowing flexibility in design and construction and thereby allowing a
variety of large-span options-potentially beyond 40m.
The truss member will then be over clad
in a range of alternative cladding materials.
The eaves on most building also include
soffits and fascias which, to ensure the integrity of the whole structure,
is integrated as part of the Somundy Roofing System in the same material
and design framework.
Roofing System will be tailored to meet individual requirements,
is fully planned and pre engineered,ready for quick and easy site
A variety of designs can be achieved
including the use of hips, valleys, dormer, dual and mono-pitch
Benefits of using light steel construction
Light steel construction utilizes
cold formed galvanized steel sections, which are either assembled
on site from individual components or pre-
assembled into trusses or sub-frames to support cladding and floors.
Light steel construction is widely used in low-rise and domestic
construction, and two SCI publications give comprehensive information
on such applications. Design using light steel sections is covered
by BSjfjfjf and Euro code 3-1.3. Design tables are included in the
SCI publication Design of structures using cold formed steel sections
and design examples are given in the SCI publication Building design
using cold formed steel sections: Worked examples.
The main benefits of using light steel
construction and components in over-roofing applications are listed below.
- The high strength to light weight ration of light
steel means that the existing building is normally able to support additional
load without modification.
- Long spans can be achieved between supports (particularly
when constructed in truss form)
- Existing services, equipment and plant rooms can
be left in their original location within the new roof space.
- New habitable roof space can be created at modest
cost, and independent of the existing roof structure.
- The flexibility of the framing system means that
it can be adapted to suit a wide range of building with different structural
problems and complex layouts. Furthermore, interesting roof appearance
can be created (for example the incorporation of hips, dormer and other
- A wide variety of cladding materials, finishes and
colors can be applied to the steel structure to achieve the appearance
- Steel is available in a wide range of standard components,
such as galvanized sections, coated sheets and composite panels which
can be incorporated using conventional technology.
- The good durability and strength properties of galvanized
steel creates a long life and robust constructions. Components are not
susceptible to shrinkage, deterioration, fungus, or insect attack.
- Light steel panels can be pre-fabricated and partly
clad off-site, thus minimizing site installation and disruption.
- Light Steel provides the most economic solution to
many over-roofing and roof extension problems, and is the only practical
way of solving more complex problems.
- Steel is non-combustible and can be easily be protected
against the effects of fire.
- The connections between members are simple, economical,
very reliable, and can be disassembled if required.
- Light steel construction can be adapted or extended
easily in the future.
Design considerations for over-roofing
In principle, there are five main considerations to
be taken into account in the design of an over-roofing system.
- The design of the new roof structure to support the
new roofing materials
- The adequacy of the existing structure to sustain
its new loading.
- The means of supporting and securing the new structure
to the existing building.
- The choice of the new roof cladding materials together
with their direct supports, insulation and membranes to achieve weather-tightness
and thermal insulation.
- Modifications to the existing rainwater disposal
and other services.