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Over-roofing 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- cladding.

Reason for Over roofing of existing buildings

  • 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.

The Somundy Roofing System will be tailored to meet individual requirements, is fully planned and pre engineered,ready for quick and easy site installation.

A variety of designs can be achieved including the use of hips, valleys, dormer, dual and mono-pitch structures.

Benefits of using light steel construction on over-roofing

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 features.)
  • A wide variety of cladding materials, finishes and colors can be applied to the steel structure to achieve the appearance required.
  • 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.