An insight into roof systems
Roof system refers to the component of a roof including but not limited to covering, framing, insulation, sheeting, ventilation and weatherproofing.
IT includes all materials contained within the roof assemblies installed by the contractor, including all materials above the deck including membrane flashing and related sheet metal work. A roof protects everything inside a building, and it can be used to install mechanical equipment and solar panels. It’s therefore critical to have a reliable roof.
Components of a roof system
Roof substructure: the trusses, rafters, purlins or other supporting framework to roof coverings, designed in accordance with requirements of the by-laws. The strength, dimensions and spacing should be as shall adequately support the roof covering applied to them. It is, therefore, imperative that one engages a competent and skillful person for this.
Roof coverings: these can be of different materials as specified in the designs. The coverings should be secured to withstand climatic conditions e.g. wind pressure, wind suction and changes in temperature.
Roof underlayment: some is applied below roof covering e.g. allucushion for insulation purposes. Others like asphalt felt are applied on top of the roof covering mainly for waterproofing purposes.
Flashings and other finishings: these include flashings on junction of roofs with chimneys, parapets, etc.; valley gutters and ridges which help to render the roof weatherproof.
What determines roof and truss design
The roof design is a key factor that affects the overall performance of a building.
Each roof system has its own characteristics, advantages and disadvantages and recommended applications to consider when making a decision. A high-quality roof design requires highly qualified and skillful workmen.
A roof design should minimise water accumulation (ponding) which can cause leakages, mold growth and rotting which may eventually lead to roof collapse. Roofs can be constructed in a wide variety of shapes and designs depending on:
◆ The requirements they have to satisfy ◆ Availability of materials and skills ◆ Span to be covered
◆ Cost
◆ Roof design aesthetics
◆ Loads acting on the roof
Common roof designs
To come up with a roof, we use trusses. A roof truss is a prefabricated structural framework which is used to support the roof of a building. They are predominantly triangular in shape, but the configurations vary to suit the roof’s pitch, size and design.
Triangles are used mainly because of their ability to support forces and they are easier to make than other shapes. There are two main roof truss categories namely flat roof and pitched roof. These two categories can be subdivided into more specific truss types which can be used depending with construction application. Roof trusses are typically made from timber or steel joined together using various means. Some are made from concrete.
Roof supports
A roof can be supported using different methods as per the designs. The type of roof to be erected determines the support that is needed. It is therefore important to build proper walls that can safely support the roof load. Some of the common supports that can be used include the following
◆ Load bearing walls
◆ Concrete columns
◆ Concrete beams (ring beams)
◆ Timber upright supports (poles)
◆ Steel upright supports (columns)
Roof cover materials
As contained in the Model Building By Laws Chapter 6 Section 11 to 24, there are various roof cover materials that can be used as roof cover. The different roof materials have different properties and hence are also used for different applications.
Their supporting structures (e.g. trusses) also differ therefore its important to have a specifically designed supporting structure for a specific covering material, with the appropriate angles as required.
Some of the materials as contained in the by-laws include the following
◆ Asbestos cement corrugated sheets
◆ Asbestos cement roof slates
◆ Steel roofing sheets
◆ Corrugated galvanised sheets
◆ Galvanised inverted box rib (IBR) sheets
◆ IBR chromadek (coloured sheets)
◆ Metal roofing tiles (e.g. Harvey tiles)
◆ Q-tiles (cement tiles imitation)
◆ Concrete roofing tiles
◆ Clay roofing tiles
◆ Grass thatching
◆ Glass
◆ PVC (translucent sheets)
◆ Wooden shingles
◆ Rock slates
◆ Copper roof sheeting
◆ Aluminium and aluminium alloy roofing
◆ Mastic asphalt roofing
◆ Bituminous felt built up roof coverings
◆ Concrete (as in decks)
When selecting roofing material, we consider a number of factors such as
◆ The pitch of the roof
◆ Roofing material budget
◆ Architectural style required / aesthetics
◆ How long you want your roof to last
◆ Weather endurance
◆ Complexity of the roof/ difficulty in construction
◆ Material weight
◆ The roofing application/use /functionality municipal requirements and residential community agreements / covenants
The role of the client
The client is an integral part in ensuring that the roof has been properly done
Have background check of the contractor to check whether they are skilled for the job at hand
◆ Reviewing and approving product requests and designs
◆ Provision of the adequate funds required for the materials and skilled labour in a timely manner
◆ Should supervise to ensure whether he is getting the desired quality for what he is paying (certifying of works)
◆ Witness and monitor performances and tests
◆ Approval of drawings and documents
This week’s questions
Describe some of the ways used to deal with roof leakages
◆ This article was first published in October last year, in Business Weekly, but given the importance of this structure of a building and some questions being asked, we running the article again this week.
◆ Innocent Chatikobo is an entrepreneur and engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in structural engineering and construction. For your comments, views, questions and feedback he can be contacted on the following platforms. Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732; Email: eng.chatyk@gmail.com; Facebook: AtroServe Engineering Zimbabwe