Roof framing is much more complicated than either floor or wall framing because of the many styles, many shapes, variable angles and the types and weight of roofing materials which could be chosen.
An oversimplification would be to describe the principal types as gabled, hip, flat, and perhaps skillion, but this certainly would not be a complete list.
The primary function of the roof framing is to provide and maintain support for the chosen roofing material which, of course, is expected to protect the house and contents underneath.
The roof framing is essentially or simply made up of spaced rafters to which may be attached battens to which, in turn, the roofing material is attached.
Rafters are attached to the top plates of the wall framing, and where appropriate to ceiling joists.
Depending on design, span, and other styling components, the roof framing may include ceiling joists, collar ties, ridgeboard, bracing, roof struts, outriggers, hanging beams and various types or forms of supplementary short rafters
Illustrative drawing of traditional or conventional roof framing for hip roof with boxed-in eaves.
While the majority of prefabricating factories (See Prefabricated Frames) would be capable of producing components for unique or individual one-off house designs, significant savings would be achieved if some standardisation or rationalisation of designs were achievable - as is the case with major repetitive projects or some broad-acre developments.
ROOF FRAME PERFORMANCE
||Roof framing of either type of construction, as with wall framing, must be adequately braced so as to resist distorting forces and must be adequately fixed together using established nailing patterns and/or a system of prefabricated framing anchors or nailing plates or similar.
All of these normal requirements are intended to provide a strong roof system, capable of supporting the roofing materials and resisting lateral wind forces and most importantly wind uplift, particularly in cyclone areas and areas of high winds.
TIMBER GRADES FOR FRAMING
A feature of AS1684 Timber Framing Code is that it provides an opportunity for the designer/builder/owner to minimise the cost of timber and minimise the amount of timber used in the framework. To achieve the benefits, the designer or builder would assess the alternative grades of structural timber available and by reference to an extensive tabulation in the Code arrive at an economical combination of timber properties.
Timber availability, stress grades (a measure of structural strength for design of each component), costs (per cubic metre or lineal metre of timber), design spans, and spacings and cross section of the many timber components. All this without diminishing the performance of the total structure in terms of the principal needs of load support and occupant safety.
TIMBER SIZES, SPANS, SPACINGS IN FRAMING
The foregoing gives no indication of sizes or grades of timber to be used in timber framing. Many such details are available from the publications listed later but the main authoritative document would be AS 1684Timber Framing Code. This Code and its Supplements provide a large number of tables from which timber grades and sizes could be readily obtained for conventional spans and spacings for such components as joists, studs, bearers, beams, lintels or headers, flooring etc.
Unconventional constructions, or where components of a planned framework cannot be interpreted from AS 1684, require the expertise of someone approved by the local authority to calculate or derive or authorise the appropriate details.
This is not an unusual occurrence since owners frequently seek to have wider balconies than the Code prescribes, wider or larger uncluttered floor areas, bigger garages and many similar variations from the "convention" when the Code data was originally compiled.
Similarly, new building products such as SCRIMBER, LVL (Laminated Veneer Lumber), LVR (Laminated Veneer Radiata), OSB, Waferboard, timber-steel composites are "unconventional" in these terms and their inclusion in a frame requires that the designer uses the technical data usually available in brochures from the manufacturer, the fabricator (e.g. roof or floor truss manufacturer) or the supplier (e.g. framing anchors. joist hangers, etc.).
Although house building with a timber framework is viewed as a traditional industry, in fact, a closer study will show that it is in a state of constant change with regard to materials, equipment, prices, availability of materials, manpower and skills.