The purpose of a roof frame is to provide a structure that spans the walls of the building and supports the roof covering. Most roof designs also incorporate a slope of pitch to allow rain water to run off and be drained away from the dwelling. The roof frame also assists in bracing the structure from wind forces from various directions. Roofs can be constructed from timber, steel concrete and other materials and come in many different styles.
For a conventional house, a timber frame could be described as a skeleton of timber components to which are attached wall claddings (the exterior skin), internal linings (the interior skin), flooring (the walked-on surface) and roofing (to protect the structure from rain etc.) and windows and doors.
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 is expected to protect the house and contents underneath.
The roof framing is essentially 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.
Roof Frame Performance
Roof framing 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 Sizes, Spans, Spacings In Framing
Sizes or grades of timber to be used in timber framing are outlined by the main authoritative document 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.
Similarly, new building products such as LVL (Laminated Veneer Lumber), LVR (Laminated Veneer Radiata), 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.).
A truss is a structure made up of one or more triangular units made with straight members whose ends are connected at joints. There are two basic types of truss. The pitched truss or common truss is characterised by its triangular shape, and is most often used for roof construction. Some common trusses are named according to their web configuration. The chord size and web configuration are determined by span, load, and spacing. The parallel chord truss or flat truss gets its name from its parallel top and bottom chords. It is often used for floor construction. A combination of the two is called a truncated truss and it is used in hip roof construction. A metal plate connected wood truss is a roof or floor truss whose wood members are connected with metal connector plates.
Trusses are generally premade, triangulated wooden structures used to support the roof. They are incredibly strong, cost effective, just about any shape can be custom built, and can span a large distances. Trusses transmit all of their weight to the exterior walls meaning that none of the interior walls are load bearing. Trusses also go up quickly. One disadvantage of trusses however, is that they do not leave any usable attic space.
When installing nail plated roof trusses, there are a few factors to consider:
Accurate location and dimensions of supporting structures must be provided to the truss manufacturer.
Consideration must be given to the reactions from any girder truss.
Supporting structures must be stable with temporary bracing where needed at the time of truss installation.
Lintels must be designed to carry the load applied from the trusses. Point loads from any girder truss are to be checked.
Non-load bearing walls must not carry any truss loading and must not be packed to touch the underside of the truss.
When fixing trusses to top plates of non-loading bearing walls bracing units, trusses must be fixed in such a way that the bottom chord of the truss is restrained horizontally but still allows for deflection when the truss is loaded. Refer to Australian Standard (AS) 1684.2, AS 4440 and manufacturers’ information for fixing details.
Truss installation must comply with the truss layout supplied by the manufacturer. All trusses must be braced and stabilised throughout the installation of the roof truss system as transfer blocking and has ceiling installation. Trusses must be temporarily braced during installation to ensure they are held straight and plumb to the installation tolerances, prior to fixing the permanent bracing.
Intermediate ceiling joists and hangers are to be fixed in accordance with the approved specifications. The connection of the hanger or intermediate ceiling joist should be with a metal connect designed for the application or a timber block fixed with two 65mmnails to each member. Where plasterboard is fixed directly to the bottom chord of trusses, noggings must be fitted as follows:
- At hip ends where bottom chords of hip-end system do not run parallel to the main run of trusses, spacing to suit plasterboard.
- Noggings to be fitted between trusses where internal walls run parallel to the common trusses.
Roof bracing must be suitable for restraining loads imposed by the direct action of the wind and to prevent rotation or buckling of trusses under the weight of roofing and ceiling materials. Roof bracing consists of lateral restraints to truss top chords, i.e., roof battens; diagonal bracing to truss top chords using steel brace; bottom chord bracing and web ties.
Roof battens must be fixed to every truss and splices in battens should be arranged so that in any top chord, no more than 1/3 of battens are spliced and no two splices are adjacent. No splices in battens are allowed over girder trusses.
In general, roof battens prevent truss top chords from buckling. This buckling action is resisted by the roof battens, which in turn transfer it to the steel roof bracing and down to the supporting structure. Each element and fixing along this sequence is essential for the stability of the roof structure. Steel brace, once installed, is to be tensioned to a minimum of 8.4kN and be prevented from sagging between supports and be installed in accordance with manufacturer’s recommendations. Typical steel brace layouts are generally supplied by the truss manufacturer.
Bottom chord bracing where required, is to restrain truss bottom chord against lateral buckling. Where battens are fixed directly to the bottom chord, lateral restraint is achieved. Suspended ceilings require steel brace installed on top of the bottom chord and attached to sidewalls. Where the ceiling is direct fixed to the underside of the trusses, bottom chord ties can be spaced at a maximum of four metres. Web bracing, where required, is achieved by the use of longitudinal ties, T-stiffeners or other supplementary members.
The connection of jack or creeper and hip trusses at a hip-end roof for wind classifications N1, N2, N3or C1 are to be in accordance with details provided in AS4440 and manufacturer’s recommendations. Where wind classification is N4, C2 or C3 details are provided in AS4440.
It is important to establish the appropriate girder bracket to connect a truss to a girder truss. Girder brackets are:
- Framing bracket – to support standard trusses with a small span
- Standard truss boot – to support standard and truncated standard trusses
- Anti-twist truss boot – to support standard and truncated standard trusses
- High-load truss boot – a welded bracket to support heavy loading from large standard and truncated standard trusses
- All facings to be installed in accordance with manufacturer’s specifications and AS4440.
A walling plate for a Dutch-hip girder is to be fixed to every intersection of the top chords and webs of the supporting truss. Fixing details are to be in accordance with the approved specifications. Valley trusses are to be fixed in accordance with AS4440, figures 5.6 and 5.7; again this is dependant on the wind classification.
There are two types of standard truss overhang:
- Raking eaves overhangs which is designed to carry the small loads from the end creeper rafters or trusses. The fascia can become a structural member.
- Boxed eaves can be propped or non-propped. The propped type is attached to the end of each top chord and to a ribbon plate at the wall. The non-propped type has the eaves trimmer omitted or a dropper is used outside the brick work to support the eaves trimmer.
A verge overhang is formed by outriggers extending to the outer edge of the external wall and fitted in accordance with AS4440.