Roof TrussesAdvantages of Timber Roof Trusses
Prefabricated timber roof trusses have been part of Australian building practices for over 30 years. Builders who regularly use roof trusses claim the following benefits:
(i) Cost effectiveness
(ii) Peace of mind for building authorities and certifying engineers
(iii) Design flexibility
(iv) Ease of installation
(i) Cost Effectiveness
Long clear spans can be achieved at an economical rate. Trusses can easily span 10-15 metres or more without the need for internal supports. This is particularly useful for ‘open plan’ designed homes.
Achieving a ceiling line as shown below via a “cove truss” does not effectively utilise the benefits of truss triangulation and consequently results in:
- Large top chord sizes and
- Large defections
Large Bending Force
A much better and more economical alternative is to use a Scissor Truss which will provide the following benefits over the cove truss:
- Smaller timber sizes
- Minimal deflections
- Longer spans
Trusses are individually engineered which optimises the timber truss components to suit the design loads applied to them.
Manufacturing of trusses optimises timber stock, leading to minimisation of timber waste.
There are many home designs which are conducive to roof trusses and deliver an overall cost saving compared to a traditional stick-built roof.
(ii) Piece of mind for building authorities and certifying engineers
There are stringent requirements associated with the development and use of Pryda’s design software, Pryda Build, which delivers the associated benefits:
- Alleviates the need for designers and project engineers to prepare detailed specifications associated with the roof, floor and wall components. This can be supplied by Licensed Pryda Fabricators.
- Building authorities can often provide building consent based on the documentation supplied by the Licensed Pryda Fabricators without the requirement for independent third party certification, particularly in residential construction.
Scope of use
Pryda Build is used by Licensed Pryda Fabricators to produce designs and manufacturing specifications for joined and unjoined timber and steel components in the construction of residential and light commercial and industrial structures. All timber components and connectors are individually designed to suit the specific loading conditions for the structure.
Independent Engineering Assessment
Pryda Build has been independently assessed by professional consulting structural engineers for compliance with the Building Code of Australia, BCA 2010, and its referenced documents.
Pryda has also demonstrated compliance with the requirements set out in the ABCB Handbook “The Use of Structural Software for Building Design Approval” (2007).
Software referenced documents
Pryda Build implements the requirements set out in the Building Code of Australia, BCA-2010, and the following referenced documents:-
AS/NZS 1170 Structural design actions
Part 0: 2002 General principles
Part 1: 2002 Permanent, imposed and other actions
Part 2: 2002 Wind actions
Part 3: 2003 Snow and ice actions
AS 1649 - 2001 Timber - Methods of test for mechanical fasteners and connectors - Basic working loads and characteristic strengths
AS 1684 Residential timber-framed construction
Part 1: 1999 Design Criteria
AS 1720 Timber Structures
Part 1: 1997 Design methods
AS 4055 - 2006 Wind loads for housing
AS/NZS 4600 – 2005 Cold-formed steel structures
Software User Requirements
All users of Pryda Build are required to have undertaken a formal training course in the use of the software. This training is provided by Pryda and takes the form of group training courses at centralised locations, and/or one-on-one training at a Licensed Pryda Fabricator’s premises.
Pryda organises intensive training programs for Pryda Build on a regular basis. Initial courses are for 3-days at centralised locations around Australia and New Zealand. At the completion of this course, attendees are able to install and operate Pryda Build. Training must be completed prior to installation of Pryda Build
Users are issued with a Certificate of Training if they have demonstrated an acceptable understanding of the features presented during the course. Evidence of this training to any fabricator using Pryda Build may be obtained on request.
Documentation supplied by Pryda Licensed Fabricators
The following reports are available and they can each be produced by a truss fabricator using the software:
- Producer Statement Report – a statement of design compliance for the whole job with overall and nominal design criteria, and BCA referenced documents.
- Plan Layout - showing the roof and all trusses laid out; all bracing(input by users); special notes for installation; all truss-to-truss connections
- Detail Sheet – a drawing of each truss with all relevant design parameters associated with that particular truss
- Design Report (summary) – all general loads; all applied distributed loads; truss serviceability displacements for major loads; support reactions; critical member timber designs details; bearing requirements; and nailplate design details at critical joints
Design Report (detailed) – as for the summary report, plus the results of the analysis for the 4 most critical combined load cases; all timber member designs; all nailplate joint designs
(iii) Design Flexibility
- In most cases the design and layout of trusses can easily accommodate air conditioning units, hot water systems, skylights and other roof features.
Trusses can accommodate a variety of architectural specifications – such as coffered ceilings, box gutters and raked ceilings.
(iv) Ease of Installation
- Installation times are reduced by trusses being delivered to site ready to install.
- Unlike materials supplied on site for stick-built construction, trusses are designed specifically for each job, thereby reducing the prevalence of material theft.
- Trusses are made in precision jigs to ensure uniformity of roof planes.
- A truss roof is supplied as an entire package as distinct from stick-built roofs supplied by different suppliers. i.e. steel beams, LVL beams and timber roof framing. This also provides scheduling benefits.
- Manufacture of trusses is undertaken off-site so work progress, other than final installation, is not limited by site conditions (i.e. weather conditions; or limited space for ‘cutting out’ on small sites and multi-residential sites).
Trusses are design specific for each house so its is uncommon for trusses to be stolen from site. They are often delivered to site and stacked on top of the wall frames.
Trusses are made in precision jigs so there is uniformity of the roof planes.