Specification is the process of communicating the designer's intentions to the supplier of the material. There must be a clear and unambiguous specification for all structural timber (including framing) required in their design. Specifiers must be able to communicate to suppliers the appearance properties required for internal applications including linings, joinery, floors and stairs.
The performance required of different types of timber elements, in turn determines the properties required. When specifying timber, the following aspects need to be considered:
- Application - the relevant functional requirements for the timber to be used in the building or structure.
- Performance requirements - appearance, structural performance, service life, initial cost, ongoing costs.
- Environmental hazards - each application will have a service environment that will enable the estimation of Environmental Hazard Level. This can be used as a guide for selecting a species with the right natural durability or for selecting an appropriate treatment type and level.
The product specified takes into account all of the performance requirements and the Environmental hazard level. The specification is an unambiguous statement of the material that best achieves all of the requirements.
Timber is generally specified by cross-section, length, stress grade, as seasoned or unseasoned, by species or timber group, and treatment. For example:
- 90 × 35 5.1 metre MGP10
- 190 × 45 4.2 metre F8 seasoned Radiata Pine treated to H3
- 150 × 38 3.3 metre F11 unseasoned hardwood.
It is imperative that specifiers check availability before describing any product. In practice, this is as simple as asking some local suppliers to fax a copy of their current price list. Having determined local availability, the specifier should choose from only those grades, species and sizes that are actually available.
Supervision ensures that the specification has been satisfactorily implemented. Specification generally considers the following criteria:
- Grades - the grading of timber ensures that all timber is able to perform to a minimum performance requirement and to provide a reliable standard against which a designer /architect can create a design and specification.
- Quality assurance - to ensure that timber is graded consistently across regardless of manufacturing source and location a number of quality guidelines and criteria have been established.
- Span tables - a number of resources exist to ensure that the correct size of timber is chosen for the design intent of the designer/architect. Span tables are critical to ensuring appropriate dimensions and performance is achieved.
- Durability - durability is one of the key performance factors used to assess the suitability of a timber species for a specific application. The durability rating of a species is based on the natural ability of the heartwood of that species to resist decay and insect pests. However, for many species, the durability of sapwood which is usually not durable, may be increased by preservative treatment.
This section contains the following resource:
- Design Guide 05 Timber service life design - design guide for durability - this guide will give building and construction industry professionals the confidence in determining the service life timber in a wide range of applications, from sole plates to suspension bridges.
- Structural Timber Products Guide - prepared by Wood Products Victoria, this technical resource provides a comprehensive guide to the range of timber products for available structural applications. If there is any doubt concerning availability, your specific suppliers should be consulted before completing design documentation, specification or ordering.
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