It is not necessary for floor timbers to be hardwood or LVL. Bearers and joists of untreated pine comply with Australian Standard 1684, Residential timber-framed construction, as long as the sizes are adequate for the span and load conditions. For example, 90 x 45 pine floor joists of MGP10 stress grade - that is the most common grade and is probably stamped on the timber somewhere - can span 1.3m in a single span, or 1.8m in a continuous span, spaced at 450mm centres. A 'continuous span' means the joists are supported at three points, ie. by at least three bearers. It also assumes the joists are not supporting any loadbearing walls, ie. walls that carry roof load down to the footings. It is also important that there is adequate sub-floor ventilation for your local climate, and that there is the required clearance between the underside of the lowest timber and the ground. We suggest that a pre-purchase inspection would be a good idea to check these details.
Pine floor bearersI'm looking to purchase a house. It has double 90 by 35 pine bearers and 90 by 45 pine floor joists for the home (both are untreated pine). I thought these were supposed to be hardwood or LVL. Is untreated pine okay on a house subfloor?
For the bearers and joists of the jetty over a fresh water wetlands pond, I'm specifying Jarrah.
The bottom of the bearers are 100mm above the water, do I need to treat the Jarrah bearers and joists to H4 or H5?
If not treated, what would be the design life that I could expect out of the untreated Jarrah in this environment?
Jarrah is a Durability Class 2 timber, and has a probable service life of 15 to 40 years outdoors above-ground, according to Australian Standard 5604, Timber - Natural durability ratings. This rating assumes the timber will be suspended over earth, not water. The reason for the rather broad time scale is that the upper level (40 years) depends on the timber being able to dry out after wetting, and assumes it will be in a situation where ventilation and drainage are adequate. Over a wetlands pond presumably there won't be any wave action or direct wetting from splashing. However, condensation on the underside may be a hazard with the bearers only 100mm above water, and we would therefore estimate the service life to be closer to the lower end of the scale. Treatment of jarrah to H4 or H5 is not an option since hardwoods are difficult to treat. This is explained in more detail on our website here: https://www.woodsolutions.com.au/articles/timber-preservation. Pine could be treated to these preservative levels and might therefore be a more reliable choice for a long life.
Jetty timberI have a project for a single storey extension using simple bearer and joist construction on brick piers. The project is in a flood prone area. What would be the best timber species/treatment to use for the bearers & joists that may be inundated during a flood event?
The denser the timber the more slowly it will absorb water, although all timbers will absorb water if immersed for long enough. However, specifying a dense hardwood such as ironbark will at least slow down moisture absorption and will be beneficial if the flood event is short lived. Ironbark is also a naturally rot-resistant species and therefore not prone to develop decay when exposed to moisture. However, if flooding is a regular event it would be worth using non-corrosive fasteners for the bearers and joists. Also the flooring material is likely to suffer if inundated.
Flooded floor
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