By Adam Krajc
As a builder primarily using prefabricated timber frames and trusses for the structure of our homes we’ve noticed increasingly over the past couple of years a decline in our perception of the performance of these frames and trusses. To be clear we have no concern over any potential structural failure, the performance issues we’ve identified relate to deflections of timber members under load, which despite compliance with structural design criteria, will provide for deflections that in certain circumstances and locations may be (or become) visible in other elements of the home as it is built. The deflected frame and / or truss members, although not necessarily a defect themselves can create issues that may be classified as a defect in other elements of the home as it is built.
Frame and truss fabricators rely on their engineered design software to specify the timber members that go into their frames and trusses. In a competitive marketplace these fabricators are constantly trying to minimise their manufacturing costs to keep their pricing down, and this is often complimented with their standard design criteria allowing for maximum deflections allowable under structural performance requirements specified by the National Construction Code. This enables their use of lesser timber members which in turn helps them keep material costs down while still maintaining minimum structural performance requirements. According to most frame and truss fabricators deflections within their design criteria (where that criteria meets with specified requirements of the National Construction Code) are not a defect.
The potential issues that may arise in a new home as a result of these frame and truss deflections include-
- -Ceiling level variations and wavy cornice
- -Inconsistent roof plane (wavy roof)
- -Hips
- -Fascia
- -Eaves level and line irregularities
- -Required variance in brick gauge
- -High or low spots in structural flooring
The above potential issues that can result directly from the ‘acceptable’ deflections of frame and truss members may be to an extent that sees them classified as a defect as defined by the Office of Fair Trading Guide to Standards & Tolerances, which in turn becomes the builders responsibility to fix. This is why builders need to better inform themselves of the design criteria their pre-manufactured frames and trusses are manufactured to.
Rosewood Homes have identified this industry wide issue and have subsequently specified the design criteria for our frames and trusses to provide for tighter tolerances than the manufacturers typically work to. We do this to achieve a better quality home for our clients.
