Hardwood Timber

It is the policy of Genneral Staircase to see that care is taken in the procurement, selection and manufacture of all the timber and timber based materials, used in the manufacture of our timber components, to insure to the best of our capability, the expected long term performance of our products.

Some of the many variables that have to come together to achieve this are, adherence to the relevant Australian timber standards and accepted trade practices, along with a sound knowledge of published wood technology practices.

While all of the above are important and within reason, they are able to be controlled in our factory, but once the product arrives on site, it becomes a different situation, the control of the prevailing moisture content (MC), due to fluctuations in the prevailing weather conditions, being the most difficult and usually outside our control. 

In the factory, we endeavour to acclimatise the moisture content in the timber and the same on site,  but should the product be installed in a new building with a tight time restraint, then the majority of the building components will still contain varying amounts of moisture, that will be gradually released over a few months, after the building is occupied. The timber components will absorb that humidity, swell in width and to a lesser degree in thickness, and could produce compression forces at joints or walls etc.

Depending on the type of building involved, whether an office block, unit or house, there will generally be large expanses of glass, with a resistance to shield the view with blinds or curtains and sunlight penetration onto the timber and associated radiant heat, will lower any absorbed MC and some of the equilibrium MC, present at the time of installation. The loss of that combined moisture and especially if the premises are equipped with a commissioned, reverse-cycle air-conditioning system, will dramatically reduce the MC in the timber components and could result in

width shrinkages, edge cupping, longitudinal splits and cracks which is still deemed as acceptable quality.

The general rule is :-

Residential house or unit. If only used in short bursts for comfort reasons in hot or cold weather, should not cause a problem.

 Offices, commercial buildings:- Usually programmed set times during the week and not on week-ends, which depending on the settings, will reduce the MC in the components, produce cupping and longitudinal edge gaps and splits, that will slightly reduce over the week-ends.

Hotels and resorts :- Usually programmed to run 24 hours per day, 7 days per week and 52 weeks per year. It will depend on whether the air-conditioning unit was continuously running for about 1 month, with the timber component in the area to be installed and the setting of the A/C, then there should not be a problem. If not, then big shrinkage problems which cannot be warrented. 

Under normal circumstances, occupants have only the opportunity to control the required temperature with air–conditioning systems, and experience has shown that most people’s comfort level is about 21°C. The manufacturers of air-conditioning systems target the relative humidity range between 40% and 60%. As a guide, timber subjected to a setting of 40% relative humidity and a temperature of 21°C, over a period of time, would result in a MC of about 7.75%, which is too low to expect that any manufacturer of timber components, could achieve that, at the time the component left the factory. 

At a setting of 60% R.H and a temperature of 21°C, the equilibrium MC would be around 11%, which gives the manufacturer a chance to meet that target and satisfactory long term product performance. At 70% R.H and 21°C, the timber will acclimatise to about 13%.

The wider the width of a single board, or the multiple width of a number of jointed boards, the greater will be the movement and the possibility of edge cupping, which is one of the reasons why we prefer where possible, to manufacturer our wide stair treads out of two independent boards and deliberately leave a central movement gap. In stair landings and winders it is common practice to use smaller sections like flooring to reduce the movement in large areas which have a tendency to open up in varying conditions. Published unit shrinkage figures for most of the Australian timbers used in our products, are much the same and as a guide, a 1.0mm loss in an 80mm wide board would represent a MC loss of about 3.3%; a 1.0mm loss in a 130mm cover board would be a MC loss of about 2%; a 1mm loss in a 180mm cover board would be a MC loss of about 1.5% and a 1mm loss in a 240mm cover board would be a MC loss of about 1.1%.

In certain situations where wider boards are required with lower movements per MC change, we have the choice of at least three Asian species e.g. a 0.64mm loss in a 240mm cover board would be a MC loss of about 1.1%.

One result of moisture loss, for any reason, is the opening up of joints and as there is no current Australian standard that gives a guide as to when a gap becomes excessive, the Guide to Standards and Tolerances was produced in collaboration with the Victorian Building Commission, the Office of Fair Trading NSW, the Tasmanian Government and the ACT Government in 2007. The only reference to allowable gap sizes, is in Section 14, Clause 14.03 Gaps in exposed timber flooring, which states :- ‘Except where affected by exposure to sunlight, cooling, heating or other heat generating appliances, flooring is defective if it has a gap of more than 2mm between adjacent boards that extend for more than 1m. Flooring is defective if it has gaps of more than 5mm in total of three gaps between four consecutive boards’. 

For your company’s consideration.

 The NSW Office of Fair Trading had published a similar ‘Guide to Standards and Tolerances for home builders and renovators’ in July 2003, in which there was a clause that stated ’The builder shall not be liable for gaps considered as defects, where the builder has made the owner aware, as acknowledged by them in writing, that the flooring system installed could suffer significant shrinkage, leading to visually obvious movement resulting in gaps, well in excess of the tolerances listed above’.   

‘ When conditions are extremely wet or dry and rapid changes in humidity occur, the timber may distort, or display surface checking, due to extreme expansion or contraction. These conditions are beyond our ability to control and therefore, where our products are to be installed in a building, the relative humidity must be maintained between 40% to 60%. In order to achieve this, humidifiers/evaporative coolers may be required in very dry weather, and air conditioners may be required in very moist and humid areas. Failure to maintain relative humidity within these levels will void this warranty’.