Many timbers (including pine, kauri and kahikatea) can decay, especially if wet. One early treatment to prevent rot involved painting timber with creosote, a preservative made from coal tar. Creosote, almost black with a strong pungent smell, was used until the 1940s. It was replaced by tanalising – a system of pressure treatment with water-borne copper, chrome and arsenic. This extended the life of the timber for decades, even when in contact with soil. Different grades of treated timber are available with different abilities to resist moisture.
Timber dimensions
Construction timber is sawn into standard dimensions, but over time the range of sizes has decreased. The introduction of metric measurement in 1967 and the desire to maximise timber production reduced thicknesses of many timbers.
Traditional dimensions were often exact multiples of inches. A one-inch (25.4 millimetres) board was reduced to 19 millimetres thick, and old sizes such as one-and-three-quarter-inch-thick (32 millimetres) boards, or nine-inch (225 millimetres) framing are no longer available. A piece of framing which was once four by two inches (roughly 100 by 50 millimetres), popularly known as the ‘four-by-two’, was reduced to 90 by 45 millimetres.
New timber uses
In the early 2000s it was difficult to obtain good-quality timber in large dimensions. Large sizes were usually formed by laminating small timber strips. Strips glued and pressed together can form a large, very stable piece of wood. Large frames can be made using laminated timber, sometimes referred to as structural timbers.
Trusses enable large spans to be achieved using quite small timber members. Early trusses were made up from quite large pieces of wood, 75 or 100 millimetres thick, and from 150 to 300 millimetres deep. Modern trusses use much smaller pieces of wood, such as 100 by 60 millimetres, joined with nail plates – flat pieces of steel punched to form a pattern of spikes on one side, which hold adjoining pieces of wood together.
Plasterboard, plywood, chipboard and MDF
New Zealand Wallboards began producing a plasterboard in 1927 in Mt Eden, Auckland. Winstone bought the company in 1930, increasing production fivefold that year to 5 million square feet of plasterboard. It was termed Gibraltar board. Internal wall linings began changing from scrim on deal (wooden boards) to plasterboard.
Dressed for timber
Traditionally timber was sold rough sawn or dressed (smoothed), depending on how it was to be used. Today, framing timber is sold as machine gauged – but finished timber is still dressed.
Sheet materials can economically use timber as a finishing material, can strengthen framed structures and are also used for temporary works in building. Plywoods use comparatively low grades of timber to form very strong sheets, which can be finished with a visible high-quality surface layer. Plywoods are made up of thin (2 millimetres) sheets of timber peeled from a log and glued and pressed together in layers. Special grades of water-resistant plywood are commonly used to make form work for concrete or for cladding buildings. Decorative plywoods can also be used for furniture and doors.
Other wood-based sheet materials are made from shredded wood combined with resins and pressed into sheets of varying thickness and hardness. These chipboards or particle boards are commonly used for flooring, wall panels, doors and furniture.
MDF (medium-density fibreboard) is another type of panel formed from resin-bonded wood pulp. Its very fine texture allows precise machining. The resulting smooth surface allows a high-quality paint finish.