Engineered wood includes panels, plywood and lumber. Also called mass timber and composite wood, there exists a huge number of types, formats and dimensions, which span architecture and construction, interiors, furniture, packaging and transport applications. They are utilised in both structural and non-load bearing applications (as finishing layers and decorative products), indoors and outside.

Products include wood fibre insulation board, particleboard (PB), medium density fibreboard (MDF), hardboard (HB), plywood (PW), oriented strand board (OSB); and structural timber products including glued-laminated timber (glulam), laminated veneer lumber (LVL), laminated strand lumber and parallel strand lumber.

Non-structural and decorative panels may utilise a high proportion of recycled contents, and wood that is otherwise unusable (small diameter trees, thinnings, twisted or distorted trees).

Wood fibre products are made from softwoods, principally spruce and pine, but can contain by-products from any type of wood production. Developed as a means of using up waste from sawmills, it has evolved into an industry supporting architecture, interiors and furniture. These are non-structural timber products used as an alternative to timber or plywood. Wood waste from timber production is chipped and used to make particleboard, or further processed into fibres for insulation, MDF or hardboard. The production of bats and panels involves some other additives, such as waxes, plastics, dyes and other ingredients that help make the panels efficient to manufacture and resistant to water, fire and insects.

Plywood is a structural panel made with layers of veneer bonded together with high strength adhesive. The direction of the layers is alternated, and balanced, to produce a dimensionally stable product. A huge number of variations exists, with different thicknesses of veneer, wood types and cores; including both softwood (SWPW) and hardwood (HWPW) varieties.

Plywood is graded according to quality and visible defects, from A through D, by the American National Standards Institute (ANSI). A sheet of plywood will have two grades, such as AB, or BB, for example. The first letter refers to the ‘face’ veneer, while the second one is for the backside.
– A-grade is the highest quality and most expensive. It is smooth and easily painted. Also called clear-faced.
– B-grade is slightly less smooth and usually has minor flaws, which have been repaired or patched.
– C-grade has some knots and inclusions up to around 50 mm in diameter.
– D-grade sometimes called builders plywood, is the cheapest and has flaws and knots that are left exposed.

In addition, plywood is rated as interior (class 1), moisture resistant (class 2), exterior (class 3) and marine. This determines the durability of the wood (against rot, decay and insect attack), as well as the weather resistance of the adhesive. Marine plywood is the most durable, and can be twice the price. It may also be referred to as water boil proof (WBP) or boiling water resistant (BWR) – the only difference between these and marine grade is that the latter is guaranteed to be free from voids. Marine plywood is manufactured from timber that is graded as at least moderately durable (beech, eucalyptus and gaboon, for example), bonded with suitable resin, which is typically phenolic (PF) or a modified melamine formaldehyde (MF).

The layers (veneers) are either rotary cut or sliced. Rotary cutting produces a continuous sheet of veneer – the log is peeled with a long knife. It is the most economical method of production and used for all types of plywood, except decorative face layers. Where a certain grain pattern is desired, such as to mimic solid wood, the face layers is produced from sliced veneer. In this case, the veneer is sliced from the log as a strip – like a very thin plank of wood. This produces grain patterns that you would expect from plain sawn or quarter sawn lumber. The veneers are then laid out to make a larger sheet, side by side. The pattern is either book match (the veneers are opened up to create a mirror effect), or slip match (there veneers are placed alongside one another, same side up, to produce a repeat pattern).

Structural engineered wood products take advantage of all the benefits of wood – lightness, stiffness and strength – without the natural variation, anisotropy (it is much less strong perpendicular to the grain), and the maximum cross-section and length that can be achieved. Knots and other weaknesses are removed, or dispersed, through the product and so more reliable mechanical properties are achieved. Compared to solid timber, structural engineered timber can span larger distances unsupported, up to 40 m, and smaller sections can carry more load in a highly predictable way. As a result, it provides a suitable alternative to steel and concrete in many types of projects, from home construction to commercial buildings and industrial products.

The downside of these manmade composite materials is that they consist of synthetic adhesive, typically based on formaldehyde. Adhesive selection is important, because engineered wood and wood composites made with formaldehyde have been identified as a source of indoor formaldehyde emissions and reduced air quality.

Formaldehyde can cause irritation of the eyes, skin and throat. While most of the off-gassing from engineered wood happens in the first few months, high levels of exposure to formaldehyde may cause cancer – it is classified by the World Health Organisation (WHO) as a known human carcinogen. As a result of pressure from consumers and legislation on the industry, there has been a great deal of development to reduce emissions, such as ultra-low emitting formaldehyde urea formaldehyde (ULEF-UF). However, the fundamental chemistry remains the same and the panels are susceptible to hydrolysis. This means that even under normal conditions, humidity will cause the panels to off-gas formaldehyde freed by the presence of water.

The worlds most stringent regulations for formaldehyde release in wood-based panels include Carb Phase 2 in North America, F***** in Japan and AS/NZS in Australia. For example, Carb Phase 2, which was first introduced in California and then adopted across North America, limits emission to 0.05 ppm.

There are two main types of formaldehyde adhesive used in composite wood: phenol formaldehyde (PF) and urea formaldehyde (UF). PF tends to off-gas less formaldehyde, and it does iso over a shorter period, making it the preferred choice for interior applications – it can reach non-detectable levels within months. In some cases, manufactures label their product as no added urea formaldehyde (NAUF) to differentiate the two. A third type is based on melamine-urea formaldehyde (MUF), which is urea-formaldehyde modified with melamine to reduce off-gassing and yield a composite with superior water resistance suitable for exterior use and marine application.

Density of the composite will also affect how it off-gases, with higher density materials like MDF releasing the formaldehyde slower than particleboard for example. While MDF may take years to off-gas to non-detectable levels, particleboard may reach acceptable levels in a matter of months.

Alternatives binders exist, such as polyurethane resin (PUR) and soy-based formulations. If no formaldehyde is used in the adhesive then the panel is labelled as no added formaldehyde (NAF) and will be <0.04 ppm. It cannot be classed as formaldehyde-free, because wood naturally off-gasses formaldehyde, like all plants and animals. Wood releases enough formaldehyde (around 0.01 ppm) that a dwelling made of wood alone may not pass the strictest emissions requirements. The disadvantage of PUR is that it off-gasses methylene diphenyl diisocyanate (MDI). Isocyanates are also hazardous and negatively affect indoor air quality. It is not considered as harmful as formaldehyde, but not much of an improvement, either. There are many formulations of soy-based adhesive, such as based on isocyanate, each with their merits and drawbacks. They may not be suitable for structural and load-bearing applications, for example, and they may not be resistant to water and humidity. All of the timber used in these products will come from plantations - PEFC or FSC certified - the demand for manufactured boards outstrips production in some countries and so plantations elsewhere are used to bolster stock. The quality of management of the plantation affects whether they contribute positively or negatively to the local ecology. Like intensive farming, plantations can reduce the benefit to the environment trees have, diluting the positive impacts of removing CO2 from the atmosphere.