Timber

What is Timber?

• Timber, or simply wood, is the material making up the body of trees.
• Timber is widely used for its compressive strength, stiffness, toughness, accessibility and cost-effectiveness.
• As wood comes from trees, it is relatively sustainable, despite being difficult to recycle, as it is biodegradable and trees can be grown back.

Categorizing Wood

• There are lots of different types of wood, each with different properties.
• Timber can be categorized into:
  • hardwood
  • softwood
  • bamboo (bamboo is actually a family of grasses, but it shares many properties with woods!)
  • solid wood
  • man made/engineered timber
  • fresh wood
  • recovered/recycled wood

Hardwood

• Hardwood is a wood type that grows from deciduous trees, which means they are leaf-bearing and lose their leaves during the winter.
• This usually applies to trees that grow around the equator and moderate zones of the planet.
• As they grow slower, grains are denser and tightly packed, and they are harder to replenish/regrow.

• They are tougher and harder to manipulate than most softwoods.
• As the grains are denser, hardwood is less absorbent than softwood.

Softwood

• Softwood is a wood type that grows from pine trees, which means they carry needles and have leaves all-year.
• This usually applies to trees that grow around the subpolar and temperate zones of the planet.
• As they grow faster, grains are less dense and loosely packed, and they are easier to replenish/regrow.
• They are less tough and easier to manipulate than most hardwoods.

Bamboo

• Bamboo is not a wood type but can be considered one when designing.
• It is in fact a type of fast growing grass.

• They grow in moderate, subtropical and tropical zones.
• They grow even faster than softwoods, so they are even easier to replenish/regrow.
  • Some species of bamboo have been recorded growing up to 120 centimeters in a day!
  • Bamboos can spread very quickly and can even grow in sub-par soil.

• They are very tough and can withstand great tensile and compressive forces.
• They need specific manipulation to create sheets, rods and tubes for construction.

Engineered Timber

• Man-made, or engineered timber is modified in some way or for from the original timber.
• Unlike solid wood, which is cut from trees in one piece, man made timber consists of layers or particles of solid wood, that are embedded in glue.
• This is why they can also be considered a composite.

MDF Board

• MDF is particle based: wood dust is glued and compressed to a panel.
• It’s dimensionally stable and easy to manipulate, but lacks a lot of qualities and needs finishing.
  • MDF is often used for simple shelves and cupboards.

Chip Board

• Chip board consists of wood shavings, glued and compressed to panels. They have some benefits of solid woods, as it contains shavings with intact wood grains in it.
  • They can be used in flooring, and are often laminated to be used for cheap furniture, like IKEA kitchen panels.

Plywood

• Laminate/ply boards have the best properties of man made timber, as it consist of cross laminated sheets of solid wood.
• By alternating sheet (thus grain) directions, it increases in strength and rigidity.
• Different types of wood (hard and soft) can also be alternated to achieve better results.
• Laminated board is applied to flooring, roofing, structures, stands etc.

Wood Life Cycle and Treatment

Harvesting and Treating

• Growing rates differ between tree types.
• Harvesting/cutting is/can be performed illegally, mostly in tropical areas.
• When done more sustainably, every cut tree is replenished at an equal rate (reforestation).
• Cutting then happens selectively and responsibly, so no fields/forests are cleared all at once.
• FSC is a stewardship label that guarantees the origin of the wood.

• Harvested wood needs to then be cut from gross sizes to net sizes.
• These are internationally standardized.
• Theoretical net sizes can vary (tolerance) a few mm in reality.

• Cut wood needs to go through a drying process to achieve its EMC (moisture balance).
• Wood can be ((semi-)open) air dried and/or kiln dried ((non-)heated).
• Drying happens to release free and absorbed moisture.
• EMC level means the wood is dimensionally stable and predictable in use.

Distortions

• Wood has natural and artificial ‘defects’, like knots, resin, bowing, cupping, twisting and warping.
• Knots and resin are naturally present elements in wood, as they refer back to their origin as a tree.

• Warping, cupping, twisting happens due to undesired growth direction inside the wood, along with imperfect conditions during drying/kilning.
• This is why only perfectly straight trees are selected for A-class wood.
• B quality wood will see more knots and deformations.
• Resin tends to be a bigger problem in soft woods than in hard woods.
• Knots, caused by branches, form holes in the grain of the wood, and can fall out, leading to unwanted holes.

Wear on Wood

• Untreated wood is more subjected to wear and tear, and weather conditions.
  • Impacts, UV and moisture affect wood in multiple ways, basically causing rot and deterioration.
• The correct treatment/finishing happens for aesthetics and protection.
• Wood can be sanded, cut, glued, painted, varnished, oiled, waxed, pressure treated and creosoted (fully impregnated wood e.g. garden fencing, furniture, decking, etc…).
  • Paints, varnishes and creosoting adds polymer based chemicals to the wood, making them harder to recycle.

Recovering Wood

• For a high wood recycling rate, a number of things need to be optimised:
  • Wood recovery
    • Used/discarded wood needs to be recovered, collected and sorted correctly.
    • Unrecovered wood will decay or be incinerated.

Joining Methods

Nails

• An old fastening method, using a smooth metal pin that is hammered into the wood to secure two parts together.
• Hard to undo, can slide off over time, allows rotation of parts.

Screws

• Threaded, tapered ‘nails’, a spiralling outer rim drives the screw into the wood, joining one or multiple parts together.
• Easier to undo, does not allow slipping and to an extent rotation.
• Screws have different types of heads and a pointy tip to drive into the wood (or plastic, metal etc.)

Nuts and Bolts

• Also threaded, but only to be connected with a nut on the other end of the wood, not to drive into wood.
• This secures panels of wood, without damaging the wood.

Wood Glue

• Strong glue that creates very strong bonds between wooden parts.
• Hard to undo, irreversible.

Wooden Joints

• A large range of wood joints exists, ranging from slotting to dowel joints, finger joints, mortise and tenon joints, etc.
• Disassemblable, but can also be secured with supportive glue.

Metal fittings

• Metal fittings can be used for additional support.

Glossary

Wood Types

• Hardwood
  • Wood from a deciduous tree.
  • The slower growth of the tree makes the wood more dense and stiff.
• Softwood
  • The wood from a coniferous (evergreen) tree.
  • The faster growth of the tree makes the wood less dense and softer.
• Laminated boards
  • Sheets of material made from layers of veneers (e.g. plywood)
• Man-made timber
  • Also known as engineered wood or composite wood.
  • Wood products that are made by binding or fixing strands, particles of fibers, veneers of boards of wood together with adhesives or other fixing methods to create composite materials.
  • Typical examples include MDF, plywood and chipboard.
• Particle boards
  • A material made from different sizes of wood chips and joined with glue.

Properties

• Absorbed moisture
  • The moisture within timber that is contained in the cells walls.
• Equilibrium Moisture Content (EMC)
  • EMC the moisture content where wood achieves an equilibrium with the environment
  • Can be affected by humidity and temperature.
• Free moisture
  • The moisture within timber that is contained within the cell cavities and inter-cellular spaces.

Treating

• Air-drying
  • Air-drying places the stacks of sawn timber in the open or in large sheds hence there is little control over the drying process.
• Creosote
  • A material that penetrates the timber fibers protecting the integrity of the wood from from borers, wood lice and fungi.
• Kiln-drying
  • Kiln-drying places the stacks of cut timber in a kiln, to reduce the moisture content in wood, where the heat, air circulation, and humidity is closely controlled
• Kiln-seasoning
  • Thermally insulated chamber, a type of oven, which produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes.
• Lamination
  • Covering the surface of a material with a thin sheet of another material typically for protection, preservation or aesthetic reasons.
• Seasoning
  • Seasoning is the commercial drying of timber which reduces the moisture content of wood.

Distortions

• Warping
  • A distortion in wood caused by uneven drying, which results in the material bending or twisting.
• Bowing
  • A warp along the length of the face of the wood
• Cupping
  • A warp across the width of the face of wood, in which the edges are higher or lower than the center.
• Knots
  • Imperfections in timber, caused by the growth of branches in the tree that reduces its strength.
• Twisting
  • A distortion in which the two ends of a material do not lie on the same plane.

Wear

• Dry rot
  • When timber is subject to decay and consumption by fungus.

Sustainability and Environment

• Reforestation
  • Reforestation is the process of restoring tree cover to areas where woodlands or forest once existed.
  • If this area never returns to its original state of vegetative cover the destructive process is called deforestation.
• Wood recycling
  • Wood recycling is the process of turning waste timber into usable products.
  • Recycling timber is a practice that was popularized in the early 1990s as issues such as deforestation and climate change prompted both timber suppliers and consumers to turn to a more sustainable timber source.
• Wood treatment
  • Treatment of wood can involve using solutions, which make the wood poisonous to insects, fungus, and marine borers as well as protecting it from the weather.