Timber

(from the Whole House book, CAT)

Introduction

It is estimated that across all the industrialised countries, timber frame accounts for 70% of all housing stock, representing some 150 million homes.  Britain shows the lowest percentage of timber frame houses, but conversely the greatest potential for increasing market share.

Fig 7:27  from: ‘Timber Frame Housing’, Hastoe Housing Association

Environmental advantages

Timber frame is much more prevalent in the ‘eco’-housing sector, and the environmental benefits of timber construction are many: 

·        Timber is the only renewable structural building material, Timber has a low embodied energy, compared with other structural building materials.                                                                                       

·        Because growing trees absorb carbon dioxide, harvested timber can be seen as a ‘carbon sink’, locking up the CO₂ which the tree has absorbed, until it is burnt or rots away. One kilogram of dry timber contains about 50% carbon, which binds in 1.8kg of CO₂. If the forests that are harvested are then replanted, timber becomes a carbon-neutral material.  The growing tree will take up as much CO₂ as the harvested one will eventually release.

·        In general, it is easier to achieve higher insulation values, and therefore lower energy use, with a timber construction than with a masonry one. It is difficult to increase the cavity depth in a masonry wall substantially, without extra-long ties or additional structural support. As the Building Regulations call for higher standards of energy efficiency, so contractors and developers are turning to timber frame construction to minimise extra works and cost.

Structural and Performance Advantages

·        Contrary to many people’s expectations, timber performs well in fire as it burns steadily at a predictable rate. In the process charcoal is formed on the surface of the timber, which serves to insulate and protect the core. It is possible therefore to make precise calculations of the dimensions that structural timbers need to be, in order to hold the building up for half an hour, or an hour, to allow the occupants to escape. By contrast, steel structures behave unpredictably in fires, appearing to be unaffected and then failing suddenly. 

·        Timber buildings are inherently durable and easy to maintain.  In the U.K. there are many examples of timber frame buildings still standing, from the 15^th century onwards. The oldest timber framed building in the U.K. is an 11^th century stave church in Essex, and the oldest building in the world is the Horiuji temple in Japan, built in AD 607 out of cypress.

Homegrown Hardwoods

About one-third of the six million cubic metres of timber produced annually by British forests is hardwood such as ash, beech, birch and oak, the majority of which is used for furniture. There are, however, two species generally considered suitable for structural purposes: oak and sweet chestnut. These are the traditional materials used in timber framed houses in Britain, and they are very strong and long-lasting.

Usually, these hardwoods are worked on and incorporated into a building while still in their ‘green’ or unseasoned state. As they dry out, many timbers have a tendency to warp or twist. Oak is especially susceptible to this (hence the wavy beams characteristic of many old buildings). However, this drawback can be overcome by the use of ties and braces at frequent intervals.

There is no question of their needing chemical timber treatment - indeed the structure of the heartwood is so dense that no preservative would penetrate very far.

Homegrown softwoods

These timbers, mostly spruce and pine, are used in the main for pallets, packaging and fencing, and for paper and chipboard manufacture. The poor quality of timber that is acceptable for such end uses goes hand in hand with a system of forestry oriented exclusively towards short-term financial return. Here, trees are planted close together in vast monocultural plantations where few other species can thrive. They are left unthinned and then harvested while the trees are still immature by clearfelling whole areas at a time, leaving a barren desert of stumps. The acidification of the soil resulting from such dense conifer plantations means that few other plants can start to grow before the new rotation cycle begins. Still, blanket spraying of plantations, often with organophosphates, to treat disease or suppress growth before planting, is common practice.

Where homegrown softwood is used in the construction industry, it is mainly for carcassing - studs, joists and roofing timbers - and is assumed to be relatively poor quality and non-structural. Chemical preservatives are relied upon to give it any sort of useful life. But it is possible to look beyond this scenario when searching out suitable timber for structural use.

European Larch and Douglas Fir are excellent framing timbers which grow well in our climate.  The heartwod of Larch is classified as ‘semi-durable’ by the Timber Research and Development Association (TRADA).  Both these softwoods are naturally dense, resinous and therefore relatively rot- and insect-resistant.  In our experience at C.A.T. they can be used without the need for chemical timber treatment, which cuts down on both cost and environmental impact. Once seasoned, they are also suitable for use in doors, window frames and floorboards.

Imported Timber

In the U.K., we import 87% of our timber requirements, mainly from Canada, Sweden and Russia. Only 10% of the 6 million m³ of sawn softwood used yearly by the U.K. construction industry is home-grown. Indeed, timber is the second most widely traded commodity in the world, second only to oil.

The energy costs of transporting such a heavy, bulky material are enormous, ranging from 0.1GJ per tonne from Sweden to 2.4GJ per tonne from Papua New Guinea (Multiply these figures by 278 for kWh). The financial costs of timber transportation can likewise be considerable.  An increase in local timber production and marketing could not only save on the costs of imports - some £5 billion a year of the U.K. trade deficit in 1997 - but also reduce the environmental impact of timber, and promote an important rural industry.

Environmental costs in the producing countries include tropical deforestation (2.5m hectares of tropical forest were lost in 2002) and loss of livelihood for indigenous peoples. It is true that in tropical areas, more trees are felled for agricultural development geared for export, than for timber harvesting. Nevertheless, the extraction of valuable trees for export (an extremely wasteful process, involving the felling of huge numbers of ‘noncommercial’ trees) serves to open up virgin areas for further clearance. Remaining small pockets of trees are often are not ecologically viable.

Importation of illegally felled timber is a major impediment to establishing sustainable timber industries in producer countries.

The U.K. is one of the worst offenders, and knowingly supports the illegal timber trade in Indonesia, by importing large quantities of tropical plywood, door blanks and paper pulp – a trade worth around £128m. The illegal timber industry in Indonesia inevitably brings with it corruption and human rights abuses.

Friends of the Earth who have produced the table below

7:50 Figures produced by Friends of the Earth, published in Sustain mag vol 2, no 2

Timber Treatment

The assumption that all structural softwood used in buildings needs to be chemically treated to protect it from insect or fungus attack, has become established for a number of historical reasons and now needs radical re-evaluation.

London Hazards Centre argues, ‘the adoption of chemical timber treatment as the norm is directly encouraging bad design practices ... and gives a false sense of long term security which may encourage inadequate maintenance’.

·        By contrast, the only sure way to avoid timber decay is to use good quality, well-seasoned timber, to ensure that the building design provides protection or ventilation of all timbers, and to institute regular inspections of all vulnerable timbers and adjacent building fabric. It

·        The main formulation used in this context is a mix of chrome, copper and arsenic (CCA), first produced in 1933.

Once timber is chemically treated the process is irreversible and a natural, healthy material is turned into a toxic product, and destined to become toxic waste. Tests conducted in the USA on soil under CCA treated decks or verandahs showed arsenic levels 80 times greater than the surrounding soil and 35 times greater than the legal limit. Subsequent studies have shown an increased cancer risk for children using playground equipment made from treated timber, because of exposure to arsenic.

Wood preservation through heat treatment is being pioneered in Finland and one U.K. timber supplier is importing Finnforest ‘Thermowood’ profiled cladding boards. The timber is treated in special kilns at temperatures of 200°C, which reduces equilibrium moisture content by 50%. The result is stable and durable timber with a high resistance to fungus attack. The enhanced durability applies to sapwood and heartwood, with surface coatings lasting up to five times longer.

Timber certification

·        Even with renewable resources, we need guarantees of sustainable production, including replanting first and foremost, but also the promotion of biodiversity, soil  and water quality, and the rights of local people. Timber certification by bodies such as the Forest Stewardship Council (FSC) offers guarantees of sustainable production for consumers, as well as an acceptable future in international timber trading for producer countries.

The FSC publishes a set of criteria – Ten Principles of Forest Stewardship – on which its certification scheme is based, and which incorporates environmentally appropriate management, social benefit and economic viability. Genetically modified organisms are prohibited and the use of chemicals is minimised. Regular inspections of certified forests are carried out by independent organisations accredited by the FSC, which is represented in 28 countries.

The FSC relies on consumer demand to provide the incentive for forest products manufacturers to seek certification. Their label is now on thousands of ‘Home and Garden’ products in U.K. supermarkets and DIY stores, including items such as garden furniture, doors and flooring. For construction timber, it is worth doing a ‘product search’ on the FSC website [www.fsc-uk.org]. There are now nearly 40 million hectares of FSC certified forest worldwide, and over one million hectares in the U.K., representing more than 70% of all U.K. commercial forestry.

Preference

¥          Use reclaimed timber where appropriate.

¥          Use homegrown certified timber, either hardwood or semi-durable softwood, and be prepared to spend some time searching out suppliers.

¥          If your source of homegrown timber is not certified, try and satisfy yourself that the forestry management practices used to produce your timber are acceptable. Encourage your supplier to seek certification.

¥          If using imported timber, try to source FSC certified. Scandinavian timber has the lowest transport energy costs.

¥          Support retailers committed to supplying sustainably produced products, such as B&Q, Do It All, and Homebase.

¥          Reduce consumption and wastage, by careful ordering and site practice, and by innovative use of smaller section timbers (e.g. spaced studs – see Section…).

¥          Use no chemical treatment. Instead, choose well-seasoned durable or semi-durable wood. Ensure adequate ventilation and regular inspections.

¥          If you decide that some treatment is necessary, use boron-based products.

¥          Avoid pre-treated joinery items such as windows and doors.

¥          If you are having a property surveyed, try to find a genuinely independent surveyor who is sympathetic to non-invasive and non-chemical treatments (AECB).

¥          If you are concerned about a particular pesticide or want more information, contact the London Hazards Centre or the Pesticide Action Network <www.pan-uk.org>

Timber products

Composite boards

an extremely economical alternative to solid timber. In some instances their manufacture uses the waste products of felling and their use in construction means there is less demand for virgin timber in low grade applications.

However, they do take more energy to produce (about 15 times that required to produce rough sawn timber) and there are health hazards associated with the glues and binders used in their manufacture.

Plywood

This is made by soaking the whole log and peeling off thin layers or veneers, which are then dried and glued together using formaldehyde or, more recently, isocyanate resins. The finished sheet can contain as few as three veneers (3-ply) or as many as 15, and the grains of adjacent layers are laid at right angles to each other. WBP board (weather and boil proof) used to refer to the type of glue and meant it was suitable for external use. This classification has been superceded by new European standards and there is now no direct equivalent to WBP. Instead, the bonding classes for veneer plywood are specified according to their end uses. Class 1 is for dry conditions, Class 2 for humid, and Class 3 for external.

There are numerous grades of plywood:

¥          Shuttering ply is the cheapest grade. It is used for formwork and made with poor quality softwood, but it is often the best environmental option where a good quality finish is not important.

¥          Middle-of-the-range ply, often used for shelving, counters and internal doors, is usually made from tropical hardwoods and has a characteristic reddish colour. The bulk of the tropical hardwood we import is in this form (over one million cubic metres annually) and should be avoided completely. There is no justification, apart from cost, for using tropical hardwoods in board manufacture, and in environmental terms it is scandalously expensive. Importing and using this plywood makes a substantial contribution to the destruction of the tropical rainforest.

¥          Marine ply is top of the range plywood. It is usually made from non-sustainably harvested tropical hardwoods such as mahogany or gaboon, and resorcinol glues which prevent it from delaminating even in very wet conditions. Also available in this price range - and a good ‘eco’ alternative - are decorative veneered boards such as oak or birch-faced ply.

¥          Blockboard or laminboard is basically a variation of plywood used largely in furniture and shopfitting. Between the two outer veneers - either hardwood or good quality softwood - is a core of small section softwood. The same synthetic resin glues are used.

Oriented Strand Board (OSB)

Commonly known as Sterling Board after one major  manufacturer, this is made from wood shavings or ‘strands’, oriented in a random fashion to give maximum strength, and glued together under heat and pressure. It makes efficient use of low grade timber and is used mainly as an alternative to shuttering ply. It can also be used for flooring, in a tongue-and-grooved (T&G) pre-sanded version.

Chipboard

Suitable for internal use only, chipboard is made from small particles (or ‘chips’) of wood, bound together with urea-formaldehyde glue. Although chipboard manufacture ought to be an appropriate end-use for forestry waste, most manufacturers insist on using the whole log and even on logs of a minimum diameter. Chipboard manufacture can cause extremely unpleasant air pollution, with local residents complaining of skin irritation, breathing difficulties and an increase in cases of serious asthma.

Because the particles of wood are so small that they cannot impart much strength to the board, correspondingly more glue is necessary, and this will typically form 7%-10% of the board’s weight. A low-formaldehyde chipboard, marketed as Kucospan, is being imported from Germany, but it is expensive.

Fibreboards

There are two basic types of fibreboard:

¥          Medium Density Fibreboard (MDF) is manufactured with urea-formaldehyde as the bonding agent, which accounts for 14% of the board’s weight. It is commonly used for internal finishes, where off-gassing from formaldehyde-based glues is most dangerous due to lack of ventilation. Workers involved in the cutting and shaping of MDF for furniture, fittings and theatre sets, are thought to be at particular risk from inhaling dust. There is now a zero-formaldehyde MDF (Medite ZF) available, but it is more expensive.

¥          Softboard, Mediumboard, and Hardboard are all made by felting wood fibres and then bonding them into a sheet using heat, pressure and the wood’s own resins. No synthetic glues are needed and the use of this type of fibreboard is associated with a very low environmental impact.

Softboard can be used as pinboard and to increase the insulation value of a wall. Impregnated with bitumen, it can be used for sheathing timber framed walls, providing racking resistance temporary weather protection and vapour permeability. Medium board can also be used for sheathing, but do not confuse it with MDF, which is much more common. Hardboard can be used for internal flush doors and as a smooth, high quality finish over shuttering ply for desks, counters, shelves, etc. As ‘Masonite’ it can be used structurally, and in its oil-tempered version it is moderately moisture resistant.

The Trade Association for the Fibreboard Industry, FIDOR, have a full range of information and can help with technical problems.

Alternative fibreboards

Non-wood fibres such as flax, hemp, or sugar cane fibre can be used for board manufacture. Boards made from flax shiv are currently imported from Belgium at the rate of 50,000m³ a year for use as the core material in internal doors.

Stramit or strawboard, which has been made in this country since the 1940s, is still being used for self-supporting internal partitions, doors and wall/ceiling panels. It has National House Building Council (NHBC) and British Standards (BS 4046) approval.

Tectan board is made in Germany from pre- and post-consumer-waste, from drinks cartons. It consists of 75% paper, 20% polyethylene and 5% aluminium. The waste is shredded and heated to a point where the polyethylene melts and acts as a bonding agent. No toxic glues are used.

Re-use/disposal

The more durable boards, such as plywood, can be re-used several times over. This is made easier by screwing rather than nailing boards together, and by the use of release oil on formwork. If they are burned, harmful gases such as hydrogen cyanide from isocyanate resins may be produced. If dumped on a landfill site, the timber will biodegrade but the constituents of the glues will remain in the ecosystem.

Synthetic resins

Common to the manufacture of most of these boards is the use of a synthetic resin binder which turns relatively weak and flimsy materials into strong, rigid sheets. Often this resin is a form of formaldehyde, which is a known animal carcinogen and a probable human carcinogen, according to the World Health Organisation (WHO). As a constituent of internal fixtures or finishes, it can off-gas inside a building where the amounts may be small, but the exposure can be long term and constant. It is a major source of indoor VOC emissions.

Recommended maximum exposure levels vary from two parts per million (2 ppm) for occupational exposure in the UK, to 0.1ppm for the general population, as advised by WHO. There are no recommended maximum levels for exposure to formaldehyde in domestic situations in the U.K., although it is acknowledged by the Department of Health that homes with new carpets or furniture could have levels of up to 0.3ppm – or three times the WHO general population limit .

Painted or melamine covered boards will off-gas much less. Unsealed boards in warm spots, such as near a cooker or immersion heater, are particularly dangerous. Urea-formaldehyde contains more of the hazardous ‘free’ formaldehyde, which has not chemically bonded with the timber. Phenol and resorcinol formaldehydes tend to be more stable.

Over recent years, the industry has begun to respond to Health & Safety concerns by reducing the quantities of formaldehyde used. An alternative binder, methylene bisphenyl di-isocyanate (or MDI), is beginning to replace formaldehyde. It is now used in over 20% of OSB production worldwide, and in some MDF production in North America and Europe. It is a very efficient binder and so accounts for only 3% of the board's weight. However, it is more expensive and has been associated with respiratory problems and skin irritation suffered by workers in the industry.

The production processes of these glues is energy-intensive and polluting, and the raw materials, mainly oil or gas, are non-renewable resources.

Preference

¥          Use sheet materials only when the use of solid timber would be extravagant or inappropriate. For decorative internal finishes such as floors, skirtings, doors and shelves, natural wood is always preferable to composite boards for reasons of health and aesthetics.

¥          Use plywood or OSB rather than chipboard or MDF. Plywood probably contains the least glue and so is the best environmental option, although it cannot be made from waste products.

¥          Avoid all tropical hardwood ply. It is possible to find a plywood for all applications made from softwoods or temperate hardwoods. Always assemble and install with a view to facilitating re-use.

¥          Avoid chipboard and MDF as much as possible, unless you are prepared to search out, and pay for, the low to zero formaldehyde versions.

¥          Maximise the use of the naturally bonded wood fibreboards.

¥     Seek out and experiment with non-wood fibreboards.

Composite/Laminated Beams

We have probably all seen examples of laminated beams, dramatically arching across a school hall or sports stadium, spanning a distance that would be impossible with conventional timber. They are made from small sections of softwood, glued side by side and end to end with staggered joints,

. The energy cost for the production of a glulam beam is one-sixth that of a steel beam and one-fifth that of a concrete beam of equal strength.