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Timber

January ’13

Advantages of timber

FSC logo

Constructing with FSC timber (or another accreditation organisation such as PEFC) is potentially one of the greenest ways of building for several reasons.

  • Timber itself captures and locks up CO² as it grows and keeps it that way till the end of the building’s life, in fact until the timber is burnt or rots away when it releases the CO² into the atmosphere
  • Timber has a very low embodied energy. It basically consists of the energy to process the timber and deliver it. Even when imported it travels mainly by ship which is an advantage in terms of embodied energy. This contrasts sharply with the energy that goes into making bricks, blocks, metals, plastics etc.  Embodied energy figures for timber are available on the ICE website. Typically timber is 8.65 MJ/Kg. However the figures on the ICE web site don’t differentiate between timber which on the one hand may have travelled thousands of miles and been kiln dried, and on the other hand may be locally grown timber used green such as with some types of oak frame. There don’t yet seem to be studies which compare the two.
  • It takes up very little of the wall thickness of a building compared with, say, blocks or brickwork so it allows more space for insulation.
  • Timber is structurally very efficient, especially when in the form of engineered timber.
  • Extremely few hazardous chemicals need to be used in timber construction.
  • The waste from timber manufacturing is useful rather than being a problem. For instance it may be used for manufacturing MDF or for powering the sawmill and kilns or be converted into wood pellets for heating. At the worst it may go for paper making.
  • It can be reused several times. For instance, many of the ancient houses in the Shambles in York, which attract so many tourists, actually incorporate structural timber member which previously came from dismantled ships.
  • On your building site there doesn’t need to be much wastage of timber. Short offcuts can usually be used for noggins or bits of outbuildings or landscaping features. Or finally to make the compost heap.

Accreditation

Since the introduction of the FSC (Forestry Stewardship Council) Woodmark scheme and PEFC accreditation it is possible to verify that any given piece of timber has been sourced sustainably. This is because it relies on a ‘chain of custody’ scheme which follows the timber from where the tree was felled right through to the point of final purchase and stamps each piece of wood as it is sawn into smaller pieces or put through a manufacturing process. This system works well in practice as can be seen, for instance, from the way that B&Q source their timber. Various other timber merchants are now becoming serious about the Woodmark.

Timber from tropical rain forests is a tricky matter.  See for instance the FOE report about timber sourced in Malasia on this.

Currently EU legislation is being introduced to prohibit the sale of illegally logged timber. See EU Timber regulation coming into force 3rd March 2013.

Why is timber house building so neglected in the UK

The UK seems to have a rather backward, or at least over-cautious attitude towards timber construction and this is outlined here.

Grading timber

There are tens of thousands of species of hardwood in the world and several hundred species of softwood so it’s a big subject.
Timber is mainly graded in two ways:

  • visual qualities
  • strength

Traditionally it has been initially based on a hardwood / softwood distinction with hardwoods only being graded according to visual qualities and softwood according to either visual or strength (depending on whether it was to be used for structural work or furniture/visible joinery).

The traditional ways of grading timber have been rather inconsistent and are being replaced by European standards. You might be confronted by either the new or old standards (or both) for some time to come.

The old standards were based on the exporting country’s historical standards.
The hardwoods tended to follow the American system and the softwoods mainly the Nordic system with a bit of the slightly different Russian system thrown in.

American National Hardwood Lumber Association. The US tends to export high quality hardwood in the two combined grades of firsts and seconds (FAS). Below that come the Common grades. This is also used quite generally by UK timber merchants and there are several web sites with excellent images of different timber grades. Try John Body.

The Swedish / Nordic grading of softwoods. These systems are very similar and divide the boards into 7 visual grades. The first four top grades are usually lumped into one known as unsorted (US). Grade 5 is the sort of thing knotty pine furniture is made of, grade 6 is about fencing grade and grade 7 is rougher stuff suitable for packaging or crates.

The timber may also be stress graded for strength when it enters the country. The Swedish grades tend to be SC (strength class)14, 18, 24 and 30 whereas it is more normal in the UK to use SC 16 and 24.

The best softwoods available in the UK are usually collectively called ‘Upper Gulf’ which means they come from areas like northern Sweden, Finland, northern Russia etc. around the Gulf of Bothnia (northern part of the Baltic sea). Timber which is grown in cold climates, especially if it is on the north facing side of a hill, grows more slowly and hence more densely with a higher resin content. It is much stronger and also more durable than timber grown in the UK. There also happen to be a lot more of the older larger trees in these areas than we have in the UK.

The Russian softwood grading system (GOST), (which tends to tie in with Finnish export grades) is slightly different from the Swedish/Nordic one in that it has 5 grades. The top three grades usually come as ‘unsorted’. (Russian 4ths is roughly equivalent to Swedish 5ths) They have slightly different grading for pine/larch and spruce/fir. Stress grading is normally carried out after the timber has reached the UK.

The new EU rules for grading can be found on the Fordaq site along with several other national grading systems. It gives some explanation about how the old standards relate to the new ones.

The naming of softwood timber by suppliers can be a bit confusing

Generally the following holds true:

Redwood means Scots pine (Pinus sylvestris). It may also be called red deal or yellow deal. Pine is generally stronger and heavier than whitewood.
Whitewood means Spruce, Fir, Douglas fir and Larch
sawfalling generally refers to mixed grades of whitewood. (rather than ‘unsorted’)

Lumber is an American term not used in the UK

Sawing your own timber

Although it will not usually pay to cut and saw your own softwood structural timber, it may well make sense to process hardwood for some joinery and built in furniture, possibly stairs and flooring. If you should decide to have your own timber sawn, either because you have access to felled local timber or you have a local sawmill then there is the opportunity to be a bit more creative with your grading

Visual qualities

Here are some of the things that might influence your eye when looking at timber:

  • Uniformity. Some people want total uniformity in terms of colour, texture etc. Almost as if it were computer designed and machine made. At the other end of the spectrum are those who relish variation, character and contrast. This shows up best in the distinction between sapwood and heartwood. Some people like a mix and others want to separate them
  • Flaws. What people call a flaw in timber can range through the following:
    • knots (alive and dead) Knots are incredibly subjective: They are a natural part of the growth of trees but some people seem to find them offensive and want what is called “clear” timber, ie. without knots. However the knots and burr found in walnut veneer are considered beautiful. Much of this can be traced down through the class system of Europe and its predeliction for demanding what was elitist and difficult to produce unless labour was cheap. Live or firm knots can be very attractive whereas dead knots (which tend to be of two types (loose and rotted) are a bit more difficult and tend to need polyester filler to fill the gaps, (- unless you don’t mind gaps). Knotty pine is usually the Swedish 5ths grade where small live knots occur about every half metre.
    • splits, also known as shakes, if you accept them, are best dealt with using GRP filler with a slightly darker tone than the timber around. Otherwise the board can possibly be resaw along the lenth of the split to effectively reuse it. This is sometimes an option on wide split boards.
    • discolouration due to fungal growth. Interesting one. Habitat furniture did a very smart range of furniture some time ago using Scots pine which had discoloured blue veins caused by fungal attack. Briefly trendy but hard to get right.
    • lead shot from gun fire lodged in timber (there’s more of this than you might think due to pheasant shooters missing pheasants and hitting trees. Most woodland in the UK is at present used primarily for pheasant shooting in economic terms. In fact it is hard to find a piece of woodland that is not shot)
    • resin pockets. Many timbers such as cherry naturally exude resins which cause slight marks on the surface. They may interfere with applied finishes such as varnishes but otherwise can add attractive character.
    • woodworm and other creepy crawlies such as wood wasps. Providing the infestation is not active this may not be much of a problem with items such as floors but could ruin the look of furniture even if carefully filled.
    • sticker marks which come from the way timber was stacked when it was drying. This is quite hard to handle aesthetically because of the straight cross line patterns that show through. This is one of the problems with sycamore which is a very light timber, almost white. It has to be air dried by boards being stood on their ends under cover without the boards touching. After that it can be kiln dried using nylon stickers: wood stickers leave marks which go quite deep into the timber.

Strength

If you should decide to process your own structural timber then if necessary you can employ a stress grader to grade timber visually. They will assess each piece and stamp it accordingly. This will normally be to SC16 or SC24 to meet with Building Regulation strength requirements. There are also requirements regarding the moisture content of the timber during grading. See more here. It will normally only make economic sense to employ a grader if you have enough timber to warrant  calling them out for at least a full day’s work. To find a certified strength grader ask around local timber yards. They usually know who is operating in the area.

Qualities of timber

strength

Loosely used, the term ‘strength’ of timber can mean three different things

  • tension. (this is mainly the Strength Class grading which is used in the building regulations Part A ( and see comments on span tables here), mainly for floors, ceilings, roofs and structural members. It is usually C16 or C24)
  • shear. this is not usually a major problem with timber but structural elements such as roof trusses are checked by an engineer
  • hardness. This is mainly to do with damage on surfaces such as floors. The effects of impact and point loads on floor boarding (and to a lesser extent other wearing surfaces) need considering. For instance a pair of high heal shoes exerts a massive pressure and can be quite damaging to the surface of flooring. This is where the hardness of timber (rather than strength) needs considering. It is measured on the Brinell scale with the following values being typical
  • Whitewood 1.2
  • Scots pine 1.9
  • Norway spruce 2.3
  • Birch 2.5
  • Alder 2.5
  • Douglas fir 2.7
  • Maple (European) 3.2
  • Pear 3.2
  • Mahogany 3.2
  • Larch 3.2
  • Cherry 3.5
  • Teak 3.5
  • Walnut 3.5
  • Hornbeam 3.5
  • Red Oak 3.7
  • Beech 3.8
  • Oak 3.8
  • Rose Gum 4.0
  • Ash 4.1
  • Wenge 4.2
  • Sapele 4.2
  • Rosewood 4.6
  • Maple (Canadian) 4.8
  • Merbau 4.9
  • Jarrah 5
  • Kempas 6.0
  • Jatoba 7

fire resistance

The degree of fire resistance necessary for various parts of a house is given in the Building Regulations. This covers both internal and external aspects of the building. The internal part is divided into -

The external part is extremely complicated in the way it deals with the exposure of different wall situations

essentially timber is fire proofed in two main ways

      • for structural timber members the thickness is slightly increased. Timber tends to form a layer of char as it burns and this slows down further combustion. This is all built in to the design of timber structures so that the time it takes before a fire causes structural collapse is carefully calculated
      • for fire spread, which is to do with how quickly fire can spread across the surface of a wall or ceiling, there are rules about how much timber can be used in various situations. It is possible to treat the surface of timber to achieve a better rating. For instance timber boarding can be upgraded quite easily to class 1 (required for most wall and ceiling situations) using intumescent varnish.

thermal insulation

while timber is a reasonable insulator (K value about 0.14) it is not as good as people sometimes assume. Until recently, timber used in external walls was ignored for thermal calculation purposes but it is now considered to constitute a thermal bridge and has to be included in the calculations. This is one of the reasons for the recent increase in the use of timber I beams which have a thin web.

durability

This is quite a complicated issue because it takes into account the timber species

the ratings are -

      1. very durable – European oak
      2. durable – Western red cedar
      3. moderately durable – Douglas fir, European Larch ( and Western red cedar – home grown)
      4. slightly durable – Scots pine, Norway Spruce, Sitka spruce
      5. not durable – sapwood of all the above

It also depends on the cut from the log, whether it is heartwood or sapwood. (In most cases sapwood is not considered durable unless chemically treated)

The hazard (or use) class is taken into consideration (to what extent the timber is exposed to water and humidity) -

      1. when the wood is under cover, not exposed to the weather and wetting.
      2. when the wood is under cover and not exposed to the weather but where high environmental humidity can lead to occasional, but not persistent wetting
      3. when the wood is not under cover and not in contact with the ground. It is either continually exposed to the weather or is protected from the weather but subject to wetting
      4. when the wood is in contact with the ground or fresh water and thus is permanently exposed to wetting moisture content 20% or more
      5. when the wood is permanently exposed to salt water

The durability class and hazard class can be combined to check whether preservative treatment is necessary -

  Natural durability is always sufficient and there is no requirement for preservative treatment
  Natural durability is normally sufficient in these conditions, but for certain uses where condensation may be severe, preservative treatment is advised
  Natural durability may be sufficient in these conditions, but depending on the wood species,its permeability and end use, preservative may be needed
  Preservative treatment is normally advised in these conditions but natural durability may be sufficient in some cases
  Preservative treatment is always necessary in these conditions

From the above table it can be seen that Douglas fir and larch are suitable for external cladding provided only the heartwood is used. It also shows that Scots pine, Norway Spruce and sapwood generally is suitable provided severe condensation is not likely or it is not being used in a very exposed situation. Highly exposed situations in the UK are mainly towards the West side of the country (rather than the North) as can be seen from the exposure map in the Building Regulations. This ties in with building practice in Norway where lower grade timber is often used for external cladding, albeit often with a moisture permiable surface coating in exposed areas.

Preservatives. Timbers vary considerably as to how easily they take up preservative. There are four classes of treatability ranging from difficult to easy.

1 – very resistant
2 – resistant
3 – moderately resistant
4 – nonresistant

Wikipedia have an excellent page on timber treatment, although it is based on the US market.

There’s a useful durability guide by Inwood which gives ratings for over 150 species.

Remedial timber treatment products suitable for use in bat roosts. Natural England have an extensive list of suppliers of preservatives.

UV and oxidation. Ultra violet light does change the colour of timber. For instance Larch cladding goes from a typical softwood colour to a sort of silvery grey within a couple of years in sunlight (and probably to do with oxidation as well). With oxidation (which is simply the affect of oxygen in air on timber) some timbers darken a good deal, particularly Douglas fir and many other softwoods will go to a rich brown from the almost honey yellow colour when new. Neither of these two effects (uv and oxidation) significantly reduce the durability of timber

workability

  • sawing. This is now almost a historical problem. It is still quoted a lot but modern saws with tungsten carbide teeth do not struggle with hardwood in the way that circular saws used to
  • planing. Almost a ditto but some timbers do plane better than others which means that there will be less sanding to do to achieve a good finish
  • ability to take a finish. This is about how easy it is to achieve a good smooth finish of paintwork or varnish.
  • grain lift. Some timbers, when wetted with paint or varnish tend to lift tiny fibres on the surface and produce a rough finish when dry. This then needs sanding down and recoating. It can happen a second time and need coating again. What furniture makers do (and builders and decorators never seem to know about this) is to first apply a light coat of cellulose sanding sealer. This is easily sanded very smooth using garnet paper and then the top coat of varnish or lacquer is applied. Possibly a second coat for wet areas. Alternately, and more ecologically, the timber can be finished with boiled linseed oil or beeswax. These coatings do not raise the grain and do not need cellulose sanding sealer.

Reflectivity

Particularly with timber floors the lightness of the colour of the wood can be responsible for a great deal of the reflected light and brightness in a room, especially in the winter months. This can help save on lighting energy.

Timber as a cladding

As rain screens become more popular timber is being increasingly used as an external cladding. An excellent set of articles about the use of timber cladding in Scotland is on the Scottish government web site. They are particularly good because they address the subject of timber cladding in an area with the harshest climates in the UK and also because they draw on Norwegian timber cladding experience. See Timber Cladding in Scotland

There is a “Desktop study for benchmarking experimental cladding designsdone by the BRE for the Forestry Commission, which looks at the potential for using home grown Sitka spruce for cladding. This document also includes some of the best detailing for horizontal boarding

Timber treatments

There is a general misconception about timber treatment. i.e. that almost all timber is better if treated (meaning treated against woodworm, dry rot and wet fungal decay). This is a blanket approach which does not properly address the needs of the building industry. Instead it tends to promote the widespread use of toxic chemicals when they are not needed. If you are talking about telegraph poles, marine timber structures or agricultural fencing then there may be an argument for using timber which is heavily chemically treated (and there may not). For house building there is a much simpler approach.

In fact timber which has been treated with creosote or CCA is now considered to be a toxic waste if you take it to your local reclamation centre where it has to be dealt with separately. There is a major critique as to whether most timber treatments actually work for very long in the Green Building Handbook page 160. To quote -

Timber which is kept at a moisture content of below an average value of about 18% (equating to an indoors relative humidity of about 60%) is extremely unlikely to attract infestations of woodworm or dry rot. If timber is protected from rain and also from internal leaks such as leaking gutters or leaking pipes then timber will last indefinitely; probably many hundred years. There is ample evidence for this. Some of the oldest buildings in Europe are of timber and they have not been automatically treated with the sort of chemicals which are now commonly used on timber. Timber in an inhabited modern house normally has a moisture content of 12%-15% which is well below what will support dry rot or woodworm. However there can be problems with House Longhorn Beetle which has special provisions in the Building Regulations covering a few counties mainly South West of London.

The third possible source of problematic damp is rising damp but this is covered by the Building Regulations part C which insist on a damp proof course (DPC) between damp ground and the main habitable area of a house.

Having said all this there may be an argument for treating the main timbers in a timber frame house such as a post and beam structure. The reasoning goes that maybe there might be a long standing unnoticed leak (roof, plumbing or bridging to damp ground) somewhere which could set up conditions which could cause dry rot. This sort of leak might happen if the dwelling was unoccupied for a long period or a small but persistent leak occurred. Then at least the main structure would be protected even if a limited area got affected. This would be similar to the risk with most masonry construction.

Probably the best way of guaranteeing this level of protection is by using a boron based timber preservative (often called Borax for short). ‘Borax’ is a relatively benign but effective method of treating timber because it has a general toxicity which is on a level with common salt but protects timber particularly well. The only thing to notice is that because it is water borne it must not leach out from the timber. This is prevented by protecting the timber from water penetration in some way. The suppliers of various borax based products have information on this. Borate treatment of timber has to be carried out within the industry

Another possibility is to use acetylated timber.

Accoya bridge at entry to Sneek, Netherlands, by Achterbosch Architectuur

Acetylated timber is softwood timber that has been treated to make it as durable and resistant to rot as some of the best hardwood species such as teak. The technology is based on treatment with a form of acetic acid (which is simply a concentrated form of vinegar and therefore pretty safe).  It is marketed as Accoya by Titan Wood Ltd. This is a relatively recent development in timber technology and has strong green credentials. The main drawback, as with much of the timber trade is the long distances travelled by the timber. Titan’s timber sources are global and the only treatment plant so far is in the Netherlands. Things would improve if there were a plant in the UK.

Another process for treating timber to improve its dimensional stability and durability is thermo modification such as Thermowood and the Plato treatment sytem which uses no chemicals, simply a special cycle of heating, moistening and drying of the timber.

building regulations

Site preparation and resistance to contaminants

This section,  Part C, covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved Document specially for houses.

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

Structure

The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Suppliers

An excellent web directory for timber suppliers is Timber Merchants Locally

See also the section on sourcing local timber

In Wales there is The Wales Forest Business Partnership which promotes timber as a building material and as a fuel

The National Wood Recycling Project is a network of social enterprises which recycles wood and wood products

The TRADA suppliers directory

New developments

The UK has lost the plot slightly on its policy towards quality timber production but things do seem to be improving slowly -

  • recently there has been an appreciation of the water resisting qualities of sweet chestnut and a number of companies in the south east are beginning to harvest and mill it
  • Scottish companies such as James Jones have started supplying structural timber to the timber merchants (including I beams with OSB webs)
  • some of the larger estates, particularly in Wales are producing good sizes in structural timber
  • there is a considerable amount of larch and Douglas fir, originally destined for pit props, coming onto the market. This is good, moisture resisting structural timber.
  • quite a lot of Western Red Cedar (Thuja Plicata) normally sourced from abroad, particularly Canada, is coming on stream from UK sources. It is particularly good for external cladding.
  • the dreaded Lawson’s cypress is proving to be a useful structural timber
  • organisations such as Woodlots are beginning to link local timber resources together (in the South East of England)
  • engineered timber such as Glulam is becoming available and finger jointing is becoming common

finger jointing

Finger jointing (the method whereby pieces of timber can be jointed together in an extremely strong manner shown to the right) along with timber lamination which also strengthens and stabilises timber, is revolutionising the way elements of timber are being made. This ranges from small pieces of high quality joinery such as doors and windows to large structural items such as I beams and huge structural laminated beams.

laminating

Laminated timber is generally well sourced environmentally. However, due to the poor understanding of timber building culture in the UK it has been marketed mainly towards large structures such as offices, swimming pools, theaters etc rather than the housebuilding market, so it can be difficult to find merchants who are supplying off-the-peg structural members. The Glued Laminated Timber Association has a list of member companies. Also try Panel Agency Limited and LamisellHow all this may affect the home grown timber market is as yet unclear. Hitherto, home grown timber has been generally of very low market value, particularly with softwood. This has been because of several factors, including the poor planning of which species needed to be grown, the way woodland is used as a tax dodge and the intrinsic lower quality of softwood grown in a warmer climate.

upgrading

With computerised cutting and sorting of timber it is possible for a machine to cut out and reject all the knots and irregular bits of timber and then glue back together all the good bits, even if they are in very short lengths. These can then be laminated with other such pieces to produce very reliable timber sections. A few joinery companies are already moving into this field. For instance Coed Derwen Joinery. See more about this at Woodknowledge Wales.
‘Reliable’ is the important word. For an engineer who wants to calculate the structural strength of a piece of timber it is more important for it to be reliable and consistent in nature than for certain bits of it to be extremely strong. This is what finger jointing, comb jointing and lamination can achieve.

The possibility that erstwhile low grade UK timber (which is usually shipped to Sweden for paper making) may become seriously useful and valuable could have an enormous impact on the timber industry within the context of higher carbon prices which affect the way timber is imported. We are sometimes jokingly known as the ‘firewood merchants of Europe’ due to our low timber quality but automation of timber production could make timber as a crop much more valuable.

timber as fuel

Another factor which is changing rapidly is the rate at which wood stoves are being purchased. People are looking at burning timber for heating and this will become a competing factor with structural timber, along with pheasant shooting, pulp production and general amenity value. A further factor is the relative value of woodland compared with agricultural land. In the UK, woodlands have generally been the areas which could not be used for agriculture because they were too hilly and/or north facing. Hence their value for pheasant shooting. If structural timber becomes a valuable crop, (partly due to computer based handling of timber) then it may even become commercially viable to plant timber on agricultural land. This would pit timber production against meat production.
Sufficient to say that for the green self builder, the way that local timber is made available in the UK could change rapidly over the next few years

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