by Roger Galpin, The Wood Shop Consultancy
The uses for timber are almost endless. Its excellent environmental credentials, and widespread feature in makeover programmes and lifestyle magazines,
architectural press etc. has lead to something of a renaissance for wood in recent years, a trend which seems set to continue.
However, to make the most of the unique appeal of timber in our homes and buildings, we need to be aware of its natural characteristics which, if overlooked, can cause projects to fail and disputes to arise.
Beginners Guide to Timber Technology
As one of the oldest materials known to man, wood has provided us with fuel wood, pulp and paper, utensils, furniture and a first class, environmentally friendly building material. So, given our familiarity with timber, why is it that some attempts to use it can result in expensive litigation?
In some cases, it would seem that it is our very familiarity that is the problem. So often, our failure to specify the correct species, treatment and condition of the timber can result in problems later. Likewise, poor manufacture, installation or incorrect maintenance can also result in timber projects falling short of expectation. As a consequence, it is often not the timber itself that is at fault but the way in which it is used.
An understanding of some of the basic properties of timber will help to ensure that it is used correctly, whether it is for a multi-million pound building or a weekend DIY project.
One of the most important and yet least understood factors, is that of moisture content.
Timber contains a large amount of water when it is freshly felled and it needs to be dried so that it can be used without excessive shrinkage and distortion. The
amount of water in a piece of wood is referred to as its moisture content. This is expressed as a percentage of the dry weight of the piece, not of the total weight; hence it is possible to have moisture contents in excess of 100%.
The amount of water in wood rises and falls in response to changes in temperature and humidity, so it is not possible to prevent wood from expanding and contracting in service. However, this movement can be minimised by drying the wood to a level that it is likely to achieve in service, referred to as its equilibrium moisture content. As an example, indoor furniture would be expected to have a moisture content of around 8-10%, whilst garden decking can be expected to attain an equilibrium of around 16% moisture content; this level would be subject to seasonal variations, becoming higher in winter and lower in the summer.
One other significant factor related to moisture content is the susceptibility of timber to decay. Wood destroying fungi, often responsible for timber decay in buildings and wooden structures, require timber to have a moisture content in excess of 22% for a prolonged period in order for them to become established and sustain their attack. However, a higher moisture content is one of the factors required by the fungi for optimum growth. For example, the true dry rot fungus, Serpula lacrymans, favours a moisture content of around 30-40%, while many of the wet rot fungi require a moisture content of around 50%.
Moisture content is of course just one factor when considering using timber. There is a wide range of industry standards and guidelines relating to timber specification to ensure that the correct timber is used in respect of durability, strength, appearance, movement characteristics etc.
The following case studies illustrate what can go wrong if care is not taken with specification, material selection and manufacture.
Case Study 1
In line with the growth in popularity of wood flooring, the client wished to combine the traditional appeal of 200mm wide oak floorboards with modern under floor heating
Following installation, the client began to notice gaps developing between the floorboards coupled with distortion of the boards themselves. A dispute subsequently arose over the fitness for purpose of the flooring, in respect to its specification, its manufacture from a solid section, and its condition at the time of
The architect’s specification referred to the correct British Standard but detailed the recommendation for intermittent heating of 10% to 14% moisture content, rather than a more appropriate condition of 6% to 8%.
By considering the known movement characteristics of oak it is possible to first establish the existing condition of the flooring and then to determine by calculation the original condition of the flooring at the time of installation. It can then be established whether the flooring complied with the specification.
In this case, it was found that a 4% reduction in moisture content due to drying out of the flooring in service had resulted in shrinkage of 2mm across each of the 200mm wide boards. Not only had this resulted in unsightly gaps between the floorboards but distortion of the boards themselves.
Whilst the condition of the flooring was found to have complied with the specification, the specification itself was found to be at fault as it did not take account of the low moisture content required for such an end use. The specification also failed to recognise the difficulties in installing solid, wide section floorboards over under floor heating where either narrower boards or flooring made from an engineered, or multi-layered, construction would have been more appropriate.
Case Study 2
As part of a barn conversion project, the client specified bespoke oak windows and doors which were designed, manufactured and installed by a local joinery company. Shortly after the owners had moved into their dream home, they unfortunately experienced various problems with the joinery. This included doors that jammed, windows that leaked and frame sections that had cracked.
Following a detailed examination of the joinery, the issues arising were found to relate to the manner in which the oak had been seasoned by kiln drying and its moisture content at the time of manufacture.
Oak has been used for centuries for external joinery and is detailed as being a suitable species in the relevant British Standard. However, it is considered to be a refractory species in relation to its drying characteristics, regarded as being difficult to dry, with a tendency to check, split and honeycomb.
One of the other characteristics of oak is that it has pronounced medullary rays. Whilst these are of decorative value, as in “silver figure”, they also present a plane of weakness along which rupture of the timber may occur if dried too quickly, which results in fissures radiating out from the centre of the log, or pith, across the growth rings.
The difficulty is that these fissures may not be apparent at the time of manufacture but with external joinery can subsequently appear as a result of movement in service when subject to the elements of the weather.
In this case, it was also found that the timber had been dried to a level that was too low for external use. When subject to wetter weather, the wood components expanded which resulted in some distortion as well as swelling of the doors and windows. Not only did this have an adverse effect on their operation but in some cases they were no longer weather-tight.
The extent of fissuring and excessive movement exhibited by the joinery was considered to render it not fit for purpose and it had to be replaced.
Case Study 3
Promoted by numerous garden makeover programmes and lifestyle magazines, the growth of the timber decking industry in the UK in recent years has been dramatic (from less than £5M in 1998 to a projected £200M by 2018 according to industry sources).
In this case, the householder had enlisted the “expertise” of a decking company to install a simple ground level deck in his garden. After only 3 years, excessive deflection was noted to one end of the deck. The householder found that the supporting joists appeared to have rotted where they had been partially built into the ground, necessitating the replacement of the deck.
Although there is currently no formal British Standard specifically for decking, there are various industry guidelines as well as formal standards concerning the use of timber in external situations. In this case, where timber was used in ground contact, it must be either of a naturally durable species or otherwise pre treated with preservative applied by pressure impregnation; this is a commercial treatment process and should not be confused with brush applied treatments.
Non durable species such as European whitewood and redwood (spruce and pine), are commonly used for decking and must be pre-treated with the correct
levels of preservative in respect of their end use.
In this case, untreated timber was used in a high hazard situation where it rapidly attained a high moisture content that enabled decay fungi to flourish, with the result that the joist sections failed in a relatively short period of time. The timber used, in its untreated condition, did not comply with the recommendations of the relevant Standards and would not therefore be considered fit for purpose.
About the Author
Roger Galpin is a graduate in Timber Technology and is an Associate of the Institute of Materials, Minerals & Mining (Incorporating the Wood Technology Society, erstwhile Institute of Wood Science). He has been involved with the technical issues concerning wood products for over 30 years, and, and has prepared hundreds of reports in relation to the use of timber and wood products, particularly in construction. His daily involvement as an industry consultant enables him to provide an opinion based on current best practice as well as providing technical information on wood products.
He has been involved as an Expert Witness for over 25 years, and has attended many of the training courses, promotional events and conferences that have been run by professional bodies within the legal industry; most recently in 2015 when he was certified as an Expert Witness by Cardiff Law School having
completed the Cardiff University Bond Solon Expert Witness scheme. As such he is conversant with the requirements of CPR 35 and has gained a broad experience in preparing legal reports, dealing with solicitors, conferences with counsel and giving evidence in court.
Mr Galpin can be contacted via www.timberconsultancy.co.uk,
or by telephone: 01480 469367.