Timber for Repairs

Peter Wilson

 

  A painted historic timber ceiling with superimposed acoustic data graphs
  Sound waves give a measure of timber strength and stiffness. Shown here are the results from non-invasive acoustic testing of historic ceiling timbers in the traditional Scottish tower house home of John Napier (the inventor of logarithms), which sits at the heart of Edinburgh Napier University’s Merchiston Campus.

The science involved in the conservation and repair of timbers in existing buildings and structures has made many advances in recent years, with far more attention being directed towards understanding the mechanisms involved in the aging of wood.

It has long been recognised that the strength and stiffness of timber change over time, as do the moisture absorption characteristics. However, the desire to better understand why these changes occur and what their implications are has drawn archaeologists, architectural historians, conservation architects, timber engineers and wood scientists around Europe towards an examination of some less obvious but related areas of research.

More attention is now being given, for example, to other areas of cultural heritage, such as musical instruments, in order to finds clues to the processes involved in wood degradation.

Identifying the various species used in the manufacture of traditional stringed instruments is only the first task: the provenance and age of the trees involved are also critical factors when assessing the age of an instrument and the acoustic qualities it derives from the almost alchemical combination of wood, traditional glues, design and form. Developing a better understanding of the first two of these has been fundamental to the modern techniques used in instrument restoration, and this knowledge has led historic building repair specialists towards a deeper examination of the historic forest resource and the changes that have taken place over time in the physical and mechanical properties of the material derived from older trees.

An EU science and technology network called COST (see further information) is bringing together specialists in all areas of forest and timber research, with the assessment, reinforcing and monitoring of timber structures having its own distinct grouping and regular meetings at which research findings can be shared, discussed and subsequently disseminated to the wider timber buildings conservation sector. This sharing of knowledge and experience is critical to current thinking about the conservation of existing fabric and the reversibility issues involved, specifically the need to ensure that any repair made today based on current scientific understanding and technological capability will not further damage the historic structure or inhibit any future intervention necessary to preserve the original fabric of the building.

IDENTIFICATION AND ASSESSMENT OF HISTORIC TIMBER

Increasing attention is being given to non-invasive assessment of structural timber in historic buildings which avoids the removal of existing material as samples for the strength and stiffness testing necessary to identify suitable replacement timbers. Research at Edinburgh Napier University’s Forest Products Research Institute (see further information) on how to measure the strength and stiffness of historic timbers in situ has applied the experience gained from other work in the forest environment to the world of historic building conservation.

Rather than applying traditional, semi-destructive testing methods, hand-held acoustic tools are now being used to measure the changes in physical and mechanical properties involved when older timbers are placed under new stresses, whether from material degradation or from changes in use that can adversely affect the loadings on traditional timber structural elements.

A woman holding a heavy stone block stands in the centre of a piece of timber which is raised on blocks at either end  
A simple but effective approach to strength testing: the timber shown here is from 18th-century properties at New Lanark.  

Acoustic tools use measurable sound transmission within timber. The sound wave generated by a simple knock on the timber will travel the length of the sample (a floor joist or roof timber for example) and the time this takes permits the material’s strength and stiffness to be calculated.

This very simplistic description of the process involved perhaps underplays the potential of this technique to aid timber conservation and repair. This type of testing not only obviates the time, cost and possible damage involved in traditional sampling and testing, but also provides immediate results that allow the specialist to make informed decisions as to the best replacement material to use and the repair method required.

Investigations are ongoing in this area. For example, Edinburgh Napier University’s Forest Products Research Institute is working with Historic Scotland to combine dendrochronological data from the latter’s considerable store of timbers retrieved from older buildings with acoustic testing in order to build a detailed catalogue of available and appropriate replacement or repair material.

Given that hand-held acoustic devices are relatively inexpensive when measured against the specialist nature of conservation work, other historic buildings agencies may also start to apply these techniques to gather accurate information about structural and non-structural timbers, allowing them to make better-informed conservation decisions.

When it comes to the best repair timber, it is not always a case of like-for-like. If, for example, the existing timber is pine, the prevailing wisdom has long been that the repair should be of the same species. But is this, arguably ‘purist’ conservation approach, always the best way to repair historic timber? Research has shown that the simple introduction of a matching species can introduce new stresses on an older timber with physical and mechanical properties that have changed significantly over time and which, although of the same biological species, has an entirely different provenance.

Again, there is much work to be done in this area of investigation, but with research being carried out in different institutions around Europe, the scientific information increasingly available to historic building specialists allows for more evidence-based decisions to be made.

PERFORMANCE ISSUES AFFECTING THE SELECTION OF REPAIR TIMBER

Buildings are not the only type of historic timber structure that require continuing maintenance, occasional alteration and eventual repair or replacement. Britain’s industrial revolution generated many new types of structure that were often built from extremely hard woods only available from overseas.

Recent years have seen a renaissance in the use of the country’s waterways, for example, and with this the need to regenerate our canals and related infrastructure. This has thrown up many new conservation and repair challenges because many of the timber elements involved are part of structures that have been listed as having significant heritage value and require careful conservation. While this may seem a relatively straightforward exercise – simple like-for-like species replacement – environmental conservation legislation precludes the use of many of the imported tropical hardwoods that would otherwise be the replacement materials of choice.

Lock gates are particularly susceptible to attack from specialised fungi and as such fall within Hazard Class 4 or 5 (depending on fresh or salt water conditions) and Durability Class 5 and were traditionally built using European oak. By way of contrast, the balance beams to the lock gates are not themselves in continuous contact with the canal water but their structural design means that elements of their construction are in ground contact and likely to be almost continuously subject to wetting (Hazard Class 4).

  A traditional timber lock gate
  The recent renaissance in the use of the UK’s waterways is generating a host of new timber conservation and repair challenges, especially as environmental conservation legislation precludes the use of many of the imported tropical hardwoods that would otherwise be the replacement materials of choice.

These and other on-land elements have over the years been replaced using species such as ekki that, with its interlocked grain, is rated as very durable, with good resistance to insect attacks and good weathering characteristics.

Unfortunately, despite its excellent performance in marine situations, this slow-grown example is now also on the International Union for Conservation of Nature’s red list. Over-exploitation has resulted in large scale destruction of wet evergreen forest throughout its range (across west Africa), causing a population reduction of over 20 per cent in the past three generations, and ekki is now deemed to be vulnerable.

Given that our waterways are managed by quasi-public bodies, finding replacement timber in these circumstances can create real headaches. Current UK timber procurement policy requires that all timber and wood-derived products for use on the government estate must come from independently verifiable legal and sustainable sources (including licensed Forest Law Enforcement, Governance and Trade partners) with appropriate documentation being required to prove this.

The policy is mandatory for all central government departments, executive agencies and non-departmental public bodies. Local authorities, other public bodies and the private sector are also encouraged to adopt sustainable timber procurement policies – hence the procurement challenge for our, and indeed other European, canal networks.

In such circumstances, radical alternatives are being explored for situations where listing criteria either do not apply or do not specify the nature of the species required in a historic timber structure. In the Netherlands, for example, chemically modified timbers are now being used to line canals.

One such modified timber is Accoya, a product made by submitting radiata pine to an acetylation process. First developed in the 1920s, acetylation increases the wood’s natural acetyl content through impregnation of acetic anhydride at high temperature and pressure. This process produces significant improvements to dimensional stability (up to 75%) and durability, effectively imparting the physical properties of valuable hardwoods to relatively inexpensive softwoods.

With its ease of machining, non-toxicity, resistance to UV degradation and perhaps (as the manufacturer’s claim) enhanced mould and insect resistance, the product is likely to be of interest to those specifying replacement timber in non-listed historic structures due to its longevity, future reversibility and after-life use.

The process is now being applied to other species and in due course a broader palette of acetylated timber products may well appear on the market. In the meantime, and although they would be inappropriate in many conservation contexts, sustainable modified timbers of this sort are clearly worth more considered investigation.

REVERSIBILITY

The testing and conservation of historic timber structures is a constantly evolving area of research and practice and there is an ongoing need to acquire deeper insights into the factors (physical, mechanical, biological, chemical and environmental) that affect the ageing process and their interactions. From this knowledge, methods for studying the various forms of deterioration that take place over long periods of time can be established, together with criteria for evaluating the long-term compatibility of interventions, treatments and products designed to improve the conservation of wooden structures and objects.

To achieve this, however, techniques capable of predicting future behaviour are required to ensure that ‘retreatability’ is considered at an early stage, thus avoiding the use of any conservation and repair method that might impede future interventions.

The triangular overhead trusses of a modern timber road bridge  
Accoya road bridge at Sneek in the Netherlands: Accoya is produced by submitting radiata pine to an acetylation process which produces significant improvements in the wood’s dimensional stability and durability.  

A good example of this is the timber bridge, a structural type that has been universally employed for hundreds, if not thousands of years. The UK has relatively few timber bridges (although there are remarkable examples such as the mathematical bridge in Cambridge) and most are quite short in span. In other countries, such as Switzerland, the numbers run into the thousands with structural design solutions ranging over several centuries from the simple to the ingenious.

The biggest problem with timber bridges is not their fitness for purpose (there are many motorway and other heavy vehicle supporting timber bridges around Europe) but the widely held perception that timber constructions such as these are not long lasting.

The challenge therefore, for historic structures as much as for new bridges, is to demonstrate how they can be repaired or built in such a way that any possible deterioration is slowed down. For new bridges, parametric computer modelling and new, engineered timber products can facilitate the construction of durable, long-span structural solutions never before possible.

At the same time, historic bridges can be repaired using appropriate timber species and experience-based details that have been designed to protect the structural members from the prevailing weather and water conditions. The reversibility of any repairs or restoration work to historic bridges is crucially important since the constructional and historical integrity of the structure is part of a visible continuum and is no place for compromised or overtly hybrid technical solutions.

Looking to northern Europe, the Norwegian Public Roads Administration requires all bridges to be built with a 100-year lifespan and minimal maintenance requirements, with no exceptions for timber bridges. The solutions employed involve a combination of constructional and chemical protection measures. The former normally involve the use of copper sheeting on all upper surfaces, an approach which is also used in exposed situations where chemical protection proves unsatisfactory.

There are many lessons to be absorbed from this example. Because of their low environmental impact, the number of structures built from sustainably sourced timber is on the rise internationally and this is generating a future need for good maintenance and repair practices. There is also a growing emphasis on the need to avoid the use of chemical products for which ambitious but often unproven claims are made.

The repair and restoration of historic structures has always tended towards a sound understanding of the material properties and technologies used in their original creation, which is as it should be. The need to transfer this philosophy to the contemporary projects that will become the historic structures of the future is the bigger, industrywide educational challenge, and one that needs to be confronted sooner rather than later.

~~~

Further Information

COST (European Cooperation in Science and Technology) www.cost.eu

Edinburgh Napier University, Forest Products Research Institute www.napier.ac.uk/forestproducts

FPS COST Action FP1101: Assessment, Reinforcing and Monitoring of Timber Structures www.costfp1101.eu

 

 

 

The Building Conservation Directory, 2014

Author

PETER WILSON is a registered architect and director of the Wood Studio within Edinburgh Napier University’s Forest Products Research Institute. The Wood Studio has a remit to promote research and innovation in the use of timber in architecture and construction.

Further information

RELATED ARTICLES

Joinery and Cabinetmaking

Structural

Wood

 

RELATED PRODUCTS AND SERVICES

Architects

Cabinet makers

Joinery

Structural engineers

Structural timber testing

Timber repairs

Timber suppliers

Wood carvers and turners

 

 

 

BuildingConservation.com
Site Map

© Cathedral Communications Limited 2014