Lessons in history
Dr John Williams, Senior Technical Consultant, TRADA Technology explains some of the options for non-destructive assessment of timber strength and condition in historic building.
Historic buildings are part of our built environment. Many are important icons of our past and form part of our heritage and culture. They also provide excellent opportunities for reuse and development and can serve as the focal point for the regeneration of many rundown, post industrial areas.
A holistic approach to the investigation of historic timber using non-destructive techniques to inspect and examine timber in situ for strength reducing defects, combined with repairs undertaken with the minimum of interventions, where practicable, can facilitate the re-use of original structural timber and can contribute to the 'recycling' of historic buildings.
The remedial treatment of timber decay in British buildings with chemical formulations is a comparatively recent activity. Previously, if any action was deemed necessary, infected or infested timber was cut out and replaced. Construction professionals, contractors and specialists from the wood preservation industry employed basic and often damaging methods to the assessment and treatment of fungal and insect infestations in historic buildings.
For example, the slightest suspicion of dry rot often resulted in masonry walls being peppered with bore holes for injecting fungicides, while plasterwork and other decorative features were damaged during invasive survey works. Often, perfectly sound timbers were treated or removed unnecessarily even though historic outbreaks of decay and/or insect attack posed no threat to the timber. Misidentification of insect damage also resulted in unnecessary treatment of timber. The inability to determine between active and historic beetle attack, has and can continue to result in unnecessary treatments.
Structural timbers are inherently durable and resistant to most biological degradation, provided they remain free from wetting. In general, fungi in buildings will not damage timber with moisture content below about 22%. However, since timber is an organic material, prolonged exposure to damp increases the risk of decay or degradation by wood-destroying fungi and insects. Unfortunately, some of our historic buildings fall into disrepair and dereliction. Subsequent water and damp ingress can trigger decay. It should also be borne in mind that once fungal decay becomes established in structural and decorative timbers, the rate of deterioration can increase.
How do we assess the condition of structural timbers? The objectives of the survey must be to identify the type and extent of damage caused by decay fungi and wood destroying insects - with the minimum of damage. When assessing timber condition, extensive opening up works should be viewed as a last resort, or only undertaken when the non-destructive survey methods used provide robust justification for undertaking these works. All too often, the perceived risks of wood-destroying fungi and insects far exceed the actual risk to the building.
There are a number of techniques which can be employed by the surveyor. These are: good eyes, nerves of steel, an open mind and a profound knowledge and understanding of both original construction methods and materials, and historical repair techniques.
Two reliable methods of surveying structural timbers are decay detection drilling and the moisture content survey. The areas in which the condition of timber is most critical tend to be areas which are the least accessible, such as the bearing ends of, for example, beams and joists built into solid external walls and the ends of roof trusses in the eaves. In such cases a wood auger may be mounted in a power drill and driven into the timber with constant pressure at low speed.
Any changes in the resistance of the timber to the passage of the auger may indicate the presence of decay. For example, if resistance decreases, then this indicates that the timber is softer and this could be a symptom of fungal decay. On the other hand, if drilling into large section members, the surveyor may be drilling into the less dense, juvenile wood (pith). Therefore, a good understanding of the material properties of timber is essential to prevent misdiagnosis of defects. If the angle and direction of the auger are positioned correctly, then it is possible to assess the condition of the hidden bearing ends of structural timbers.
There are, too, more sensitive methods of surveying members. 'Resistograph' or 'Sibert' drills use a very fine microdrill. The advantage is that the rate of penetration through the member can be recorded electronically and its profile reproduced graphically. The remaining hole in the timber is only 2mm in diameter and is very difficult to spot. The disadvantage of using microdrills is that they are comparatively slow and more awkward to use than augers in areas where access is restricted.
Decay detection drilling using the Sibert microdrill
The search for decay causing fungi is not the only reason for justifying opening up works. Buildings may undergo a change in use, floor loadings may change and sound structural timbers may need to be evaluated for their load bearing capacity, which can be done through strength grading – with an important caveat.
With particular reference to structural softwoods, modern grade stresses have been based upon the testing of structural timber sourced from modern forests and are believed to have been based upon a moisture content range of 16% - 20%. For structural purposes, modern timber is strength graded either visually or by machine and supplied in strength classes and is clearly marked by a stamp denoting its grade and strength class, e.g. C24. Therefore, one can surmise that modern grade stresses for softwoods, when applied to older historic softwood timber, can be viewed as conservative and, arguably, even punitive. The challenge is to try to find ways of using knowledge and expertise to prove these advantages in strength that are present in historic seasoned timber.
In summary, the assessment and examination of the condition and strength of historic timbers requires specialist knowledge. Timber condition surveys and in situ strength assessments should not be carried out without considering the condition of other materials in contact with the timber. In almost all cases, significant savings in construction time, materials and costs can be achieved by a detailed condition survey and strength assessment of historic timbers.
But it's not all about money. We often hear the term 'taking a holistic view'. There are other benefits to take advantage of by retaining and re-using existing timber and other building materials. We put less pressure on our natural resources and, in an ever increasing environmentally conscious industry, this in turn can help reduce our carbon footprint.
It has been a century or more since doctors routinely cut open their patients to find out what was, or was not, wrong. Perhaps the same consideration should be bestowed upon our heritage buildings, especially if they are of architectural and historical importance?
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