Timber in historic buildings
Dr John Williams, TRADA Technology Consultant, discusses how to assess old timbers in situ and how best to preserve them.
Often, there is a perception that timber in older, derelict buildings is in poor condition and requires total replacement. In most instances, this is simply not the case. Generally, the timber will appear to be in a far worse condition than it actually is.
When undertaking conversion or refurbishment works, the developer must answer two questions.
Are the original timbers in good condition?
How strong are they?
Simple questions, on the face of it, but very often poor advice or a lack of knowledge can result in unnecessary and expensive remedial/replacement works.
Structural timbers are inherently durable and resistant to most biological degradation, provided they remain free from wetting. However, since timber is an organic material, prolonged exposure to damp increases the risk of decay or degrade by wood destroying fungi and insects.
Fungal decay, be it wet rot or its more sinister companion, dry rot, has just the same requirements as we do. Fungi need oxygen, food and most importantly water to survive. Timber will only be vulnerable to fungal attack if its moisture content persistently exceeds the decay threshold of 20%. In other words, provided the timber within a building is kept dry and well ventilated, it cannot be affected by fungal decay. Mitigating the risk of insect attack is a different matter, but in most cases, the effects of insect attack are not as damaging as those caused by fungal decay.
Historically, the construction industry has had a simplistic approach to controlling and eradicating fungal decay in buildings. The emphasis was on extensive opening up in the hunt for decay organisms, followed by extensive preservative treatments and/or replacement works. Many perfectly sound structural timbers were replaced and masonry very often deluged with preservatives. Costly and time-consuming works are still undertaken, sometimes unnecessarily, throughout the construction industry.
The use of timber preservatives is still widespread and is often viewed as an insurance policy. For example, how many of us, when purchasing a house, have been advised that woodworm is present and insecticidal treatments are necessary? When we consider how often many of us move house, particularly in areas with a transitory population, the same house could end up being treated every few years simply because the survey has failed to differentiate between active and historic insect attack.
The same is true for fungal decay. Often the survey will not differentiate between active and historic decay, with the result that the decision to carry out remedial works is taken.
All too often the perceived risks of wood destroying fungi and insects far exceed the actual risk to the building, simply because the construction professional does not have the necessary understanding of the biology of these organisms. At best, remedial treatments are often justified on a precautionary basis.
When we consider refurbishment and restoration projects, there has to be a balance between the ‘environmental’ approach of managing and controlling timber decay organisms and what is commercially realistic. A derelict building may well have serious problems associated with penetrating damp and consequent wood decay. Adopting a chemical-free ‘environmental approach’ usually requires isolating timber from sources of damp and drying out of the building. However, this can cause considerable problems, particularly with construction timetables. In other buildings, it may be problematic to maintain sustained low moisture contents and it may prove necessary to undertake targeted preservation treatment or removal of the ‘at risk’ components.
Understanding the biological requirements of decay organisms and wood-destroying insects and applying this knowledge on site leads to less potentially destructive opening-up and less disruptive remedial/replacement works. The specification and application of preservative treatments will be better targeted, with the result that there is greater control of the use of chemicals. This is especially important when undertaking restoration and conservation works, particularly in listed buildings.
Timber condition surveys should not be carried out without considering the condition of other materials in contact with the timber. All potential sources of moisture and damp penetration must be identified and these areas thoroughly investigated. Non-destructive surveys use a range of techniques that cause minimal disruption and allow the inspection to proceed with the minimum of opening up works during the preliminary phase of the timber investigation. However, it must be borne in mind that these non-destructive techniques form the basis for justifying opening up works when areas of decay are located or where it is established that there is a risk of fungal decay.
The search for wood-destroying organisms is not the only reason for justifying opening up works. Buildings may undergo a change in use, or sound timbers may need to be evaluated for their load bearing capacity. Opening up works for structural appraisal can and should be limited to representative areas. At the very least the structural members must be exposed along their entire length. If consistent results are obtained, opening up can be kept to a minimum but variable results can require additional works. The key consideration is to carry out opening up works only if they can be justified and their extent can only be determined a case by case basis.
Timber elements should only be exposed, treated or replaced if there is good reason to do so. In other words, if the timber has not been and is unlikely to be affected by fungal decay, and it can still function structurally, it should be left undisturbed. The necessary knowledge to make such decisions can be gained by a close inspection of the building using a range of inspection techniques.
Having determined the condition of structural timbers, the next stage is to determine their strength. For structural purposes, modern timber is strength graded either visually or by machine and supplied in strength classes. The strength class depends upon the species and grade of the piece. There are a number of strength classes defined in the European Standard BS EN 519 ranging from C14 for the weakest softwood strength class up to D70 for the strongest hardwoods. Typically, engineers work with strength classes C16 and C24 for softwoods and D30 and D40 for oak.
However, where original timbers are to be retained, the developer is not in a position to select timber of the appropriate strength class, especially where alterations could impose new loading or where the timber has been affected by deterioration, particularly insect attack in perishable sapwood bands. Equally, medieval carpenters had no recourse to BS EN 519 and modern design codes such as Eurocode 5. Our experience shows that designers tend to adopt a conservative stance when estimating the strength of existing timbers.
Modern timber grading rules are designed so that all visible surfaces of the timber are assessed in the stockyard. Clearly this is not possible in a historic building. However, the principles laid down in modern strength grading standards can form the basis for the skilled surveyor to apply a strength class to the timber. Furthermore, the experienced surveyor can consider the actual effect of strength-reducing features. For example, are knots above or below the neutral axis of a beam and how does their position affect strength? Where are the points of maximum stress and what is the residual sound cross section of the element at these points?
This expertise can also be supplemented with the very latest ultrasound non-destructive testing technology. Specialist equipment provides direct readings of member strength and stiffness, which can often result in engineers being able to justify the use of enhanced design values.
In almost all cases, significant savings in construction time, materials and costs can be achieved by a detailed condition survey of historic timbers.
TRADA Technology’s Consultants provide a range of surveying services to the construction industry. For further information visit the Services section of the website.
