TRADA Technology's Dr John Williams looks at historic timbers and the methods used to inspect them. There are, he argues, a number of good reasons to recycle and re-use our heritage buildings...
Historic buildings and the materials therein are part of our history. They are a record of our built environment and how it has evolved. Furthermore, many of our historic buildings are icons of our industrial and architectural heritage. What about the redevelopment of some of our waterside sites such as Woolwich Royal Arsenal on the Thames by Berkeley Homes, or Royal William Yard in Plymouth and Manningham Mill in Bradford, both redeveloped and brought back to life by Urban Splash?
In recent times, the methods and approaches used to assess and repair timber in historic buildings have evolved considerably. Previously, the industry employed basic and often damaging methods to assess and treat fungal and insect infestations. 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 and/ or removed unnecessarily even though historic outbreaks of decay and/ or insect attack posed no threat to the timber. Unfortunately, these practices still persevere.
Very often, poor advice or a lack of knowledge can result in unnecessary and expensive remedial and/ or replacement works. 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: think of the timber suffering from a nasty skin complaint that is easily remedied by stripping back to the bare timber: often this will reveal beautiful, tight grained slow grown timber. Of course there may be decay but often this is localized. In most cases decay will be found at the bearing ends of the timbers where they have come into contact with damp masonry.
Structural timbers are inherently durable and resistant to most biological degradation, provided they remain free from wetting. 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.
A survey will allow the condition of structural timbers to be assessed. Its objectives must be to identify the type and extent of damage caused by decay fungi (very simplistically - dry rot and wet rot) and wood destroying insects. Understanding the biology of these organisms can contribute vastly to less disruptive and less damaging inspection and remedial measures.
Conventional methods of opening up structures can cause extensive and unnecessary damage. Therefore, the objective of any survey in an historic building is to undertake the survey with the minimum of damage. 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 prior to essential interventions.
There are a number of techniques which can be employed by the surveyor. The most important piece of equipment is a good set of eyes supported by an enquiring mind. The first tool to come out of the bag is often a hammer. The 'TAMO' – thump and move on! - method is a fast and effective survey strategy but it does have its limitations especially when assessing large cross section timbers. 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.
The use of wood augers is a fast and efficient method of surveying members although there are more sensitive methods. 'Resistograph' or 'Sibert' drills use a very fine microdrill. Their advantage is that the rate of penetration through the member can be recorded electronically. The remaining hole in the timber is only 1mm in diameter and the profile of the member can be reproduced graphically. Furthermore, the residual hole is very difficult to spot. The disadvantages of using microdrills is that they are comparatively slow and not as easy to use as augers in areas where access is restricted and awkward.
Checking moisture content
In buildings, internal structural timbers should not have a moisture content in excess of 20%. If such readings are obtained then this is indicative that there are building defects allowing water ingress into the structural timbers. When undertaking the moisture content survey it is essential to measure moisture content at the most critical point, i.e. the bearing ends. Conventional hammer probes or pins are not suitable for surveying large section structural timbers. The most effective method is to insert long, insulated probes into the holes drilled using the auger to test the condition of the hidden bearing ends.
The search for decay-causing fungi is not the only reason for justifying opening up works. Buildings may undergo a change in use and floor loadings may change and sound structural timbers may need to be evaluated for their load bearing capacity. The timber then needs to be graded.
In the UK, all structural timber, whether hardwood or softwood, should be strength graded in accordance with the umbrella standard BS EN 14081 Timber structures. Strength graded structural timber with rectangular cross section. The UK national standards which comply with BS EN 14081 are: BS 4978: 2007 Visual strength grading of softwood - Specification and BS 5756: 2007 Specification for visual strength grading of hardwood.
The strength class is based on grade, timber species and where the timber was grown. There are 18 strength classes - groups of species/strength grade combinations with similar properties - 12 for softwoods, prefixed C, and 6 for hardwoods, prefixed D. The most commonly used in the UK are C16/C24. The numbers represent the characteristic bending stress of the timber.
The evolution of the modern strength class system has simplified procurement and construction as engineers can now design by strength class as opposed to specifying a particular species. The engineer no longer requires detailed knowledge of timber species. Furthermore, the specification of strength classes can allow the timber supplier a certain amount of flexibility in supplying the right species for the job. Clearly this is not possible in an historic building.
With particular reference to structural softwoods, modern grade stresses have been based upon the testing of structural timber sourced from modern forests. Furthermore, grade stresses are believed to have been based upon a moisture content range of 16% to 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.
Therefore, we can surmise that modern grade stresses for softwoods, when applied to older historic softwood timber, can be viewed as conservative and even punitive. The challenge is to try to find ways of using knowledge and expertise to prove these advantages in strength inherent in historic seasoned timber.
Before we do this it is important to explode a myth. We often hear phrases like 'They don't build 'em like they used to.' The construction industry has and always will be governed by price. Design and build contracts in the 19th Century industrial Britain were no different to design and build contracts today. Buildings were built to a budget and a thorough structural investigation will almost always reveal that the building would probably have been built to the limit of its design. However, if we recognise this fact, we can still find advantages in the strength properties of older, historic timber.
This can be achieved by a number of ways. Firstly the timber needs to be visually graded.
One of the major challenges facing the timber surveyor when grading in situ timber is access.
BS 4978 and BS 5756 both require that all six surfaces of a piece of timber need to be inspected before it can be assigned a strength grade. This is rarely possible in a historic building and very often the surveyor will only be able to assess three visible surfaces. Therefore we are only able to assign an indicative visual strength grade although very accurate predictions about grade and strength can be made if only three surfaces are visible. One of the major challenges facing the timber surveyor when grading in situ timber is access. We can only grade what we can see and the more timber we can see, then the more confident we are with our recommendations. It is far better to assess a small number of structural timbers along their entire length than to look at many small exposed sections of many structural timbers. Opening-up works for in situ visual strength grading can and should be limited to representative areas. 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 on a case by case basis.
It is inevitable this will cause some damage but this can be mitigated. The owner of the building is faced with either lifting the flooring or removing the ceilings. If there are ornate plasterworks and the ceiling is plaster and lathe then lose the floor. If the ceiling comprises plasterboard, then lose the ceiling. Sometimes, we are faced with soundproof pugging. If the ceiling cannot be removed then once floorboards have been lifted, access holes should be cut into the pugging to allow the timber to be assessed with the aid of a torch and mirror. I have come across a few sceptics that have used the phrase surveying using 'smoke and mirrors' but the resultant time and cost savings by being able to retain the original timber without the need for interventions usually allays such scepticism.
Once the timber has been visually graded we can start to use our judgement. We can begin to find advantages in strength by assessing moisture content. Drier timber is stronger so there are increases in strength to be found using this route of assessment. Denser timber is stronger timber. By assessing density we can find additional increases in strength. We can also consider the size and location of strength reducing defects. All these factors can be considered on site in order to develop and assign bespoke grade stresses. The key word here is bespoke.
Back in 2001, TRADA Technology worked alongside Thomasons consulting structural and civil engineers on the Colonial Buildings project in Bradford, converting an old wool sorting warehouse into office space. The initial inspection, using the modern codes, indicated that the beams could be assigned strength class C24 although it was clear that the timber was of very high quality and superior to structural timber extracted from modern forests. When modern codes were applied the design analysis indicated the structural timbers failed in deflection so there was a requirement to introduce strengthening. The estimated costs (back in 2001) for strengthening works was about £150,000. When these potential costs were presented to the developer, the design team recommended that we undertake a bespoke grading programme with the objective of assessing the feasibility to upgrade the existing beams beyond strength class C24. The results of this investigation proved that the original timber was far stronger than strength class C24 and that there was no need for any strengthening works to the timber. We recently undertook a cost analysis for the same project and the interventions would have cost in excess of £250,000.
Ultrasound, a hi-tech version of the TAMO method, is a widely used technique for non-destructive investigations throughout the medical and industrial worlds. However, its routine use in timber inspections is limited in the UK. TRADA has undertaken a small study whereby we have demonstrated that that there is potential for the use of ultrasound as a tool in to support bespoke strength assessments and to assign higher strength classes to timber than would normally be permitted.
In summary, the appraisal of the condition and strength of historic timbers requires specialist knowledge. Nowadays, physicians avoid cutting open their patients unless it is unavoidable. Perhaps our historic buildings, especially if they are of architectural and historical importance, deserve the same consideration?