Structural Analysis of Historic Construction – D’Ayala & Fodde (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-46872-5

Repairs to historic timber structures: Changing attitudes and knowledge

D.T. YeomansICOMOS, UK

ABSTRACT: The conservation of timber structures has improved in recent years with a growing appreciationof their historical significance. However there are still difficulties presented because of the limited number ofengineers with an understanding of timber structures and the failure of design codes to consider the kind ofdetailing often used in historic structures. This paper will point out a number of areas where research on thebehaviour of ‘traditional’ carpentry would be valuable.

1 INTRODUCTION

Since the eighteenth century timber in Britain has beena rather Cinderella material, essential to constructionbut not highly regarded compared with masonry asan architectural material. It did enjoy something of arevival during the nineteenth century for roof struc-tures of the Gothic revival but, apart from that, ithas been a poor relation to other materials for struc-tural purposes. That is to some extent because Britain,not having large supplies of structural species hasimported most of its building timbers, but also a legacyof the Fire of London and the switch to brick. The sub-sequent ‘Georgianising’ of timber-framed buildingsacross the country was probably as much a fashionstatement as a concern for fire protection. One couldargue that it is not having a continuing tradition of tim-ber structures that has led to a lack of concern for thehistory and historic value of such structures, exceptfor exposed half-timbering. Be that as it may, the his-torical significance of timber structures has only beenrecognized relatively recently, so that when we con-sider the approaches to conserving timber structureswe have to consider the development of ideas about thesignificance of these structures as well as that of thetechnical solutions; an understanding of their historyas much as of their structural behaviour. It is true thatit has for some time been a material of interest to a fewenthusiasts, people like the late Freddie Charles, and itsimportance was recognized in those buildings whereit was a prominent architectural material: the roofs ofopen halls and EastAnglian churches for example. Butotherwise it was taken little account of.

In the nineteen sixties there was a series of studiesof the major monastic barns, some undertaken throughthe collaboration of Freddie Charles and the Americanscholar Walter Horn that recognized the significanceof what were the greatest timber structures of their

day (Charles & Horn, 1973: Horn & Charles, 1966,1984). At much the same time, Stuart Rigold (1966)did work on the many timber barns in Kent, and CecilHewett (1967), having looked at the barns of CressingTemple, went on to draw, examples of major medievalcarpentry from all over the country, his work culmi-nating in two large collections (Hewett, 1980 & 1985).His work did a great deal to draw people’s attention tothese structures but, because of the inaccuracy of hisdrawings, regrettably little to advance scholarship inthe area. (One could even argue that it set back scholar-ship since many must have assumed that this work hadalready been done.)Timber buildings were also a com-mon interest of students of vernacular architecture, sothat by the end of the 1970s there was an extensive cov-erage of medieval carpentry. In contrast little had beendone at that time on later carpentry work and for thatreason I chose to look at seventeenth and eighteenthcentury carpentry to provide an outline of develop-ment during that period that I hope others might followup on.

This still leaves the nineteenth century, both in termsof the development of industrial structures, the tran-sition to iron and architects’ use of exposed timberstructures in both Gothic revival buildings and theirlater treatment of the material. Only Booth has donework on nineteenth century laminated timber. Thereare also significant 20th century timber structures thatwe are now loosing. Oxford Road Station, Manchester,a group of conoids, was recently imitated in steel.

The histories of early carpentry provided an accountof developments in the overall form of timber struc-tures, their structural design if you will, but carpentryis a craft skill and we also need to consider its details.Again Hewett provided the general reader with draw-ings of joints but that is something requiring carefulobservation and measurement. In this the work that Iknow best is that of Richard Harris, now curator of the

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Weald and Downland Museum, and Peter McCurdy,who I would describe as a scholar craftsman. These arepeople who have worked closely with the timbers andwho have come to understand and bring to our atten-tion the significance of setting out marks that one findswithin the timbers, and have a sound understanding ofthe process from conversion of the log to the erectionof the building. Although they have published rela-tively little, in their teaching they have shown us thatcarpenters marks are more than just the numbering cutinto the timbers. For imported timbers there has beena little interest in merchant marks that indicate theirsources, significant because of the changing patternsof trade, particularly during the late eighteenth andearly nineteenth centuries, although this work is stillin its infancy.

What historical significance these clues have I leaveto others, but the significance of this work is thataspects of the surviving timbers are now being rec-ognized that were simply not seen a few decades ago.Moreover, as many timbers have been reused, it hassometimes been possible to reconstruct the design ofearlier structures that the timbers were a part of. Forexample, the earlier roof of Lincoln Cathedral has beenreconstructed from timbers that now form part of thepresent roof (Foot et al., 1986), but which were reusedfrom the previous. An example of more recent reusewas that timbers from the original gallery structure atChrist Church, Spitalfields were reused for the floorof later pews.

This means that surviving timbers are now beingrecognized as conveying evidence of the developmentof structural design, as evidence of craft practices andas evidence of the earlier form of the buildings inwhich they are found. This is a change from the scantvalue that they were given only a few decades ago.I recall visiting the roof of Norwich Cathedral to lookat the timbers in the roof shortly before they weretaken away. By that time the transepts had already beenre-roofed using reinforced concrete A frames, involv-ing the total loss of the original timbers. In today’s cli-mate an alternative and rather different solution wouldsurely be sought in similar circumstances, perhapsusing a supplementary structure rather than a completereplacement. We are perhaps beginning to recognizethat structural design is a cultural activity whose his-tory is to be valued as much as carving or wall painting.However, medieval structures still seem to be the mainfocus of attention with far less understanding of thehistory of early modern structures.

This is disappointing because without the develop-ment of new carpentry structures in the seventeenthand eighteenth centuries the forms taken by the greatpublic buildings, churches and country houses wouldsimply not have been possible. But the carpentrystructures that made this architecture possible arelargely invisible and so little attention is paid to them.

Much the same might be said of the structures of thenineteenth century and even the early 20th century, andthe danger is that we will loose historically significantstructures before their significance is recognized.

2 ANALYSIS OF THE STRUCTURE

Recognizing the value of structures is the first step totheir preservation, but the second is their proper struc-tural analysis. Here it is unfortunate to have to reportthat their behaviour is not always properly understood,with the result that unnecessarily heavy-handed repairsare sometimes specified leading to the loss of historicmaterial and hence the loss of historic character. Onmore than one occasion I have been contacted by thoseconcerned when their engineers have specified the useof steel connectors for timber structures that had beenperforming in a perfectly satisfactory manner.This canbe either because of an unwillingness to treat the struc-ture on its own terms, or simply the application ofcomputer software better suited to steel or concretestructures.

In some cases it is clear from the places where thesteel has been specified that the structural form hasbeen analysed as if it were a steel structure, result-ing in assumed tension forces that the timber jointscannot take. However, if these members are removedfrom the analysis what remains might be a perfectlysatisfactory structure. In spite of what seems obviousfoolishness, there have even been a number of paperspublished where finite element analysis has been inap-propriately applied to historic timber structures. In onecase that I was sent to referee (I will not give a refer-ence to protect the foolish.) the author came out withthe startling discovery that in a king post structure, itwas the post rather than the tie beam that would becarrying the majority of the load. A moment’s thoughtwould have told him that the principal rafter/king postassembly must be stiffer than the tie beam. I have afeeling of unreality when reading such work. I haveto assume that such studies are carried out to impressacademic colleagues rather than as contributions to thepractice of either history or conservation.

The message here is that ‘work is no substitute forthought’, but it remains a problem when the work isdone by a computer while it is the engineer who isrequired to do the thinking. It is likely to remain aproblem because the modelling of structures usingcomputer programs has become a relatively simpletask. Entering the data into the computer is a fairlymechanical operation that can be performed withoutmuch thought. It is a sad reflection that there seemsto be a large number of people for whom thought isa painful activity to be avoided at all costs. So mucheasier to just put the data into the machine and let it

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do the work. Of course, computer analysis can be usedintelligently if one realizes that large tension forces atjoints are unlikely to occur and the structure needs tobe modified accordingly. Sometimes one even findsthat the result is a statically determinate structure andwith a little thought one could have started from there.

One also needs to recognize that the nature of theworkmanship can affect the support conditions formembers that in turn will affect the overall structuralscheme.Also, the section properties that one can insertas data for an historic structure can often be little morethan a guess. This is equally true of the behaviour ofconnectors, and any analysis needs to be based on athorough qualitative grasp of the structure followingclose observation of its construction and the way thatit has behaved. Often the uncertainties are such thatmore than one structural scheme has to be considered.Of course, timber is not unique in this regard but per-haps the linear nature of its elements encourages a falseidea of simplicity.

3 REPAIR STRATEGY

This brings us to the methods of repair, and here weshould recognize that different approaches might beappropriate for different kinds and periods of structure.A simple contrast can be drawn, for example, betweenthe decorative roofs of the middle-ages and the purelyutilitarian structures of nineteenth century industrialbuildings. In spite of both being equally visible, theformer are clearly more valuable architecturally.As forall historic structures, we should begin by consideringthe possible historical value and hence the strategy tobe adopted rather than focusing immediately on thetactics of repair. This issue of conservation strategy isone in which the engineering historian has somethingto contribute and about which the engineering consul-tant might be better informed than other members ofthe conservation team.

This means making some strategic assessment thatshould take into account:

– The historical value of the fabric.– The overall condition of the structure and hence– The scale of the repairs required, but also– The options for future use.

Consider the extremes here. We might try to retainthe timbers in a condition as close as possible to theoriginal structure, i.e. as a repaired structure carryingout its original function of supporting the building.At the other extreme is provision of new structureto support the original timbers that simply remainas an historic artefact reminding one of what wasthere originally. The latter might well be the preferredoption where the decorative nature of the timbers is of

paramount importance. Between these two there maybe a choice of options that the engineer should be ableto assist in deciding between. This requires the to engi-neer have a greater involvement in the process thansimply providing the technical fix after the strategicdecisions have already been taken.

In considering the overall strategy the choices arebetween:

– repairing and restoring the as-found structure and– providing supplementary structure

And if the first option is chosen one needs toconsider the choice between

– Repairing in-situ and– Dismantling for repair, either completely or in part.

There is often resistance from conservation offi-cers (and possibly clients) to the latter, even thoughtthere are some cases where this might be the cheapestand safest approach. It is an option for timber struc-tures because of their prefabricated nature, and thereis almost certainly a failure to understand that where astructure has distorted over time, for whatever reasons,to repair it in its as-found condition is to preserve acripple. It will often have locked in secondary stressesthat have arisen through the distortion that has takenplace and which now have to be accommodated withinthe repairs, a complicating factor.

I have argued (Yeomans, 2007) that in some build-ings, particularly barns, the structural design is whatwe should be seeking to preserve. In contrast, wherethe timber has been treated decoratively this willtake precedence and we may have to compromise theintegrity of the structure in order to preserve the sur-face. Perhaps there are those occasions where thechoice is that simple, but this dichotomy means thatsome judgement must often be made. In such circum-stances the engineer responsible must be prepared toengage with other conservation professionals and toexplain his point of view. It may be necessary to offeralternative structural strategies that would have dif-ferent effects on the historic fabric. One is balancingstructural safety, the heritage value of the structure, andperhaps a choice of future use. The client and otherconservation professionals will be involved in suchdecisions, and the engineer must be prepared to explainthe engineering issues in terms they can understand.

Although I have emphasised the need for con-servation engineers to work with other conservationprofessionals in determining the overall strategy, it isalso advantageous to work with the carpenters. I regardthe simple model of professionals producing a designfor contractors to price as inappropriate for the conser-vation of timber structures. We are not dealing with anempty site on which a building is to be assembled byreasonably tried and tested methods. We are dealingwith unique situations where it is necessary to ensure

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the continued stability of an existing structure that isbeing changed while it is being repaired. This is a situ-ation in which the methods used may affect the detailsadopted, or in which the practicalities of a particulardetail might affect the scope of the work to be car-ried out. In many cases an experienced carpenter maymake an invaluable contribution at the design stageand, where possible, it is worth employing one as partof the design team.

I raise all these issues before discussing repair meth-ods in detail because both the repair recommendationsof ICOMOS UK’s Wood Committee and the Recom-mendations of ISCARSAH, while informed by suchconsiderations, do not draw the conclusions as clearlyas they might. My view is that more considerationneeds to be given to this stage of the process in theadvice given to professionals.

4 REPAIR DETAILS

When we turn to the tactics of repair, timber has alwaysseemed to be a fairly contentious material the methodsof repair have attracted rather strongly held positions.There has also been a developing of skills and thetwo together seem to have resulted in some swings inapproach. (This might be true of conservation in gen-eral, it just being my own perspective that makes methink that it is more problematic for timber.) Repairmethods in timber can be loosely divided into threekinds:

– Those using steel– Those using modified traditional methods– Those using epoxy resins with or without some

reinforcement.

The kind of repairs once commonly carried out byengineers, cutting back the decayed timber and puttingin supplementary steelwork (Fig. 1), was later dep-recated by those who favoured a more ‘traditional’approach, although that usually meant carpentry meth-ods supplemented by modern steel fasteners. It hasmore recently been recognized that while such anapproach is more visually attractive, and so appropriatefor medieval timber frames, where the results are bothvisible, and have to form a junction with secondarymaterials, it does involve a greater loss of historic fab-ric. In that respect the use of supplementary steelworkis perhaps to be preferred where the repair is not seen.Repairs using epoxy resins had a ‘bad press’ for awhile and suffer from a regrettable lack of researchinto their long-term performance, but lack of researchalso affects other areas of repair work.

One of the major repair projects carried out earlyin the 20th century was to the roof of WestminsterHall (Baines, 1914) where substantial decay had beenfound. That used steel connectors, but in locations

Figure 1. Repairs using steel carried out to the heel joint ofa Wren roof truss at Hampton Court.

Figure 2. A truss in the Banqueting House, Whitehall withunnecessary reinforcing.

where it would not be visible. One wonders whetherthere was general concern at the time for the adequacyof timber structures simply because they were timber.The roof of the Banqueting House, Whitehall, built bySoane in 1824 to replace Inigo Jones’s original, wasa fine combination of timber and iron. Nevertheless,sometime in the inter-war period it was felt necessaryto add steel to all the joints (Fig. 2) and to insert steelties to take over the job that the tie beams seem tohave been doing adequately till then. Clearly the rein-forcing shown in Figure 2 is having no useful effect.The steel structure added to the roof over the hall atGreenwich Hospital also seems to be an unnecessarysupplement to the original structure because in neithercase is there obvious structural distress. But excessiveuse of steel can still be seen in use today because itis the material that engineers are familiar with and somay instinctively turn to even though timber repairswould be perfectly satisfactory.

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In the 1970s the use of epoxy-resins as either a con-solidant, or to replace lost timber appeared to offer asolution for those who wanted the minimum loss ofhistoric fabric, but this ran into technical difficulties.Early research in the US on this method, where theywere interested in it for repairing softwood trusses inthe roofs of aircraft hangars, showed the difficulty ofcontrolling the run of what was a liquid and the obvi-ous change in the structural characteristics of joints.However, it is questionable whether or not this workwas noticed by those in Britain who were enthusiasticfor the use of the material and there are some unfor-tunate results of its early and inexpert use. BaguelyHall, on the outskirts of Manchester, is an example ofthe kind of thing that can go wrong. Epoxy-resin wasused to repair the main fames of this fine medievalhall but was allowed to run over the surface of timbersand is now a permanent disfigurement. Many earlyrepairs using this material were carried out by a firmwho appeared to have little technical knowledge andeven less interest in the proper training of their opera-tives. The result was a number of failures, sometimesbecause of the use of the technique in unsuitable loca-tions where exposure to the weather seems to haveresulted in accelerated decay of timber adjacent to theepoxy-resin repair.

The difficulty then was that with considerable anec-dotal evidence about failures following the use ofepoxy-resin, and with little scientific evidence, therewas deep concern among some over the long-term per-formance of such repairs.This was especially so wherethey might be used in timbers with high or fluctuat-ing moisture content. If there were to be deteriorationof the adjacent timber it would have an effect on thestrength of the joint. One might also be suspicious ofits behaviour when used as an adhesive to join steelreinforcing plates to timber and the effects of temper-ature changes. As the steel must move more than thetimber what is happening at the interface between thematerials? Here we have no information and can onlywait and see.

Opinions on the use of the material seem to stillbe divided between those with long memories and aninstinctive distrust of these techniques and those forwhom it is a valuable tool. TRADA (2001) have pub-lished a book on its use and at the Whitbread Brewery,London, Hockley and Dawson used it to effect in therepair what may well be the largest surviving eigh-teenth century trusses. More recently the Weald andDownland Open Air Museum has been developingskills in the use of epoxy resins for repairing struc-tures that have been dismantled. (Their techniques arenot suitable for in-situ work.) Richard Harris pointsout that this material allows one to make repairs thatretain a lot more of the historic fabric. He has alsoargued that if something is going to be lost throughdecay and epoxy-resin will prolong its life, then it is

sensible to use it – but this is an argument normallyapplied to non-structural elements.

5 FASTENERS AND DESIGN CODES

Once the forces in a structure have been determined,and unless there is particular concern over deflections,our engineering problems become a series of con-nections with members in between; member stressesare generally low. Unfortunately the jointing methodsused in new structures are not always appropriate forhistoric structures. Moreover, historic structures usedetails that are not used in modern carpentry and there-fore for which there is no guidance within design codesframed for new buildings, using modern materials andmodern methods. This was never so clear as duringthe lacuna when the newly introduced British Standard(BS5268) in 1984 failed to include oak as a structuralmaterial and conservation engineers had to continueto use the obsolete code (CP112). While that issue hasbeen resolved, there are still questions over the per-formance of traditional joints and the values that onemight use for metal fasteners.

The British code of practice is even inadequate inthe information given for steel fasteners. One exampleis that safe loads for screws in shear are only given upto 10 mm diameter when carpenters may well wish touse 12 mm diameter screws. (Even that is an improve-ment on the earlier editions of the code where loadswere only given up to 8 mm diameter.) An engineerwho is unfamiliar with timber may be unwilling to gobeyond the limits of the table, particularly as the for-mulae for determining the loads from first principlesare rather daunting. Bolts can be seen used in circum-stances where screws would have been better but whereM10s were presumably inadequate. Moreover it mightnot be clear to those unfamiliar with the code that thespacings and edge and end distances for bolts ratherthan screws apply to these larger sized screws.

There is the serious question about how we shouldview the allowable loads on bolts and screws used ingreen timbers that will dry in service. The problem forconservators is complicated by the fact that one sideof the joint will be dry timber and the other side green.I have no idea what reasoning or experimental resultslie behind rather draconian reduction factor requiredby the code. If it is to allow for the possibility of split-ting as the timber dries one would have thought thatsuch an event might well reduce the capacity of thejoint to zero.

6 TRADITIONAL JOINTS

The terminology is a little loose here because thereare traditional joints, either surviving in existing struc-tures or used in repair work that are no different from

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Figure 3. In loading to determine shear capacity of timberthe forces are applied at each end.

Figure 4. In atypical heel joint restrain is provided by thetie beam behind the notch.

joints contemporary with the original construction.At the same time there are those joints that have asuperficial resemblance to traditional joints but whichrely on modern fasteners, the most common being thescarf joint. The difficulty is that any tests to deter-mine working loads for such joints must be affectedby the workmanship in the carpentry. One can envisagethe problem of relating test results to the performanceof historic joints, both with very uncertain standardsof workmanship and the effects of time and chance –or perhaps that should be wind and weather.

One wonders whether the standard test for shearresistance is a suitable basis for the design of tradi-tional carpentry joints relying upon shear along thegrain? The test loads the timber at either end so thatone might expect a sensibly uniform shear stress alongthe plane of failure – the dotted line in Figure 3.

The joint where timber is loaded in shear parallelto the grain is a typical heel joint where the normalcondition in practice is for restraint to be provided bythe tie beam behind a notch (Fig. 4) or a mortice andtenon joint. In such configurations one would expectthe shear stress along the plane of failure to fall offtowards the unloaded end; but in what way? A com-plicating factor in some buildings is that this timbermay be rather exposed and have a higher than normalmoisture content.

Pegged joints is one of those areas where there is anoverlap between new-framed construction and conser-vation, the question being whether such joints can berelied upon to transmit tensile forces. The difficulty isthat pegs do not behave in the same way as steel bolts ordowels so that Johansen’s equations cannot be appliedto find the allowable loads. Experimental work showsthat the pegs fail in shear close to the tenon/mortice

Figure 5. A pegged mortice and tenon joint. The peg willfail at the interface between the two timbers.

Figure 6. Forces on a lap-dovetail joint used between tiebeams and wall plates.

boundary without the kind of rotation that occurs inbolts.

Unfortunately what work has been done so far hasproduced conflicting results. Drawing on the work ofJonathan Shanks carried out at Bath (Shanks &Walker,2005), Ross et al (2007) have given possible work-ing loads for pegs in oak, whereas a formula derivedfrom quite different tests by Schmidt (2004) at theUniversity of Wyoming suggests a working loads ofless than half their figures. Given such a wide dis-crepancy, prudence suggests one should use the lowerfigure for design, but science surely requires someexplanation for this large difference. The Bath testswere carried out on complete joints while those inWyoming in more ‘traditional’ laboratory experimentthat allowed the effects of varying density in both pegsand member timbers to be explored.What surprises meis that the Bath tests that used carpenter-made joints,and presumably subject to variations of workmanship,nevertheless resulted in the higher figure.

The behaviour of dovetail joints is another problem-atic area as these rely on timber loaded across the grain(Fig. 6). When there is drying shrinkage in the timbersthere is a change in the angles of the two parts of thejoint resulting in a reduced contact area and a corre-spondingly higher compressive stress.The stress mightbe well above the elastic limit with collapse of someof the cell walls. However there is no readily availabledata from which one can estimate the movement thatmight result.

A serious problem occurs when existing structuresrequiring repair incorporate details that are ‘prohib-ited’ by the present code. An example occurred whererafters had birdsmouth cuts over the supporting platewith the depth of the cuts exceeding half the depth of

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Figure 7. Birdsmouthed rafters.

Figure 8. A simple half-lap scarf with bolts in shear.

the rafters (Fig. 7). This is larger than the depth of anotch allowed by the code but any change would haveaffected the geometry of the roof. Admittedly somehad split, but these were in the minority. In the event acase had to be carefully made to allow their retentionof the sound rafters and the repair of those that hadsplit.

Scarf joints are commonly used in repairs wherebending moments have to be carried.A simple arrange-ment is a half lap with bolts or other fasteners actingin shear (Fig. 7) but a more traditional joint is oftenpreferred for visual reasons (Fig. 8). If a moment isapplied to this, one end will go into tension and thepreferred fixing is usually one or two coach screwsacting in withdrawal. (Screws are preferred to boltsso that they are visible from one side only.) However,there is currently no guidance on the detailing for sucha screw fixing and as carpenters will usually want tosink the heads below the surface, the small amount ofroom available is evident from the drawing.

Carpenters adopt a simple rule of making the lengthof the joint about three times the depth of the section.A few years ago TRADA looked at a number of differ-ent methods of making such joints deriving measuresof ‘efficiency’ for each, i.e. comparing their resistancemoment with that of solid timber of the same section.This simple, some might say simplistic, approach to theassessment of joint behaviour does not tell one how the

Figure 9. A more traditional scarf joint with screws inwithdrawal at the tension end of the joint.

joint is behaving in practice, and the fastening meth-ods were not those that would recommend themselvesto carpenters.

I have left the most difficult issue till last, which isresistance to wind loads. One might assume that build-ing that has stood for many years, possibly centuries,has stood the test of time. While that is not unreason-able for masonry structures with large masses it is moreproblematic with lighter timber structures that mighthave been subject to some deterioration, especiallyif there have been periods of neglected maintenance.Moreover we are aware that climate change will resultin higher wind speeds in future, again more of a prob-lem for timber structures rather than masonry. Theresult is that it might be difficult to demonstrate thatan existing structure can cope with the required designloads. Of course, in attempting to do this one will oftenconsider the structure acting alone whereas the framesmay have always relied to some extent upon the infillmaterial. It is, of course, possible to use the resistanceof infill panels where these are of a construction recog-nised by present design codes, but we know little ornothing about the resistance provided by historic lathand plaster or wattle and daub infills. Here we mightconsider Japanese work that has explored the resis-tance of traditional infilling material in the context ofearthquake loading. Whether such an approach can beextended from large and infrequent loading to moremodest but very frequent loading is a moot point andone that would need to be considered before embarkingon research in this direction.

7 CONCLUSIONS

This view of the present state of timber conservationsuggests that in many cases there is too little engineer-ing involvement in determining the overall strategyof repair. This might be true for other materials aswell but its effects are possibly more serious whendealing with structures often requiring extensive andhighly visible repairs. This is in part compounded bya failure by some engineers to come to terms with the

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characteristics of this material in their analysis of itsbehaviour.At the detailed design stage, there have beensome developments as a result of the present fashionfor green-oak structures. However, work in this area isstill patchy and rather inconclusive. If this seems a littlepessimistic, what I have not discussed is the quality ofthe pool of carpentry skills that has been growing overthe last few decades that facilitate good standards ofconservation

REFERENCES

Baines, F. 1914, Westminster Hall, report to the first commis-sioner of H.M. Works, etc., on the condition of the rooftimbers of Wstminster Hall, with suggestions for main-taining the stability of the roof, London, HMSO. (Cd.7436).

Charls, F.W.B. & W. Horn 1973, ‘The cruck-built barnof Leigh Court, Worcestershire’, J. Soc. ArchitecturalHistorians.

Foot, N.J.D., C.D. Litton & W.G. Simpson 1986, ‘The highroofs of the east end of Lincoln Cathedral’ in MedievalArt and Architecture at Lincoln Cathedral (B.A.A. Conf.Trans., 8), pp. 47–74.

Hewett, C. 1967, ‘The barns at Cressing Temple, Essex’,J. Soc. Architectural Historians, 26.

Hewett, C. 1980, English Historic Carpentry, Philimore,London.

Hewett, C. 1985, English Cathedral and Monastic carpentry,Philimore, London.

Horn, W. & F.W.B. Charles 1966, ‘The cruck-built barnof Middle Littleton, Worcestershire, J. Soc. ArchitecturalHistorians.

Horn, W. & F.W.B. Charles 1984, ‘The cruck-built barnof Frocester in Gloucestershire’, J. Soc. ArchitecturalHistorians.

Rigold, S.E. 1966, ‘Some major Kentish barns’,ArcheaologiaCantiana, 81, pp. 1–30.

Ross, P., C. Mettam & A. Holloway, 2007, Green Oak inConstruction, TRADA, High Wycombe, p. 163.

Schmidt, R.J. 2004, ‘Considerations for mortise and tenonjoint design’, Wood Design Focus, 14, No. 3, pp. 14–17.

Shanks, J.D. & P. Walker 2005, ‘Experimental performanceof mortice and tenon connections in green oak’, TheStructural Rngineer, (6 September), pp. 40–45.

TRADA 2001, Resin-bonded Repair Systems for StructuralTimber, Timber Research and Development Association,High Wycombe.

Yeomans, D.T. 1992, The Trussed Roof: its history anddevelopment, Scolar: Aldershot.

Yeomans, D.T. 2007 ‘Appropriate technologies for conserva-tion’, Journal of Architectural Conservation, 13 No. 3,pp. 9–20.

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