Repair of Timber-Framed Buildings

Although some repair work is undertaken under the supervision and direction of an architect or building surveyor with knowledge of the repair of timber-framed structures, most of the repairing and adapting of these buildings is undertaken, with little or no professional assist­ance, by local builders who in many cases follow the instructions of their clients. Before leaving the subject of timber-framed buildings it might, therefore, be appropriate to say something about the repair of these structures to assist those people contemplating such work either by themselves or with the aid of a builder.

First, one must distinguish between the terms ‘repair’ and ‘resto­ration’, both frequently used in connection with old buildings but meaning very different things and having very different effects on the building. The term ‘restoration’ implies an attempt to revert the building to a precise period in its architectural history. In a misguided attempt to restore an old building to an original form, it might be necessary to destroy other, later and important features, and this constitutes in many cases at least as great a threat to our heritage as decay and redundancy. It must be remembered that the form and character of an old building are a result of a long historical process in which the many changes which have occurred throughout the centuries are as much a part of the building as any old feature. Restoration work should only be undertaken when the building is unique and the work is carried out to enhance the architectural and historic value of the building. Any such restoration work should be approached with great caution and carried out under the supervision of an expert and is perhaps best left to one of the museums, such as the Avoncroft Museum of Buildings or the Weald and Downland Open Air Museum.

The repair of an old structure is another matter and is restricted to the repair of the structure made necessary by lack of maintenance and subsequent decay. It does not demand the replacement of a decayed feature – for instance, a moulded medieval beam too decayed for repair – with a replica but with something in the modern idiom which may compete, by comparison, with the old and so form a continuation of the building’s architectural development which is a fundamental part of its beauty and interest. It is the repair of the timber-frame and in

particular the frame and roof structure that are dealt with in this chapter.

Before carrying out any repairs to a timber-framed structure, it should be carefully surveyed, for no repairs will be successful unless the building is fully understood both technically and aesthetically. The internal finishes need to be stripped away, and this should be done with great care, for often much important information about the building’s history is lost by their careless and sometimes needless destruction. Beneath the layers of wallpaper, which may themselves be rare, may well be the original wall and ceiling paintings, and although these and other details may not be suitable for preservation, all should be recorded.

It is, however, the timber-frame and its condition that must be most carefully examined. It is important to understand the bay system and the method by which the rigidity of the building is ensured. Timber-framed structures were, as we have seen earlier, nearly always triangulated against racking by means of diagonal braces between the various beams and main posts and roof trusses and purlins. Mutilation of these and other structural members over the centuries has been caused, in many instances, by a complete ignorance of the structural relationship between these members. The cutting of these structural members often transfers excess stress to the joints, particularly where new doors and enlarged windows have been provided or where braces have been removed in order to provide additional headroom, particu­larly in roof spaces where new doorways between attic rooms have been formed. In some instances the tie-beam of the roof truss triangu­lating the roof members has been cut away. Another source of weakness often arises with the insertion of a chimneystack cutting through bridging joists, binders and roof members. Again structural failure often occurs in a building whose upper floor had originally been jettied, the lower supporting wall being removed and the jetty underbuilt flush with the wall above. This not only extends the span of the first-floor joists but also loses any cantilever effect of the upper walls whose intended end beam loads had the effect of an upward thrust on the joists and relieving them at mid-span.

Although all these and other defects cause structural problems, because timber adapts itself gradually to these additional and varied stresses and loads, it is important only to repair and strengthen the structure, and no attempt should be made to correct distorted frames back into place unless distortion has only recently occurred and is likely to cause further structural problems. Apart from the possibility of opening up other joints, there is also a chance that new stresses may be induced into the frame.

The jointing system is an important feature of all timber-framed construction, and it is here that any movement will accumulate. The joints were secured with oak pegs and are liable to rot, becoming loose or breaking under stress, with the result that the joints may become open and unseated, and these will require to be strengthened. Any defective pegs should be hammered out and never drilled. Where wooden pegs need replacing, they should be of oak obtained from the heart wood, should be ‘cleft’ not ‘turned’ and should be tapered from a roughly square head to a point, driven in and left projecting.

Despite the distortion and structural problems caused by these defects, timber-framed structures generally suffer more from the ef­fects of beetle infestation and fungal decay.

Infestation is ascribed to one of a number of species of wood-boring insects. First there is the common furniture beetle (anobium puncta – tum), which can destroy both softwood and hardwood, its larvae boring through the wood, digesting the cellulose. It is responsible for about three-quarters of all woodworm damage to property in Britain. The wood-boring weevil (euophryum confine) first became established in Britain after the last war and is already a widespread and serious threat to buildings, attacking both softwood and hardwood. It differs from other wood-boring insects in that it will only attack timber which is already decayed by wood-rotting fungi. A bettle which attacks only softwood is the house longhorn beetle (hylotrupes bajulus). It is found mainly in the southern home counties, particularly Surrey and Hampshire. The lyctus beetle (lyctus brunnens), commonly called the ‘powder post beetle’, came into Britain from North America many years ago. The name ‘powder post’ is derived from the very fine dust produced by the larvae which is quite unlike the frass of the furniture beetle. The larva is found only in the starchy sap woods of hardwoods; the female requires a starch content of three per cent in the sapwood before she will lay her eggs. Unlike some beetles, they will attack only sound timber and are never found in timber already infected by fungus, and are therefore often associated with replacement timbers. Lastly, there is the death watch beetle (xestobium rufovillosum) the most damaging of all wood-boring beetles in old buildings, attacking hard­woods which have at some time maintained fungal infection, often attacking the centre of large timbers which suffered fungal heart rot when installed.

With the exception of death watch beetle, wood-boring beetles can be treated effectively by one of a number of proprietary materials classified in BS 1282, most of which contain copper, chrome and arsenic compositions in accordance with BS 7072. Prior to spraying the timbers, they must be brushed down to remove all dust and other foreign matter. Because of the particular activities of death watch beetle and its habit of attacking large structural hardwoods, normal spray treatments are less effective, and other methods have been used with some success. One system is to apply a thick paste, made up of fungicides and insecticides, liberally on the surface of affected areas. Over a period of time the paste releases its toxic chemicals deeply into the timber. Another method is the development of a special fumigation programme carried out annually when the death watch beetles emerge.

Timber structures also suffer severely from damp, which can result in devastating attacks by some wood fungi – dry and wet rot being the two most commonly known terms used to describe this fungal decay. Dampness, combined with the lack of ventilation, creates the ideal conditions for these attacks, the fungi actually being plants of a low vegetable form of the saprophyte family which attacks timber in buildings in order to extract food (cellulose) to maintain growth and the generation of spores.

Of these two, dry rot (merulius lacrymans) is the more serious and consequently a greater hazard to structural timbers. It requires moist, warm conditions with the moisture content in excess of twenty-two per cent for the spores to germinate and develop. The strands or hyphae grow from the spores to spread, producing enzymes which digest the adjacent damp timber. The cellulose, which it attacks, is broken down to carbon dioxide and water, causing the timber to lose its strength, developing cuboidal fractures (longitudinal cracks) and becoming dry and brittle, and it may be crumbled by hand. Dry rot, being malignant, will spread through brickwork, following the line of damp, in search of more timber to attack. The only treatment for dry rot is the removal of all the affected timber or, on large structural timbers, the removal of all decay and burning of all defective timbers. As the hyphae will no doubt have penetrated the walls, all plastering and rendering in the area of the outbreak should be removed and the wall sterilized, either by heating with a blow lamp or by spraying with a toxic chemical such as pentachlorophenol. When contamination of the wall by the hyphae is considerable, the joints should be cut out and a deep irrigation method should be employed, using a fungicidal solution via holes drilled in the wall. All timbers which are retained and all new timbers should be treated with a proprietary fungicide as classified in BS 1282.

Wet rot (coniophora puteana), sometimes known as ‘cellar fungus’, although not as serious as dry rot, is still a common cause of structural defects. It requires a moisture content of between forty and fifty per cent – much higher than that required by dry rot – to become active. However, when the moisture is removed, all activity of this fungal growth ceases. After attack the timber is left with small cuboidal splits and dark brown in appearance. Again it is important to remove all decayed sections of timber and in addition at least eighteen inches of sound timber. However, no treatment of brickwork or stonework is usually necessary. All new timbers should be treated with a fungicide as previously described. Although both dry and wet rot can be controlled, because of the nature and biology of the wood-rotting fungi, it is always necessary to locate and remove the cause of the outbreak. Most outbreaks are caused by the absence of an effective damp-proof course, by defective roof-covering and by leaking gutters and downpipes. These should all be repaired and kept in good order to prevent possible recurrence of the problem.

The treatment of timber against beetle infestation and fungal decay is normally carried out by specialist firms using specialist equipment and protective clothing, as many of the remedial treatment products are toxic. Care should, therefore, be taken to avoid contact with the skin and the prolonged breathing of any vapour or oral ingestion. When undertaken by specialist firms they are generally prepared to guarantee the efficiency of the completed treatment.

There are several important principles which must be observed whenever possible in repairing timber-framed buildings. First it is of paramount importance that as much of the original timber as possible is retained, and timbers should be replaced only when too much of the timber is defective to make a satisfactory repair. The timber used in repair work is also of importance and must be of the same species as that used in the original structure; when repairing an oak frame, only oak must be used, likewise where the frame is of softwood, this too should be used. When oak is used, it should be unseasoned native oak and should if at all possible be selected from timbers of the least diameter to give the required cross-section of the member.

The conversion of the timber is also important: main timbers, beams, purlins, plates and posts, larger than eight inches square should be obtained from boxed-heart logs; smaller timbers should be halved or quartered depending on size, with those with their cross-sectional dimensions less than six inches heart-sawn, the heart-face always being used for the upper or outer face of the completed building. Logs cut through and through should never be used for external work. Suitable reclaimed timbers are preferred by some, but old oak is extremely hard and difficult to work and should be avoided. Added to this the use of old timbers will tend to confuse future historians.

Timber should, whenever possible, have identical properties es­pecially with regard to grain and moisture content; the latter is particu­larly important with regard to the scarfing-on or the piecing-in of new timbers, which may suffer from the results of shrinkage. When new wood is introduced into an old structure, it is always advisable to impregnate it with one of the proprietary materials previously de­scribed to discourage fungal and beetle attacks. Care should be taken

to ensure that the solution is colourless so as not to detract from the appearance of any exposed work. All new oak pegs used to secure new joints or as a replacement of defective pegs should always be impregnated, as these tend to rot and as previously stated should be cleft and not turned and tapered from a roughly square head to a point to ensure a good fit. In addition all pegs should be baked in a slow oven for a day or two, so that they may expand within the hole, ensuring a snug fit. Bolts should in general be avoided when timber-to – timber repairs are undertaken. All scarf joints should, whenever pos­sible, be skew-pegged, with each peg driven in at a different angle. The pegs should be randomly aligned and can be driven in from all sides. Above all, the repair of old timbers should be undertaken by a skilled craftsman.

As previously stated, it is recommended that timber should be used whenever possible in repair work, but it is also desirable to keep as much of the original timber as possible, and with this in mind it is often advantageous to use metal straps and plates, particularly in the strengthening of joints. Wrought iron was the traditional material used for this and should still be used whenever possible, although it is now difficult to obtain, ceasing to be manufactured commercially some years ago. As a replacement, mild steel is commonly used, but it should always be protected with an appropriate zinc-based primer or better still galvanized. It should also be remembered that all bolts and coachscrews used in conjunction with oak should also be galvanized to resist corrosion by the tannic acid in oak.

Updated: 24th October 2014 — 2:41 pm