The Conversion of Structural Timbers

Recognising Historical and Modern Techniques

Joe Thompson

 

  Timber supported on trestles is sawn from end to end with one sawyer standing on the timber and one underneath it
  The author (top) demonstrating trestle sawing (Photo: Tim Walton)

If only historic timbers could talk – what questions you could ask, what stories they could tell. While they may remain stubbornly silent you can, however, ‘read the timbers’ and learn how they were converted and what size trees they came from. This is done by examining the surfaces of the timbers, often with a low angled light, for signs of both the method and the type of conversion.

The information gained from these primary sources is invaluable for interpretation reports or conservation plans, helping to build up a picture of the chronology and development of the structure in question. Documentary evidence and scientific analysis can also be used but the initial appraisal is often based on a stylistic judgment which relies on understanding what you are looking at and putting it into context.

It is worth remembering that nearly always the part of the tree (the log) that we are interested in derives from the main trunk or bole, not the branches or roots, although sometimes portions of branch or root stock are left attached to the main trunk for specific purposes, such as jowl posts.

In the case of homegrown hardwoods – typically oak and elm from managed woodland, parks or hedgerows – the timber trees are often oval in section and crooked as well as tapering along their length. As a result, bark and sapwood are retained on the converted timber, and curved or cranked timbers nearly always come from trees with curved or cranked trunks rather than being cut out of a larger straight piece. Imported softwoods were often from virgin forest near to rivers and seas, and tended to be circular in section and straighter in length.

CONVERSION METHODS

There are three methods of conversion that have been and continue to be used to turn round logs into timbers for structural use by carpenters: cleaving, hewing and sawing.

Cleaving, splitting or riving uses wedges which are struck with tools such as beetles (mauls) or mallets to split the log longitudinally, following the grain. If required, a variety of clamps or ‘brakes’ can be used to help hold the timber and stop it moving.

Cleaving’s great advantages are the fact that it is the quickest of the three methods and that the maximum achievable strength is gained by following the grain. Its practical disadvantage is that it is relatively difficult to find a consistent supply of good quality cleaving logs. Our Anglo-Saxon and Norman forbears made extensive use of the natural wildwood (virgin forest), which contained many such trees. Based on the evidence of archaeological finds from waterlogged sites in England, they produced structures using cleft timbers, often ‘earthfast’ (built into the ground like a fencepost).

Cleft timber surface See sawn timber surface  
Cleft surface: the process of splitting tears the grain apart and leaves a distinctive, slightly ridged finish. See-sawn surface: inspection of the area where the two saw-cuts intersect shows that the last connecting piece of timber is cleft off, leaving a torn surface midway along in the shape of an equilateral triangle.  
Pit-sawn timber surface Mill sawn timber surface  
Pit sawn surface: the finish is characterised by saw-marks at about 75 to 85° to the length of the timber and a cleave-off at one end in the shape of an irregular triangle. Mill sawn surface: wind or water-powered reciprocating saw blades cut at about 90° to the length of the timber, leaving evenly and closely spaced saw marks with no cleave off.  

The process of splitting tears the grain asunder and leaves a distinctive, slightly, ridged surface finish. Because the splits follow the grain, and the grain can both spiral and deviate around knots, however small, cleft timbers are often dressed with an edge tool like an axe or drawknife to produce greater regularity. The marks from where the edge tools have cut into the surface are sometimes visible. As a result, it is extremely rare, due to the difficulty of procuring the required amount of long, clean timbers, to come across any cleft structural timbers in buildings surviving from after 1200AD and those that do survive tend to be relatively short (less than 2m long). Splitting is used, however, to produce a wide variety and great quantity of associated elements that are quite short (roughly 300-1800mm), such as panelling, boards, shingles, laths, staves and pins.

Hardwoods cleave best radially, conferring the added advantages of reducing moisture-related movement and enhancing the aesthetic qualities of oak by exposing the maximum amount of the attractive medullary rays on the wide face. In contrast, softwoods such as Scots Pine cleave best tangentially. This can often be seen in nailed-on, imported Georgian laths where a face knot runs through a series of adjoining laths.

Hewing is the technique of using edge tools to ‘square up’ logs, although the final shapes are often rectangular in cross-section as well as tapering along their length. Due to the adaptability of the axe and the hewer, cranked or curved timbers present no particular problems. Whatever the shape required, lines are first snapped on to the log, the bark being either left on or removed.

Two different types of axe are used – firstly the scoring axe, which is often the same axe that was used to throw the tree over. The log is supported on low bearers or trestles and held-fast with pointed iron dogs. The long-handled, double-bevelled scoring axe first chops a series of notches into the log at roughly 300-400mm intervals. Then the same axe is used to strike off the timber between the notches and to clean off any remaining large irregularities.

This leaves rough-hewn surfaces consisting of shallow notches, torn-out grain and axe-marks that are then cleaned up with a second type of axe, which is generally shorter handled, single-bevelled and slightly convex. It is used to remove just the final shavings, to get exactly down to the line. The final surface finish is defined by the slightly undulating, smoothly cut surface with some small amount of tear-out on one side of any knots.

The quality of some hewing from the 13th and 14th centuries has to be seen to be believed – it can appear almost hand-planed, so crisp are the corners and so smooth is the surface. In theory, no trace of the roughhewing should be visible, but in practice it becomes increasingly evident on hewn timber surfaces from the 15th to the 19th centuries. Hewing was practised widely and nearly all buildings with structural timbers have some visible hewn surfaces. With imported softwoods, which mainly date from the 17th century onwards, the hewn baulks show a similar, slightly undulating, smoothly cut surface finish. Hewing is slower than cleaving but quicker than sawing.

Sawing shows the most variation of the three types of conversion due to the different ways the saw can be presented to the timber and the motive power driving it. Sawing appears around the end of the 12th century in the form of ‘see-sawing’, where the hewn timber is inclined at an angle and supported at about mid-length on a single trestle. The frame saw then cuts from one end down to the trestle, before being withdrawn. The timber is then pivoted on the trestle like a see-saw and a second cut started from the other end.

  Circular sawn surface with evenly spaced curved saw marks
  Circular sawn surface: a product of the steam age, circular sawn timber is easily identified by its very distinctive, large, curved saw marks which radiate across the surface.
  Bandsawn surface with widely spaced vertical marks
  Bandsawn surface: recognisable by very distinctive and widely spaced saw marks that are at 90° to the length of the timber.
  Boxed heart with pronounced shakes on three of the four faces, running from centre of face towards the heart
  Boxed heart timbers have just the outside of the log removed so tend to be roughly square in section. During seasoning they stay straight but do tend to ‘shake’ (or crack) on one or more faces.
  Halved timber with shake running from centre of upper wide edge towards the heart
  Halved timbers are first boxed and then sawn down the middle. The wide faces tend to bow, with shakes developing on one or both of the two wider faces as they season.
  Quartered timber, roughly square in section and free from shakes
  Quartered timbers are boxed and halved and then each half is sawn again to produce four square sections. They tend to bow in two dimensions but do not shake as they season.

Bearing in mind the steel technology available between the 13th and the 15th centuries, it is a testament to the skill of the saw-makers and sawyers that these two cuts almost always line up. Close inspection reveals that the saw-cuts do not quite intersect but that this last connecting piece of timber is cleft off, leaving a torn surface in the shape of an equilateral triangle about midway along. See-sawing is also distinguished by the orientation of the saw marks at an angle of about 50-70° to the length of the timber.

Between the end of the 14th century and the mid-16th century, a new technique was gradually adopted across the country which consisted of supporting the hewn timber horizontally, either over a pit or on high trestles, and sawing from one end only. This technique is often generically described as ‘pit sawing’ and from the 17th century onwards it used a saw without a frame (a whip saw or pit saw). Pit-sawn timber is distinguished by saw-marks at about 75-85° to the length of the timber and a cleave-off at one end of the timber in the shape of an irregular triangle.

Evidence of mill sawing by European sawmills is mainly found on imported Baltic softwoods from the 17th century onwards. Mill sawing used wind or water-powered reciprocating saw blades which cut at about 90° to the length of the timber, leaving evenly and closely spaced saw marks with no cleave off. It can be difficult to distinguish between mill sawing and good quality pit sawing.

Circular sawn timber arrived in the late 18th or early 19th century and became established with the advent of steam power during the 19th century. It is easily identified by its very distinctive, large, curved sawmarks radiating across the surface.

Lastly, bandsawn timber dates from after the 1860s and can be recognised by sawmarks that are at 90° to the length of the timber but with a regular pattern of more widely spaced sawmarks than timber from reciprocating sawmills.

CONVERSION PRODUCTS

The following types of conversion are often encountered; the first three types were extensively used to produce a variety of timber products including posts, ties, plates and rafters:

Boxed heart or boxed timbers have just the outside of the log removed and so tend to be roughly square in section. They can be either hewn or sawn and may contain waney edge on one or more arrises. Typically, the boxed heart timbers stay straight but they do tend to ‘shake’ (or crack) on one or more faces, due to differential drying between the radial and tangential dimensions as they season.

Halved timbers are first boxed as above and then sawn down the middle. This produces a distinctive rectangular section with waney edge on a maximum of two adjoining arrises. The wide faces tend to bow, with shakes developing on one or both of the two wider faces as they season.

Quartered timbers are ‘boxed and halved’ as above and then each half is sawn again to produce four square sections with waney edge on a maximum of one arris on each quarter. They tend to bow in two dimensions but they do not shake as they season.

Quarter-sawn or radially converted timber is produced by sawing or cleaving the log multiple times from the bark to the centre, producing thin, triangular sections. The bark and sapwood are trimmed off leaving boards that tend to stay flat and not split as they season. This is the predominant type of conversion for producing oak panelling, instantly recognised by the pattern of the lighter coloured medullary rays running as flowing flecks across the wide faces of the boards.

Through-and-through or plain or flat sawn timbers are first boxed and then sawn into four or more evenly sized, rectangular pieces. Those pieces nearest the pith tend to cup more than those nearest the outside of the log but the latter can have one or two waney edges. Braces, struts and floorboards are typically converted like this.

Multiple or multi-sawing is found with larger logs (above 300mm diameter) which have first been boxed and then sawn into six or more evenly sized pieces. Four pieces contain one waney arris, the others none, and usually only the timber(s) containing the pith develop any shakes. Timbers converted like this were typically used for floor joists and rafters from the 16th century onwards. They can be distinguished from quartered timbers by the shallower curve of any waney edge and by looking at the end grain if possible.

Baulk re-sawing is restricted to large, imported, softwood, boxed heart timbers which were then re-sawn in England. In his classic 1937 book The Village Carpenter, Walter Rose described how the dirty outer faces of the hewn baulk were sawn off for joists and ceiling ties leaving a clean, sawn, boxed heart which was then sawn through and through for boards. Close inspection of Victorian houses undergoing repair work to the floors provides evidence of this practice.

REPAIR PHILOSOPHY

How should today’s conservator respond when fitting new timbers alongside historic ones?

Modern structural timbers are generally bandsawn so their surface finish does not match that found on most historic timber frame structures. Firstly, the timbers can be used as they are, with their bandsawn finish; this is by far the most commonly encountered response. If a surface finish visually closer to historic timbers is specified, then a number of options have been tried over the last 40 years or so. Hewing timbers by hand is best practised on freshly felled logs but this approach is expensive and time-consuming. While it is theoretically possible to hew just a few millimetres off the exposed faces of bandsawn timbers with a side axe, in practice it is hard, slow, highly skilled work. More commonly, planes with convex blades are used obliquely across the grain in an attempt to imitate the undulating surfaces of hewn timbers.

In the case of sawn timbers too, working them by hand is hard, slow work. Often a chainsaw mill is used instead. It cuts at a similar angle to pit sawing so it imitates the historic surface finish. Cleaving is still fairly widely practised today for the fencing, shingle and lath markets due in part to its inherent speed, so obtaining good cleft material is usually straightforward.

Philosophically, where do the above modern imitations leave us? Working the surfaces of the timbers, after the imitation historic finish has been applied, to such a level that it is difficult to distinguish between modern and historic – for example by drilling fake woodworm holes or hitting surfaces with chains to simulate wear and tear – is deliberate deception and should be avoided. This approach confuses and distorts the archaeological record and is counter to the long-established conservation principle of ensuring that new is distinguishable from old.

An approach which is widely advocated is that of ‘honest repair’, but this is a rather vague and ambiguous term in practice, with some repairs being left with razor sharp arrises in stark contrast to the weathered, more softly contoured adjoining timber. The time-honoured response is often: ‘don’t worry, it will soon weather in!’ However, the time required for this process to start can vary from a year to many decades, depending on exposure and humidity.

It is often best to follow a twin-track approach which allows the new timber to be easily identified on close inspection, while from a distance it blends in visually with its neighbours. This can be achieved in a number of ways; for example by removing any sharp arrises, by sanding off most of the visible bandsaw marks, and by careful initial selection and matching of the timber.

 

 

 

The Building Conservation Directory, 2017

Author

JOE THOMPSON of Sussex Oak and Iron has been working with historic timber frames since 1990. He is carpenter in residence at the Weald and Downland Open Air Museum in Sussex, where he also lectures and runs practical workshops.

Further information

RELATED ARTICLES

Joinery and cabinetmaking

Structural

Timber conservation

Timber-framed buildings

RELATED PRODUCTS AND SERVICES

Cabinet makers

Fine joinery

Historical research

Structural engineers

Timber frame builders

Timber repairs

Timber suppliers

Timber testing, structural

Wood carvers and turners

 

BuildingConservation.com
Site Map

© Cathedral Communications Limited 2017