Tuesday, October 17, 2017

The Coupled Roof, Ceilings and Trusses


Moving away from early roof forms that provided both wall and roof in one unit, the next development showed a true roof built on masonry or timber walls. The simplest form of roof was a coupled roof, consisting of two lengths of timber bearing against each other at the top and resting on a wall plate at their feet. The timbers, called couples, were pegged together at the top with timber dowels and were similarly pegged or spiked to the wall plate. The term ‘couple’ was used until the fifteenth century when the terms ‘spar’ or ‘rafter’ started to be used. The term rafter of course is still used to describe the piece of timber in a roof spanning from the ridge to the wall plate. paced about 400 mm apart tied only by horizontal binders and tile battens. The simple couple was adequate for small span dwellings and steep pitches, but the outward thrusting force at the feet of the rafters caused stability problems with the walls, and excessively long rafters sagged in the middle under the weight of the roof covering. The illustration in Fig: 1.1 shows the required shape in solid line and the deflected shape in dotted line.

To overcome both of these problems the ‘wind beam’ or ‘collar’ was introduced. Whether the collar acts as a tie or a strut for the couples will depend upon the stiffness of the supporting wall below. Assuming however, that the wall is so substantial that it will not be pushed outward by the bottom section of the couples, then the collar will act as a strut. If however, as is more likely with early timber framed buildings, the wall is relatively flexible then in that case the collar would act as a tie holding the couples together. There would still be some outward thrust but this would be limited by the collar to the degree of bending in the lower part of the couple only. It can readily be appreciated that in larger roofs, where the walls are relatively flexible, there is a considerable tying effect in the collar demanding a more sophisticated joint between collar and couple than could be achieved with simple iron nails. The collar was therefore frequently jointed to the couple with a halved dovetail shaped joint, often secured with hardwood pegs.
Simple Coupled rafter-roofconstruction-terminology.blogspot.com
Fig: 1.1 Simple Coupled rafter


The next development was to fit additional members to assist with the stability of the roof in windy conditions and these were called ‘sous-laces’ or braces. On roofs constructed on substantial masonry walls which were also very thick, further struts or ‘ashlars’ were introduced to stiffen the lower section of the couple. Fig: 1.2 illustrates this form of construction, the wall plate being well fixed to the wall with the bottom member of the ashlar halved over it to prevent the roof sliding on the top of the wall.

These now very substantial ‘couples’ began to be spaced further apart and became known as ‘principals’. Between these main members simple couples or ‘rafters’ were placed, but to avoid sag or to accommodate longer rafter length possibly not available in one length of timber, an intermediate support was needed and this was called a ‘purlin’. The purlin is in turn supported by the principal couples, as shown in Fig: 1.3.

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Fig: 1.2 Ashlar stiffening

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Fig: 1.3 Principal truss and purlin roof

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Fig: 1.4 Tie beam truss

The tendency for the roof to spread was now concentrated in the heavily loaded principals and it became apparent that if spans were to increase this spreading would have to be controlled. The ‘tie beam’ was introduced thus forming the first ‘trussed’ or ‘tied’ roof. Fig: 1.4 illustrates the roof form described.

As development progressed the span of the roof was limited only to the availability of long timbers used for the tie beam, but it is obvious that these long beams themselves would tend to sag under their own weight. To prevent this happening they too had to be supported and this was done with the introduction of ‘struts’ fitted to a corbel built into the wall below, as illustrated in Fig: 1.5.

With this tie beam now becoming a major structural member a different configuration of members evolved becoming more like the truss common today. Having stiffened the tie beam it became apparent that this could be used as a major structural item from which to support the principals. The major support running from the center of the tie beam to the ridge purlin was known as the ‘mountant’, now referred to as a ‘king post’ (see Fig: 1.6). A king post truss is also illustrated in Fig. 9.1, being used as part of the structure of an attic room. With two posts introduced the roofform is known as a ‘queen post’ truss, which in its simplest form is shown in Fig: 1.7. This particular roof form gave the opportunity of providing a limited living space within the roof. It should be remembered that until this stage of development all roof forms and trusses described had no ceiling and were open to the underside of the rafters and roof covering. To use the queen post roof form as an attic, a floor was needed thus creating a ceiling for the room below.

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Fig: 1.5 Strutted tie beam

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Fig: 1.6 King post truss

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Fig: 1.7 Queen post truss


Ceilings were first referred to in descriptions of roofs in the fifteenth century when they were known as ‘bastardroofes’ or ‘false roofs’ and then later as ‘ceiled roofs’, hence ‘ceiling’ as we know it today.

The ceiling supports were known as joists or cross beams again being supported by the hard working tie beam between the principals. The construction is illustrated in Fig. 9.2.

Continuing developments of the roof form itself, and demand for even larger spans and heavier load resulted in some relatively complex principals or trusses being developed. One such form was the ‘hammer beam’ roof, illustrated in Fig: 1.8. Clearly this is not a roof to be ‘ceiled’, being very ornate as well as functional.

Hammer beam truss-roofconstruction-terminology.blogspot.com
Fig: 1.8 Hammer beam truss
The hammer beam roof is generally to be found supporting the roof over halls in large mansions and of course churches. The roof was framed in such a way as to reduce the lateral thrust without the need for a large and visually obstructing tie beam. The walls onto which such a roof was placed had to be substantial and were often provided with buttresses in line with the principals to contain any lateral thrust that may develop.


Roofs in truss form developed using carpentry joints and some steel strapping, until the latter part of the eighteenth century when bolts, and even glues, started to be used to create large truss forms from lighter timber members. Such truss forms often used softwoods, as distinct from the hardwoods more frequently used in the shapes previously described. The large timber sections in oak particularly were becoming very scarce and of course very expensive. Whilst some significant advances in span were achieved, using the techniques described above, the domestic roof did not require very large spans and changed very little from the collared coupled roof. Indeed many small terraced houses built during the eighteenth and nineteenth century required no principals at all. The dividing walls between the houses were close enough to allow the purlins to rest on these walls, effectively using them as principals. Fig: 1.9 illustrates a typical terraced house roof construction.

The larger properties where the span of the purlin was too long for one piece of timber, or where hip ends were involved, continued to use the established methods of construction using principals, collars and purlins, but it was common practice to omit the principals and to support the purlins off the walls below with posts or struts.
Purlin and common roof-roofconstruction-terminology.blogspot.com
Fig: 1.9 Purlin and common roof


In 1934 the Timber Development Association (TDA) was formed, now known as TRADA (Timber Research and Development Association). The Association took up the work already being done at that time by the Royal Aircraft Establishment and progressed work on timber technology alongside the Forest Product Research Laboratories. Although the Royal Aircraft Establishment may sound a strange body to be interested in timber, it must be remembered that many aircraft of that era, and some notable ones after such as the Mosquito, used highly stressed timber structures for the fuselage and wings. Some aircraft hangars were of timber construction and utilised record breaking large span small timber section trusses with bolted joints.

After the Second World War shortages of materials resulted in a licence being required for all new building works, making economy in use of paramount importance. Imported materials such as timber were very much at a premium and TDA was given the task to find ways of economising on the country’s use of timber. Quite correctly they identified the roof structures of buildings as a high volume user of timber and developed a design for a domestic roof using principal trusses constructed of small timber sections connected with bolts and metal connector plates. The roof used purlins and common rafters similar to the systems previously discussed. These trusses became known as ‘TDA’ trusses, and with some minor modifications are still in use today. It appears that some of these designs were available shortly after the Second World War but were first published as a set of standard design sheets around 1950.

The designs were based on existing truss shapes but were not engineered in the sense that structural calculations were prepared for each design. Load testing on full size examples of the truss was used to prove their adequacy and from these tests other designs developed.


The first designs produced were known as ‘A’ and ‘B’ types, dealing with 40° and 35° pitches respectively. They covered spans up to 30 ft (9 m).

House design fashion changed during the later 1950s and early 1960s, demanding lower roof pitches. 1960 saw the introduction of the TDA type ‘C’ range for pitches between 22° and 30°. Spans were also increased up to 32 ft (10.8 m). Around 1965 the types ‘D’, ‘E’ and ‘F’ ranges were published; these later designs using a slightly different truss member layout went down to 15° pitch and up to 40 ft (12 m) span. Further designs used trusses spaced at 6 ft (1.8 m) centres and had some degree of pitch and span flexibility within specified limitations.

A range of designs for trussed rafters (i.e. each couple tied together at ceiling level) was produced also using bolt and connector joints, but these were designed only to carry felt roof coverings and did not prove as popular as the principal truss designs.

Industrial roofs were not neglected, with principal truss designs using the bolt and connector joint techniques for pitches of 22.5° spacing between 11 and 14 ft (3.35– 4.25 m) and up to 66 ft (20.1 m) span.

Whilst roofs are still constructed using these techniques, the TDA designs are no longer available from TRADA.


All of the TDA principal and trussed rafter designs used bolts and connectors at joints where previously mortice and tenon, half lap or straight nailed or pegged joints would have been used. The small timber sections used in the designs of the trusses did not allow the use of conventional carpentry joints and gave insufficient nailing area for an all nailed assembly. The connector allows the forces in the joint to be spread over a large area of the connected timber, the bolt holding the timbers in place thus allowing the connector to transmit the load from one truss member to the other.
Fig: 1.10 illustrates the typical single connector joint.
Toothed plate connector joint-roofconstruction-terminology.blogspot.com
Fig: 1.10 Toothed plate connector joint

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