CHAPTER 1
The Development of the Pitched Roof
PRIMITIVE ROOF FORMS
Man has always needed a roof for shelter. Early man used roofs formed by nature such as caves, but nomadic peoples had to be more resourceful, creating shelters of a temporary nature each time they moved. It is likely that simple tents formed with animal skins over branches were the early form of constructed roofs, with more permanent shelter being pit dwellings. These were simply a shallow excavation covered with a simple roof of branches and skins. It is an easy step from this type of dwelling to a simple wall on the edge of the pit to raise the headroom and then to use shaped branches to give a slight pitch, thus improving rain run-off and therefore the quality of the environment within the shelter.
The simple ‘crack’ frame comprised two curved pieces of timber standing on the ground at one end and meeting at the top. Across several of these ‘crocks’ were tied horizontal members onto which, again, were fixed skins or as time progressed simple thatch.
THE COUPLED ROOF
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.
The couples were generally spaced 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. Figure 1.2 illustrates this condition. 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.
STABILITY
The next development was to fi t 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. Figure 1.3 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.
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. Figure 1.5 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.
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 centre of the tie beam to the ridge purlin was known as the ‘mountant’, now referred to as a ‘king post’ (see Fig. 1.7). A king post truss is also illustrated in Fig. 9.1, being which is in the PDF. Download it and view the full document.
used as part of the structure of an attic room. With two posts introduced the roof form is known as a ‘queen post’ truss, which in its simplest form is shown in Fig. 1.8. 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.
CEILINGS
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. 1.9. Clearly this is not a roof to be ‘ceiled’, being very ornate as well as functional.
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
Fig. 1.9 Hammer beam truss.
often provided with buttresses in line with the principals to contain any lateral thrust that may develop.
TRUSSES
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. Figure 1.10 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.
Fig. 1.10 Purlin and common roof.
DESIGN FOR ECONOMY
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 identifi ed 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.
STANDARD DESIGN ROOFS
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.
BOLT AND CONNECTOR JOINTS
All of the TDA principal and trussed rafter designs used bolts and connectors at joints where previously mortise 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. Figure 1.11 illustrates the typical single connector joint.
Fig. 1.11 Toothed plate connector joint.
TRUSSED RAFTERS
In the early 1960s the punched metal connector plate was introduced into the UK from the USA and was to revolutionise the construction of domestic roofs even more than the TDA truss designs described. There are now four main plate manufacturers in the UK, the fi rst in 1967 being Gang-Nail whose name has come to be used to describe all punched metal connector trusses, in the same way that ‘Hoover’ seems to describe a vacuum cleaner.
Trussed rafters are generally prefabricated in a factory and transported to site, although with certain types of plate, fabrication can take place on site. In the case of metal plates, the manufacturer sells plates backed up to varying degrees with design aids to approved manufacturers, many of whom are also timber merchants. The timber used is both graded for strength and machined on all surfaces to give accuracy to the finished product. Trussed rafters can also be assembled using plywood gussets, the plywood being either nailed to a defined pattern or nailed and glued to the truss members to form the joint. Ply gusseted trusses are not as popular as metal plated trusses, but do offer a method of manufacture not requiring specialist equipment. Similarly the galvanised steel plates punched with a pattern of holes to receive nails can also be used to form truss joints and these too can be fabricated on site.
The punched metal nail plates used in factory trussed rafter production are mechanically pressed into the timbers on both sides of each joint to form a trussed rafter. This trussed rafter is then placed on the roof at approximately 600 mm centres taking the place of the common rafter. Hence its term ‘trussed rafter’, as distinct from the TRADA type principal truss, although it will be seen later in Chapter 6 that trussed rafters themselves can be used to form principal or girder trusses. A typical Fink trussed rafter is illustrated in Fig. 1.12.
COST ADVANTAGES
Trussed rafters are designed to carry simply the direct load imposed upon them. It is assumed that they are to be kept upright by other members, these members being the binders and diagonal bracing and even the tile batten vital to the overall stability of the roof. Whilst most trussed rafters are used for roofs of housing, their use is increasing for roofs of public buildings, commercial buildings and to a lesser extent for industrial and agricultural buildings. Clear spans in excess of 30 m can be achieved with lightweight roof coverings.
When first introduced into the UK, the designs were limited to those contained in standard design manuals, thus the duo pitch and mono pitched roofs were common but more complex roofs needed individual designs prepared. The advent of the computer both speeded up and dramatically reduced the cost of the design process, and this has been further advanced by the use of microcomputers installed in all trussed rafter manufacturers’ offices. There are now almost no limitations to the possible shape of trussed rafters, except those imposed by the practicality of production and
transportation to site. The power of computers enables not only the individual trussed rafter to be designed but also the whole roof as a structural entity. Roof layout drawings can be produced in minutes and then either plotted on to paper or sent via ‘email’ to the end user.
Trussed rafter roofs use approximately 30% less timber than a traditional roof, and can be built into a roof form in a fraction of the time taken for either a truly traditional common and purlin roof, or a TRADA construction. Factory production keeps the labour cost of trussed rafter manufacture very low compared to that necessary to assemble a bolt and connected jointed truss, thus giving further cost advantages to the trussed rafter. Almost all new housing now uses a trussed rafter form of roof construction.
LEGISLATION
Having looked at the development of the roof form, we must take account of the legislation controlling building construction in the UK. Before the twentieth century no controls existed, and it was not until the introduction of the model byelaws by each local authority area that some degree of control was placed upon the design of buildings.
The Building Regulations as we now know them first appeared in 1965, and have been amended and re-issued on several occasions since that date. Subsequent amendments have dealt with such roof related matters as the restraint of gable and walls, thermal insulation and roof void ventilation. The first major change to the Building Regulations occurred in 1985 and took the form of a two-part publication, the first part setting the standards to be achieved and the second, for approved documents, laying down approved methods of achieving them. The fourth edition of this book has been produced in the light of the latest edition of the Building Regulations which came into force in 2000, including the recent amendments. These regulations lay down the legal requirements for building and concern themselves with health and safety aspects and not the aesthetic aspects of the structure. The latter, of course, is controlled by the local planning authorities.
The National House-Building Council (known as NHBC) has its own set of standards, which although incorporating the Building Regulations requirements, look beyond the health and safety aspects and seek to set minimum standards for quality control and such items as heating, electrical power sockets, and the general fi nish given to the buildings. Formed in 1936 it was not until the mid-1960s that the council began to have infl uence on the vast majority of house builders in the UK.
Concerned by the so-called ‘jerry builders’ after the Second World War, the building societies needed some method of ensuring that the homes on which they had granted mortgages were of an adequate standard to protect their investment. These societies therefore demanded that house builders building and wishing to sell homes on which the societies were granting mortgages must belong to the NHBC and submit themselves to their inspections. Having achieved full compliance with the NHBC requirements and of course the Building Regulations, the mortgage would be granted. Consequently most newly built homes until now have had to be inspected by the local authority as well as the NHBC, although this is likely to change in the near future, and only the inspectorate of the NHBC will be involved. An alternative to NHBC for mortgage purposes in most instances, is that the house should be inspected by a registered architect, and this seems to be the only way that a non-registered house builder can build and sell a new home under a mortgage agreement.
The Building Regulations and NHBC standards in turn refer to various British Standards and it is intended here only to deal with those British Standards concerned with timber in roof structures.
Code of Practice 112 started life in 1952, and was amended in 1967 when the principle of allocating grade stresses to timber was introduced. 1971 saw further changes to the code of practice, then issued with stresses and timber sizes in metric units. This code became British Standard 5268 which itself was split into many parts: Part 2 deals with the general principles of timber structural design. Part 2: 1996 simplifi ed the hitherto relatively complex subject of stress grading by grouping timbers into strength classes ranging from C16 for softwoods to D70 for hardwoods. However, the Building Regulations approved document table still referred to the earlier issue of BS 5268: Part 2: 1991, and remained based on SC3 and SC4 grades.
The current standard recognises a special grade for the use in punched metal nail plated trussed rafters known as TR6. BS 5268: Part 3: 2002 deals specifically with the design and fabrication of trussed rafter roof construction. BS 6399: Part 3: 1998 is the code of practice for the loads imposed on roofs, dealing with such aspects as dead and live loads as well as snow loading. BS 5250 concerns itself with the roof void ventilation and was last reviewed in 1995. BS 5534: Part 1: 1997 deals with the design of slating and tiling with the recommendations for workmanship for these roof coverings being given in BS 8000: Part 6.
British Standard 4471: 1987, Sizes for sawn and processed softwood has now been withdrawn and replaced by an English language version of the European code EN 313, known as BS EN 1313/1: Part 1: 1997, Softwood sawn timber. The standard sets out standard sizes and processing tolerances, whilst BS 4978, revised in 1996, deals with the stresses allocated to structural timbers. This edition has been revised to take account of the publication of the relevant European Standards:
(1) Changes have been made to the visual grading section in accordance with BSEN 518 structural timber – grading – requirements for visual standards.
(2) Machine strength grading is now specifi ed in BSEN 519 structural timber – grading – requirements for machine strength graded timber and grading machines. The sections concerning machine strength grading have been deleted and the title has been changed.
(3) The sections concerning visual strength grades for laminations have been deleted.
(4) BS EN338: 2003, structural timber. Strength classes.
This British Standard specifies the means of assessing the quality of softwoods for which grade stresses are given in BS 5268: Part 2. This document is recommended for those wishing to have some insight into the visual appearance of the type of timber that they can expect with the various stress grading. Such factors as knots, fissures, bow, spring and twist are dealt with, giving limiting factors.
The above deals with timber from European countries. Timber from Canada and the USA is covered by their own standards which are recognised in the UK for visually graded timber. These are NLGA, Canada, national grading rules for dimension lumber, and NGRDL, USA, national grading rules for softwood dimension lumber. There is also a machine grade standard known as NAMSR set by the North American exports standard. This was introduced to give more precise selection of strength potential, thus increasing the economic use of this natural resource.
All structural timber used in dwellings must now be graded into stress limiting classes and marked with the grades. The mark must show not only the grade, but the grader and the grading station, the British Standard number and the species group. Alternatively of course it can be marked with the approved Canadian and American grading stamps. Grading can be carried out either visually by qualified visual graders, or by licensed stress grading machines operated by trained staff.
Earlier standards classified timbers within a single species and were developed from an assessment of the timber’s strength compared to that of a defect free sample, thus the old grades of 40, 50, 65 and 75 represented the percentage strength of the sample compared to the defect free sample. Thus with different species offering different strength properties it can be seen that a weak timber (say Balsa wood), at 75 grade would be much weaker than British pine at 75 grade. The strength classes simplify this by classifying by strength regardless of species, thus a piece of C14 balsa (not that it actually exists), would have the same structural ability as a piece of C14 British pine. This of course simplifies design unless visual appearance is of importance on exposed structural feature members of a roof form, in which case the designer should refer to BS 4978 to gauge for himself the visual defects likely to be allowable under the strength class selected by structural analysis.
As old strength classes are still allowed by the current Building Regulations 2000, the comparison table below may be of interest and assistance.
CHAPTER 3
The ‘Traditional’ or ‘Cut’ Roof
DESIGN
The traditional or ‘cut’ roof as it has become known is essentially a roof cut and assembled on site from individual timber members. It is most frequently a common rafter and purlin roof, the design of which can be prepared from readily available standard span tables for the individual timber members. Hips and valleys are generally constructed to what has become known as ‘good practice’ and are less well documented with span tables and specific design aids. The sizing of these members is often left to the architect or engineer and it is not always necessary to provide calculations to prove their adequacy. The design of all new roof structures in England, Wales and Inner London must of course conform with the latest edition of the Building Regulations. In Scotland the Building Standards (Scotland) regulations apply, and in Northern Ireland the Building Regulations (Northern Ireland).
The Building Regulations 2000 set out the statutory standards of construction to be achieved, and whilst the 1991 edition of the regulations contained approved documents, the new ones do not. For guidance on roof design, the Building Regulations refer to a TRADA publication entitled Span Tables for Solid Timber Members in Floors, Ceilings & Roofs (Excluding Trussed Rafter Roofs) For Dwellings. It is their publication reference TRADA technology design aid DA 1/2004. This document covers the design of all members likely to be encountered in a simple roof construction and is an extremely complete guide, giving the background to the designs, listing all current standards relevant to roof member design and to timber grading. A list of the span tables contained in the publication is set out in Table 3.1, and is a reproduction of Table 5 from that document. It is strongly recommended that anyone who is in any way involved with the study, design, alteration, conversion and repair, or the approval of roof structures should have a copy for reference. Full details of TRADA can be found in the bibliography.