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 finish given to the buildings. Formed in 1936 it was not until the mid-1960s that the council began to have influence 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 simplified 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 specified 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 gradings. 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.
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