Roof failures can occur as a result of designer error. Raised standards of education and professional training, increased knowledge of the performance of structure and materials, and regulatory control of building design, all mean that (in theory, at least) roofs have become increasingly less likely to fail. However, the introduction of new and not properly understood design approaches, the use of new materials, or of traditional materials used in new ways, even the loss of knowledge of how traditional materials perform, can each result in the roof structure and/or its covering failing to perform as required.
It also needs to be recognised that, historically, the standards of roof design before (and even after) the introduction of model by-laws in the nineteenth century were variable. This is particularly noticeable in the buildings of cheaper, and probably speculative, domestic construction which were erected in the period of extensive urban expansion of the nineteenth century.
Generally, the older the building the less likely that there will have been any scientific design criteria applied when considering structure and materials. The designer, or more likely, the master builder, would have based his construction on the accumulated knowledge of generations of previous similar builders using materials that had been 'proved by the test of time'. Furthermore, it was not uncommon for corners to be cut or design implications to be misunderstood, even in so-called quality work, resulting in the roof structure performing at the limit of, or possibly beyond, its proper structural capability. Whilst not necessarily leading to total failure, it does mean that the roof will be over-stressed and, almost certainly, showing signs of distress.
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