Structural overstressing of wood components in buildings, perhaps causing distortion or collapse, is usually due to failure to observe established design and construction criteria, currently represented by British Standard Specifications and Codes of Practice and, for example, the Approved Documents which are published in support of the Building Regulations for England and Wales. Inadequate dimensions in relation to structural requirements, that is in relation to loads, spans and component spacings, can be readily checked and need no special comment in this book, other than a reminder of the basic principles involved; in relation to cross section, the strength of a component depends on the width and the square of the depth, and when designing to support a particular load the required strength is proportional to the square of the span, remembering that the load is not the load density that is often quoted as a requirement but the product of the load density times the span. However, it is much more difficult to check that species and grading requirements have been observed, and this subject is therefore considered in more detail in section 6.2.