This section is from the book "The Construction Of The Small House", by H. Vandervoort Walsh. Also available from Amazon: The construction of the small house.
In the construction of lintels in either the wall of brick or stone, the introduction of either wood or steel is necessary for strength. Where the openings are less than 4 feet in width, timber lintels are used at the back of the lintel or arch, which are cut to serve as a centre for a rowlock or keyed arch. Any face brick may be supported by using a small steel angle. Where lintels are wider than 4 feet, steel I-beams, channels, or angles must be used. Where the span is more than 6 feet, it is necessary to build in bearing plates for the support of the ends of lintels.

Lintel Construction.
 
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