page_title: Timber roof truss
section_title: Hammerbeam truss
hierarchical_section_title: Timber roof truss / Open trusses / Hammerbeam truss
caption_attribution_description: English: Illustration from 1908 Chambers's Twentieth Century Dictionary. Hammer-beam, n. a horizontal piece of timber in place of a tie-beam at or near the feet of a pair of rafters.
context_page_description: A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. The space between each truss is known as a bay.
  Rafters have a tendency to flatten under gravity, thrusting outwards on the walls. For larger spans and thinner walls, this can topple the walls. Pairs of opposing rafters were thus initially tied together by a horizontal tie beam, to form coupled rafters. But such roofs were structurally weak, and lacking any longitudinal support, they were prone to racking, a collapse resulting from horizontal movement. Timber roof trusses were a later, medieval development. A roof truss is cross-braced into a stable, rigid unit. Ideally, it balances all of the lateral forces against one another, and thrusts only directly downwards on the supporting walls. In practice, lateral forces may develop; for instance, due to wind, excessive flexibility of the truss, or constructions that do not accommodate small lateral movements of the ends of the truss.
context_section_description: The hammerbeam roof was the culmination of the development of the arch-braced truss, allowing greater spaces to be spanned. The hammerbeam roof of Westminster Hall in London, designed by Hugh Herland and installed between 1395 and 1399, was the largest timber-roofed space in medieval Europe, spanning a distance of just over  20 metres (66 ft). It is considered to be the best example of a hammer-beam truss in England.
  Hammer beam trusses can have a single hammerbeam or multiple hammerbeams. A false hammerbeam roof (truss) has two definitions: 1) There is no hammer post on the hammer beam as sometimes found in a type of arch brace truss or; 2)The hammer beam joins into the hammer post instead of the hammer post landing on the hammer beam.
caption: Chambers 1908 single hammerbeam truss
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