Collar beam

A collar beam or collar is a horizontal member between two rafters and is very common in domestic roof construction. Often a collar is structural but they may be used simply to frame a ceiling. A collar beam is often called a collar tie but this is rarely correct. A tie in building construction is an element in tension rather than compression and most collar beams are designed to work in compression to keep the rafters from sagging. A collar near the bottom of the rafters may replace a tie beam and be designed to keep the rafters from spreading, thus are in tension: these are correctly called a collar tie.

Etymology

Collar in general comes from Latin collare meaning neck.

Collar beam roofs

The simplest form of roof framing is a common rafter roof. This roof framing has nothing but rafters and a tie beam at the bottoms of the rafters. The next step in the development of roof framing was to add a collar, called a collar beam roof. Collar beam roofs are suitable for spans up to around 18 feet (6 meters).

King post

A king post (or king-post or kingpost) is a central vertical post used in architectural or bridge designs, working in tension to support a beam below from a truss apex above (whereas a crown post visually similar, supports items above from the beam below).

In aircraft design a strut called a king post acts in compression, similarly to an architectural crown post. Usage in mechanical plant and marine engineering differs again, as noted below.

Architecture

A king post extends vertically from a crossbeam to the apex of a triangular truss. The king post connects the apex of the truss with its base, holding up the tie beam (in tension) at the base of the truss. King posts were used in timber framed roof construction in Roman buildings,medieval architecture in buildings such as parish churches and tithe barns. They also appear in Gothic Revival architecture, Queen Anne architecture and into modern times. Said to be the oldest and simplest truss, the oldest surviving roof truss in the world built between 548 and 565 is a king post truss in Saint Catherine's Monastery, Egypt. Also, king post trusses are used to construct wood and metal bridges. The post can be replaced with an iron rod called a king rod (or king bolt) and thus a king rod truss. The king post truss is also called a "Latin truss".