Jettying

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A double jettied timber framed building. The ends of the cantilevered beams supporting the upper floors can easily be seen.

Jettying is a building technique used in medieval timber frame buildings in which an upper floor projects beyond the dimensions of the floor below. This has the advantage of increasing the available space in the building without obstructing the street. Jettied floors are also termed jetties.

[edit] Structure

A jetty is an upper floor that depends on a cantilever system in which a horizontal beam, the jetty bressummer, supports the wall above and projects forward beyond the floor below (a technique also called oversailing) . The bressummer (or breastsummer) itself rests on the ends of a row of jetty beams or joists which are supported by jetty plates. Jetty joists in their turn were slotted sideways into the diagonal dragon beams at angle of 45° by means of mortice and tenon joints.

The overhanging corner posts are often reinforced by curved jetty brackets.

[edit] Vertical elements

The vertical elements of jetties can be summarized as:

• the more massive corner posts of the timber frame that support the dragon beam from the floor below and are supported in their turn by the dragon beam for the extended floor above.
• the less substantial studs of the close studding along the walls above and below the jetty.

[edit] Horizontal elements

The horizontal elements of jetties are:

• the jetty breastsummer (or bressummer), the sill on which the projecting wall above rests; the bressummer stretches across the whole width of the jetty wall
• the dragon-beam which runs diagonally from one corner to another, and supports the corner posts above and is supported by the corner posts below
• the jetty beams or joists which conform to the greater dimensions of the floor above but rest at right angles on the jetty-plates that conform to the shorter dimensions of the floor below. The jetty beams are morticed at 45° into the sides of the dragon beams. They are the main constituents of the cantilever system and they determine how far the jetty projects
• the jetty-plates, designed to carry the jetty-beams or joints. The jetty-plate itself is supported by the corner posts of the recessed floor below.

[edit] See also

• cantilever - modern buildings still use cantilevered floors, but the term jettying is rarely used. See for example the Whitney Museum of American Art, New York.