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Last edited 17 Nov 2021
A knee wall is a short wall, typically under three feet (one metre) in height, used to support the rafters in timber roof construction. In his book A Visual Dictionary of Architecture, Francis D. K. Ching defines a knee wall as "a short wall supporting rafters at some intermediate position along their length." The knee wall provides support to rafters which therefore need not be large enough to span from the Ridge ridge to the eaves. Typically the knee wall is covered with plaster or gypsum board, enclosing the useful part of the attic space (not necessarily high enough for a person to stand up), while the remaining small space under the eaves is only useful for storage.
The term is derived from the association with a human knee, partly bent. Knee walls are common in houses in which the ceiling on the top floor is an attic, i.e. the ceiling is the underside of the roof and slopes down on one or more sides.
See the following link https://en.wikipedia.org/wiki/Knee_wall
(1) Ceiling joists and collar ties of not less than 38 mm by 89 mm lumber are permitted to be assumed to provide intermediate support to reduce the span for rafters and joists where the roof slope is 1 in 3 or greater.
(4) When struts are used to provide intermediate support, they shall be not less than 38 mm by 89 mm material extending from each rafter to a loadbearing wall at an angle of not less than 45° to the horizontal.
(5) When dwarf walls are used for rafter support, they shall be framed in the same manner as loadbearing walls and securely fastened top and bottom to the roof and ceiling framing to prevent overall movement.
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