A cathedral ceiling's design affects the structure of a home's rafters and ceiling joists.
Cathedral ceilings generally are slanted ceilings that extend through more than one floor of a home. These ceilings usually follow the slope of a home's roof. A cathedral ceiling's impact on a roof's overall structure mostly depends upon the configuration chosen for the roof rafters and ceiling joists. For example, a single beam may support rafters to eliminate all horizontal supports in a cathedral ceiling design, or a roof may have multiple support beams of horizontal, diagonal or vertical position.
Flat Ceilings
A gable roof is one of the most common roof designs. It usually consists of wood rafters that meet at a center peak and slope down to rest upon a home's longest exterior walls. A traditional flat ceiling beneath a gable roof has a series of wood trusses above it. One truss consists of two rafters fastened to a horizontal ceiling joist so the three pieces form a triangle. Creating a cathedral ceiling generally involves changing the structure of traditional trusses by raising the ceiling joists or eliminating the joists altogether.
Collar Ties
Horizontal collar ties, or rafter ties, take the place of ceiling joists in some cathedral ceiling designs. The ties essentially are shorter ceiling joists that sit higher up on the rafters to expand the height of the ceiling. Fine Homebuilding magazine indicates that properly placed collar ties take up no more than one-third of the length of the rafters to achieve the appropriate tension with the walls below them. The portion of the rafters that extends below a collar tie transmits tension to the walls, keeping them from bending inward.
Collar Tie Variations
Some cathedral ceilings are created by installing collar ties across a specific number of rafters. For example, double collar ties may be installed across every third set of rafters. Installing double ties involves placing ties on both sides of a set of rafters for added support. The rafters and double ties still form a triangle, but two supports run along the bottom of the triangle instead of one.
Center Beams
Architects may eliminate all ceiling joists and collar ties in some cathedral ceiling designs. In such cases, a center beam, or ridge board, is structured so it supports the top of the rafters and carries the entire weight of the roof. Columns must support each end of the beam. The columns need to rest on solid surfaces, such as the home's foundation or steel beams that support the floor beneath them.
Tags: cathedral ceiling, ceiling joists, collar ties, cathedral ceiling design, cathedral ceiling designs