What problems do cantilevered joists have?

expert EXPERT April 26, 2007

Cantilevers are inherently weak structural details and exterior cantilevers are particularly weak. Most modern conventions restrict cantilevers to two feet or 1/6 of the length of the wood member, unless there has been special engineering consideration. In some areas, and with some older sets of rules, up to 1/3 of the length of the joist could be cantilevered.

Weak cantilevers are caused by

1. poor original design or installation
2. deterioration (usually rot) of the cantilevered members

Cantilevering creates a joist that is unsupported at one end and, consequently, subject to deflection and springiness. The other reality of cantilevers is the teeter-totter effect. If joists are cantilevered over an outside wall, for example, bouncing on the cantilevered end puts an upward force on the other end of the joist inside the house. The outside wall is the fulcrum. You'll sometimes find humps in floors at the inner end of the cantilevered joists.

The design may minimize deflection and springiness and there may be no hump. That still leaves a significant problem. Cantilevered joists are vulnerable to leakage and rot where they pass through an exterior wall.

There can be a lot of concealed water damage. Cantilevered joists may rot at the wall penetration point creating an unsafe condition which is difficult to identify.

Look very closely at cantilevers for deflection and springiness, and pay special attention to water damage at the joist/wall joint on outdoor cantilevers.