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Flooring - Deflection and Vibration

Cantilever Deformation

Problem: Cantilever Deformation


Excessive cantilever spans and/or tail joists.

When cantilevers carry both roof loading and live floor loads, movement and deformation can occur both at the cantilever and within the floor system at the opposite end of the cantilever. The length of the cantilever and the length of the tail joists, the inward extension, can affect how significant the deformation will be.


Reduce cantilever span:

  • As with all structural components in a wood frame house, joist sizes must be determined using building codes. Floor joists that support roof loads must not be cantilevered beyond a distance greater than 16″ (400 mm) for 2″ × 8″ (38 × 184 mm) joists; or beyond 24″ (600 mm) for 2″ × 10″ (38 × 235 mm) joists as shown in Figure 18a and 18b.
Reducing Cantilever Lengths - Allowable
Figure 18a - Reducing Cantilever Lengths - Allowable
Reducing Cantilever Lengths - Recommended
Figure 18b - Reducing Cantilever Lengths - Recommended

Extend tail joist length.

  • When cantilevers are at right angles to main floor joists, the tail joists, inward extensions must be extended back into the floor framing by a lengthy at least equal to 6 times the length of the cantilever. This will prevent an upward movement at the tail joists, especially in high loading situations as shown in Figure 19.
Cantilever Framing
Figure 19 - Cantilever Framing