Glossary: Deflection Limit
The maximum allowable structural sagging or bending of a joist, rafter, or beam under a uniform load, normally expressed as a fraction of the span length (e.g. L/360).
Standard Deflection Caps in Building Codes
Deflection limits ensure that structural members possess sufficient stiffness to support building loads without excessive bending. Under the International Residential Code (IRC), limits are defined as:
- L/360 for Floor Joists: Live load deflection is restricted to 1/360 of the span length. For a 15-foot span (180 inches), maximum allowable sag is 0.5 inches. This prevents drywall ceiling cracks and bouncy, springy floors.
- L/240 for Rafters and Decks: Roof elements or external joists without plaster ceilings are subject to a more lenient L/240 deflection cap.
- L/180 for Cantilevers: Cantilevered overhang beams use a deflection constraint of L/180 relative to the projecting arm.