Sags

Sag is the downward displacement or drooping of a structural element, caused by gravity, weight, moisture, temperature, or other loads. As a noun, it describes the extent of the drop, and as a verb it describes the action of drooping. The term is used in engineering, construction, and everyday language to reference any instance of sagging or bending, including multiple sags in a system.

In engineering and construction, sag describes the vertical deflection of a flexible element between supports. Examples include power or telephone cables sagging between anchors, suspension cables, and floor or roof joists under load. Sag depends on span length, weight per unit length, material stiffness, tension, and temperature. Excess sag can indicate overstress, poor design, or deterioration.

In road and railway engineering, a sag curve is a downward-curving portion of a vertical alignment, typically occurring where a road or track drops from a crest and then rises again. It affects drainage, sight distance, and load distribution and is designed with appropriate grade and curvature.

In clothing and textiles, sagging refers to fabric or garments that droop under their own weight or due to fit, such as sagging pants; in fashion contexts the term can be deliberate or unintentional.

In wood, timber, and other building materials, sag can occur when moisture content rises, loads increase, or spans are too long for the member’s stiffness, causing visible deflection or permanent deformation. Mitigation includes shortening spans, adding support, or selecting stiffer materials.