Definition
Shelf sag
The gradual downward bend a shelf develops under a sustained load, caused by creep in the material rather than by a single overload. It is worse with longer spans, thinner boards and heavier loads, and it is permanent.
A practical rule is to support a shelf at least every 30 to 36 inches, and to add a centre support on anything longer. A shelf that spans unsupported will sag over months even if it feels solid on the day it is fitted.
Tools that use this term
Shelf Spacing Calculator
Divide a height into evenly spaced shelves, allowing for the thickness of each shelf.
Furniture Dimension Calculator
Standard heights and depths for tables, desks, chairs and counters, and a check on a dimension you already have.
Wood Weight Calculator
Weight of a board or a whole project from its volume and the density of the species.