"Historically beams were not designed by mathematical means. Beams were sized empirically based on experience. There has always been a desire to build to the practical minimum. Undersized beams deflect under load. A triangular crack often occurs over the opening. Severity is a matter of degree in each case."
The same applies to joists. "Joists are often small by modern standards. Some deflection or springiness is common. Local overloading under the weight of partitions or load concentrations is also common. When a change of use occurs, floors may become more heavily loaded than they were originally designed for. Whether this defect is serious or not, is a matter of degree on the facts in each case."
"KEY FEATURES:
1. Deflected, sagging floors. May be localized if uneven load.
2. Springy floors.
3. Door opening out of square, door may bind or have been cut out of square to fit.
4. Separation of internal walls from external walls, widest at top.
5. Separation from ceiling cracks widest at mis span.
6. Ceilings crack beneath
7. Crack sometimes between door head and ceiling."
Source: Holland, M. (2012). A practical guide to diagnosing structural movement in buildings. Chichester, West Sussex [England]: Wiley-Blackwell.
