Deflection is the term applied to the physical displacement of glass from its original position under load. Excessive deflection can result in distortion of reflected images, edge pull-out, contact with interior components such as blinds, or invasion of an occupied space.
Equal thickness annealed, heat strengthened or fully tempered glass lites all deflect equally under loads. Laminated glass at room temperature will deflect the same as other glass types of the same thickness for short durations of time (seconds as opposed to minutes). Required thickness may be determined by deflection before limiting strength is reached. Rectangular glazing, supported on all four edges, does not deflect linearly under load (doubling the load does not create double the deflection). As a general rule, center of glass deflection greater than 3/4" is aesthetically objectionable. Two of the main causes of deflection are barometric changes and wind or snow loads.
For more information please refer to ASTM E 1300 and/or see the Load Resistance Program from Pilkington.