The pressure arch theory in roof action suggests that, when a narrow heading is advanced, the layers of rock in the immediate roof deflect slightly and relieve themselves 2449 of the load of the overlying strata by transferring it to the sides of the opening by means of a pressure arch. The arch width just short of that which the higher strata cannot span and transfer the load to the sides of the opening is called the maximum-pressure arch. The depth mainly influences the minimum width of the pressure arch, although the type of overburden also plays a part. The following formula has been developed for approximating the minimum width of the maximum-pressure arch (W = minimum width of arch, in feet; D = depth of coal from surface, in feet): W = 3((D / 20) + 20). The equation does not apply for overburden less than 400 ft (122 m) or more than 2,000 ft (610 m) thick.