When a chimney is not adequately supported by ties or securely restrained in any way, its height H when measured from the highest point of intersection with the roof surface, gutter and so on should not exceed 4.5W, provided the density of the masonry is greater than 1500 kg/m3 (that is: brickwork, heavyweight concrete block and so on) where:
W = the least horizontal dimension of the chimney measured at the same point of intersection, and
H = measured to the top of any chimney pot or other flue terminal
Proportion for masonry chimneys
Note, to determine the minimum height of a chimney required in relation to the roof pitch reference should be made to DSA Guidance Note "Chimney heights for solid fuel appliances"