This Guide covers the factors and conditions that influence@ favourably or unfavourably@ the effectiveness of a frangible roof joint. These factors and conditions and their effects on roof frangibility are considered from both theoretical and practical standpoints. The Guide is intended to assist designers and users of such tanks to carry out detailed evaluation@ of both new designs and existing tanks@ as to whether there is: ? a reasonable chance that the tank will fail preferentially at the roof-to-shell connection; ? a significant risk that the tank will fail at the shell-to-bottom connection; and in the event of the latter@ to assist in identifying the design@ constructional or operational parameters which may need to be changed in order to increase the probability of a roof-to-shell failure occurring before a shell-to-bottom failure.