Where the roof overhang the gable, you have the roof verge, and in building or repairing the verge you must decide whether to use dry verge or mortar. Here is some information about both to help you decide.
Mortar is the traditional choice for a roof verge. When it is applied correctly, as the work is being done rather than after, it can provide a clean stable verge detail. It does, however, have a tendency to crack and require constant maintenance if it is not used correctly. Even when used correctly, mortar can shrink and therefore become ineffective in protecting your home.
Using a dry verge eliminates the need for mortar altogether. They are often made specifically for the tile you are using, and have little to no need for maintenance once fitted in. It can require a little more work and money up front, but is much more effective in the long run.
Choosing what is right for your roof verge is entirely up to you and your needs. Considering the architecture of the home, building costs, and how much time and energy you have for ongoing maintenance is key in choosing between dry verge and mortar.