The height at which you fix your fascia boards can have an affect on the eaves course of tiles, so must always be taken into consideration when you are tiling your roof.
All the tiles on a roof need to be laid in the same plane, but if you have a fascia board attached to the roof eaves, you may inadvertently affect the eaves course of tiles so they are not in the same plane as the second course you lay.
If the fascia board is fixed in a position that is too low, the first course of tiles will be steeper than the subsequent courses of tiles so the roof line will be affected conversely. Also, the only point of contact that these tiles will have will be their top edge, which will reduce their weather stopping potential. If the fascia board is fixed too high, the first course of tiles will then be too shallow, which could result in ponding, and may be below the recommended pitch for the tile to work effectively.
Some types of tile have to be clipped into position, so if the fascia is not mounted in the right place, the tiles will simply not lock together at all.