On horizontally-run metal roof systems that use individual interlocking ridge caps, it is impossible for the seams between those caps to be completely watertight in all conditions.
The seam between the caps is not 100% watertight because it is lying very flat and prone to water build-up as well as to wind-driven rain. Therefore, watertightness at the ridge must be accomplished prior to ridge cap installation.
The ridge can be closed in and made watertight prior to ridge cap installation in one of two methods. One option is to bend the last course of shingles from one or both sides of the ridge over the other side of the peak, attaching it securely through the last course on the other side and into the decking. The other option is to use metal flashing coil to close in the ridge. Ideally, this flashing will lock into the top locks on the shingle courses on both sides of the ridge. Determining which ridge closure method is used depends upon how the shingle courses from either side fall in relation to the ridge and also on how high of a profile (butt thickness) the shingles have. If you install too short of a piece of shingle as the last course, it can make the ridge very flat and prone to water infiltration and wind uplift. This is made worse with higher profile panels and lower pitched roofs. By determining the proper ridge closure method and following it, a watertight ridge system can be easily achieved.