It would leak onto the patio below... which was what they were most likely trying to avoid.
Prior to decking, you can use Trex RainEscape... after decking you can use Timbertech Dryspace.
Other than that... you have to realize wherever the ceiling is attached to the purlin... that is the gap you have for water and debris.
Overtime, the purlins will rot, and the nails/screws will release.
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