This 3 D animation of posts and railings explains where most leaks are on a roof deck and how to correct it. All posts must have this treatment to prevent leaks. Save $$ by fixing it yourself.
Most of the time when a flat roof leaks around a post-it is because of the flashing. A 4 x 4 post with copper flashing attached to it is not complete unless the post is covered with a sleeve and cap on top of it.
A bare 4 x 4 post will always leak no matter how good the flashing is. A sleeve that slides over the 4 x 4 post acts as a counter flashing. If there is no casing or sleeve around the 4 x 4 post, then the roof is likely to spring a leak.
Waterproofing and Leak Specialists