Doing a site survey on a commercial roof. On the solar pro website it states that on commercial roof tops" a minimum of 6' clear perimeter around edges of commercial roof tops that are larger then 250' by 250'. I understand that part the next sentence on that article I don't understand. "However, if either axis of the roof is 250 feet are less then the system designer can reduce the clear perimeter around edge of roof to 4' thanks