If there are 15 boards there are 14 gaps in-between. But first, 15 boards each 9 and 1/4th inches so you get a total of 138 inches and 3/4th If you subtract that from 144 you get 5 inches and 1/4th. The remaining amount is the total of all of the distance from the spacing. Then you divide that by 14 (number of gaps/spacing in-between.) So you get .375 inches. (3/8) So width of the spacing between each board is 3/8ths of a inch