The number 13 in 1408?

Does anyone know the significance of the number 13 in 1408? In the movie on the postcard Enslin receives about the Dolphin hotel he puts =13 on the postcard. I thought he was just adding the numbers together, but later in the movie Gerald Olin the hotel manager says the hotel pretends to act like 1408 is not a problem just like they pretend the 13th floor does not exist? There may not be any significance, but if anyone knows anything let me know.