Timber Pile

Hi guys I have a question for you about the timber piles. I read on the wiki that the piles require a distance of at least 5 tiles from everything. I tried different layouts from using CTRL for put down the pile from using the automatic tile attach. This morning I collected the piles, 2 perfects and 2 not (one had 13 moldy boards and the other 20). Is there a clue for this? Is there always the chance to fail with the boards or am I missing something?