(abit of a late reply, but perhaps this will help someone else)
You can change snapping to face, that way you can move it along the Y axis and have it snap directly to the face even without a vertex located there.
Also it often matters which way the object is approaching the snapping point. If you are moving it from the inside it will snap to the inside. Likewise if you are moving it towards the planks from outside it will snap to the outside.
