I will soon replace a Woodruff roof. The Woodruff is nailed to slats. My question concerns putting in the sheathing. I have noticed that one method is to nail 1x4s perpendicular to the slats so that there is a space under the sheathing. Is the purpose for this better air circulation in the attic? The method I prefer is to nail the sheathing directly to the slats without the space. Any ideas on which is better and why?