I used 1/2 inch OSB and painted it with oil base barn paint. It has rain a lot, snowed, and we have very high humidity(in KY)! It is holding up very well!
Heres my coop page https://www.backyardchickens.com/web/viewblog.php?id=3809-Our_Coop
Heres my coop page https://www.backyardchickens.com/web/viewblog.php?id=3809-Our_Coop