Say you are caught out without a tent in mild temperature conditions, e.g. moorland/woods. What material would you look for to make a good shelter roof out of?
The same way that modern houses aren't made out of only one material (in most places), your shelter should be made up of different materials as well. Obviously this all depends on where you are and what is available. Primarily you'll need two types of materials. (I'll assume you're in a fairly standard deciduous forest):