There is a sprite for the space terrain that u can place on a map and ground units can not walk over it at all making it perfect for roof tileing.
The only problem with this tile is that when units walk under it they become unable to be hit because of that percentage stuff as is with all sprites.
Can i change this without loosing the roof cover?
You could try unchecking "gives cover" in sf, although I have no idea if this would work.
Highly doubt it Sickoe_DT_ although you can do what the other guy said and hopefully that will work. Some of the sprites that are in SF or SCMDraft wont allow you to change the percentage of it.