there is no real roofs and walls in RCT...you just make it look liek it and theres a lot of possibilities.....
1.walls can be made by using the land button using single blocks and raising the land you can make a box figure
2.now in the inside put a ride(lowering land inside the walls will make it easier to put roof in.
3. the roof is used with footpaths, cover the top of the building with footpaths.