Creating ceiling on your map.
1 - Have the map structure, wall, floor ready.
2 - Now clike the entity "Block Tool" [Shift + B].
3 - Select the entire territory of the screen "Top (x / y). And at the "Side (x / z)" so you mark the top line.
4 - Select the texture by clicking next "Browse" and select the texture, then go to the part you selected the location and click the right button, then "Create Object".
5 - Now see the result.
Any doubt post in the topic, Thanks.