Skip to step.3 if you already have a fully decorated room ready.
Step.1
Build the box/room and even add the floor while your at it, im just making a simple box as a example.
Step.2
Design the room, add some props or anything you want in the room(Its important to add everything inside of it before you make the roof).
Step.3
Choose what your going to use for the roof, im using the dark wood sign but here are some options.
Step.4
Duplicate the roof item you choose we are only going to use one right now so put the other one to the side.Click on the one of them then go on to the availability and change the visibility scope to “Player” thats the only option we are changing in that one.
Step.5
Next click on the one you put to the side and change the visibility scope to player and change it so that its not visible at the start of the game.Then go on appearance and make it 0.25 percent visible
Step.6
Click on the fully visible one then go to availability then change “show prop when receiving on” to “1” and change “hide prop when receiving on” to “2”
Step.7
On the non fully visable one do these “show prop when receiving on” to “2” then “hide prop when receiving on” to “1”
Step.8
Make a zone and change these settings"When player enters zone" “2” and “when player leaves zone” “1”
Step.10
Copy both the fully and non fully visible roofs to be the size of the room you made or scale them to be the size of the room you made either one works.
Please Comment if this was helpful if it was you can 
and if there are any problems please comment them im sorry if it was hard to understand this is my first guide.
Credit to Argo77 if you would like to check out his guide its right here->How to make roofing hide when you get close to it! (Simular to Zombs Royale)