Modularizes SD files a bit

code/modules/multiz/misc.dm

@@ -0,0 +1,20 @@
+// Invisible object that blocks z transfer to/from its turf and the turf above.
+/obj/effect/ceiling
+	invisibility = 101 // nope cant see this
+	anchored = 1
+	can_atmos_pass = ATMOS_PASS_PROC
+
+/obj/effect/ceiling/CanZASPass(turf/T, is_zone)
+	if(T == GetAbove(src))
+		return FALSE // Keep your air up there, buddy
+	return TRUE
+
+/obj/effect/ceiling/CanPass(atom/movable/mover, turf/target)
+	if(target == GetAbove(src))
+		return FALSE
+	return TRUE
+
+/obj/effect/ceiling/Uncross(atom/movable/mover, turf/target)
+	if(target == GetAbove(src))
+		return FALSE
+	return TRUE