Doorposts and Lintel:
An opening is only obligated to have a Mezuzah if it has two doorposts [one on each side], and a Mashkof [lintel] on top of the doorposts.
Must the entrance contain a doorframe to be obligated in a Mezuzah?
Yes. If one does not have an actual doorframe by the entrance, but the entrance contains a doorframe structure that is formed from the wall, it is obligated in having a Mezuzah. Nevertheless, if the entrance does not contain a door, the Mezuzah is placed without a blessing, as explained in ??.
Does a room without a ceiling require a Mezuzah by its entrance?
If the room does not have a lintel, then it does not require a Mezuzah. If the room has a lintel and doorframe, but does not have a ceiling, then it follows the law explained in ???
Wall frame without door-Place without blessing
No lintel-Exempt from Mezuzah
 Michaber ibid
 See Pischeiy Shearim 287/6 [p. 120] that many Poskim have agreed that no frame is needed, and thus even when the walls themselves form the doorposts and lintel, one needs to place a Mezuzah with a blessing if it has a door. However some rule that a frameless entrance is exempt from needing a Mezuzah.