You can use ladder from other room in Altar.
I upload screens so it will be easy to say what i mean.
PHOTO 1 : Building at Altar with Ladder on roof
PHOTO 2: Room at left has ladder. Room at right of screen has not ladder but you can use ladder from room on left in room on right.
PHOTO 3: Blue cabinet in right room - if u go next to him, you can use ladder from other room.
PHOTO 5: Using ladder

Screens : http://imgur.com/a/Y9yg7