Kick open the door leading from the room where the sniper and his buddy had been lurking. This gives you a view out onto the next street. Don't rush out. There are 5 or 6 more mercenaries ahead—2 on the ground and either 3 or 4 on the rooftops and balconies. They're armed with rifles and grenades.
If Lara falls off the roof, you could use the drainpipe on the building where the first sniper was standing (shown in the fourth and fifth screenshots above) to climb back up. However, if there are still men on the opposite roof, they can hit her as she climbs. You're probably better off on the ground using the buildings for cover.