VRoamer is a new Microsoft Research project that generates VR worlds on-the-fly while users walk through unknown building environments. Players can wear their HMD and no longer have to rely on artificial locomotion techniques such as teleportation. They simply walk through their environment and the game is built around them. This is possible through the use of a wearable tech camera that scans the environment in front of the user and visualizes a playable virtual world. The system paints its virtual environment over real world objects such as doors. The system keeps the user safe from objects in the real world, even though those objects are hidden to the user. Transitions are done through corridors that are constructed to the available space in the user's environment. Players can open real doors to progress the game and the corridors may contain weapons, enemies, keys, etc. Objects that suddenly appear in a user's environment such as other people may become skeletons or traps. In this paper, we present VRoamer, which enables users to walk unseen physical spaces for which VRoamer procedurally generates a virtual scene on-the-fly. Scaling to the size of office buildings, VRoamer extracts walkable areas and detects physical obstacles in real time using inside-out tracking, instantiates pre-authored virtual rooms if their sizes fit physically walkable areas or otherwise generates virtual corridors and doors that lead to undiscovered physical areas. The use of these virtual structures that connect pre-authored scenes on-the-fly allow VRoamer to (1) temporarily block users' passage, thus slowing them down while increasing VRoamer's insight into newly discovered physical areas, (2) prevent users from seeing changes beyond the current virtual scene, and (3) obfuscate the appearance of physical environments. VRoamer animates virtual objects to reflect dynamically discovered changes of the physical environment, such as people walking by or obstacles that become apparent only with closer proximity.