The rooftop is an exterior location and has its own fast travel marker; the Mass Fusion executive suite. It is normally only accessible as part of the main quest endgame. The Mass Fusion executive ID that allows access to the elevator down will not be present if no relevant quest is active.
This section is accessible from the roof by a quest locked elevator, or at any time from downstairs from an elevator on the highest floor of below the glass ceiling. The area is mostly comprised of destroyed offices where multiple caved in floors create one big interior space. This area will be abandoned if visited before Spoils of War or The Nuclear Option. During these two quests, the Sole Survivor will be ambushed here by synths or the Brotherhood of Steel as they exit the elevator.
This area is the building lobby and some walkways and offices. It is accessible from the front door and is occupied by Gunners if encountered before the Spoils of War or the Nuclear Option quests. When approaching from above while on one of the relevant quests, the battle with the opposing main quest faction will continue here.
The reactor room is below ground level; the beryllium agitator sought after by both the Brotherhood of Steel and the Institute is located here. After dealing with the security, including turrets and sentry bots, the agitator can be found in a large, highly radioactive central room.
- Full set of possibly leveled power armor - Worn by the Gunner commander.
- 力量娃娃 - 在大廳那個巨大雕像的頭上，從五樓可以拿到，從大廳搭電梯上來，有通往瞭望台的出口那層樓就是了。
- 逾期圖書 - 一本在四樓的桌上，必須從五樓跳下去拿。Another is on a desk on the reactor level.
- Mini nuke - On the same floor as the bobblehead in the room to the northwest where the Gunner commander comes out from.
- Fusion core - On a desk 3 floors from the top, accessed through a destroyed wall facing the central shaft using the techniques described for the Freefall Legs.
- 特斯拉科學第5期 - 9樓東南邊角落電腦桌上，可以用噴射背包一路跳上來，或是等最後主線解鎖區域後拿到，5樓有電梯通往8樓，再走上來就看到了。
- 自由落體護甲左/右腿 - 在最接近頂樓的房間保險箱內，只能透過噴射背包一層一層上來，或是using the trash can wall climbing glitch.
- Stealth Boy x 2 - One is on a metal shelf next to the chemistry station, three floors above the Strength bobblehead, at the same level with the glass floor. The other is in the same room as the Freefall Legs, on top of a small bookshelf.
- 麻州核融合廠實驗室鑰匙 - On the level with the glass floor in the northwest office room on a desk.
- 傑克·羅克佛德的日記 - 上述自由落體護甲所在房間實習生的終端機內。
- The unique damaged hazmat suit can be found in a drawer right outside the decontamination room, that leads into the reactor room.
- A hazmat suit can be found in the locker room behind the attendant's terminal, sitting on a shelf of one of the lockers. And another next to the analyst's terminal on top of a safe.
- A sentry bot spawns behind the northern security door, which can be opened through the reactor security control terminal on the south side of the room before the reactor room. It has a fusion core. Alternatively, one can get the core and trip the security, the doors will then open automatically.
- The rooftop of the building is the highest point in the entire Commonwealth and offers an impressive view across the whole game world. Despite this, the Mass Fusion building is the second tallest structure in Fallout 4, after Trinity Tower.
- Using a Brotherhood of Steel Vertibird, one can travel to the Mass Fusion building, and discover the Mass Fusion executive suite location en route. However, the elevator ID card needed for the Mass Fusion and Spoils of War quests does not spawn until the player character actually starts one of them. This may have been patched in a recent version of the game; The location does not appear discoverable before the appropriate quest.
- With proper timing and careful planning, it is possible to get a Vertibird to land and drop the player character off on the roof of the building, even before the associated quest.
- With careful jumping, it is possible to reach the top of the building with a power armor equipped with a jet pack.
- The building has multiple floors with many broken walkways. The jet pack power armor mod makes navigating the facility much easier.
- Drinking the water in the reactor room will kill the player character instantly unless they have Lead Belly at its maximum level.
- The Robes of Atom's Devoted from the Far Harbor add-on will also protect from this, however, without the aforementioned perk, radiation will still increase to a significant level.
- There is a steamer trunk in the reactor room underwater on the northwest wall, below a "Danger Radioactive Materials" sign.
- Each time the player character leaves the reactor room, they are decontaminated and lose all rads.
- Hacking the Reactor Security Control [Master] terminal in the southeast corner of the Main Reactor allows the player character to open the security doors: one leads to impassable rubble while the other opens the facilities room and reveals an inactive sentry bot.
- It is possible for provisioners and their brahmin to turn up on the roof, where they will be stranded and unable to leave. The only way to get them back to ground level is by using console commands or if they are somehow lured over the edge of the building (they will survive the fall).
- One can leap from the southwestern corner of the roof, and land safely in Goodneighbor without the use of power armor or Freefall Legs. Aim for the Old State House. The player character will be teleported to the main entrance upon entering the cell.
- If the player character did not support their faction in Mass Fusion or Spoils of War, the enemy faction will have taken over the building after the quest. On the other hand, the player character's faction will control the location if they are assisted.
- If the player character glitches into the reactor room, the Mass Fusion quest will begin.
- If Spoils of War is completed successfully, the Brotherhood of Steel will replace the Gunners and occupy the building. Synths will occupy the building if Mass Fusion is completed for the Institute.
The Mass Fusion building only appears in Fallout 4.
- The can chimes on the ground floor do not disappear after collecting them, however one can still get XP and tin cans for doing so. [已核实]
- Similarly the bathroom scale trap provides materials and XP upon disarming it, but the tesla trap will still go off. [已核实]
- Taking the large circular elevator from the lobby, back up to the Mass Fusion executive suite, sometimes causes the player to arrive in the wrong location in the exterior cell - a location which was not intended to be accessible. [已核实]