Fire Ratings at Edges
Fire separation attaches to the relationship across a wall, not to either room by itself. Two bedrooms inside one apartment share a wall that typically needs no fire rating at all. Turn that same wall to face the common corridor and it usually becomes a 1-hour assembly. Put a different apartment on the other side and it's commonly 1 or 2 hours. The bedroom didn't change. Its neighbor did. Move the boundary or change the use next door, and the required wall, and the doors in it, may change with it.
A separation property follows its boundary
This coordination example assigns 120 minutes beside a stair, 60 minutes beside a unit (each Unit is a separate dwelling) or between unlike other uses, and zero between identical corridor/service uses. These are authored example properties, not a code lookup: real separations depend on occupancy, construction type, sprinklers and the adopted code edition. Each boundary has one declared 3 ft opening whose example rating follows its wall, the pattern the original thesis stated: 90 minutes in a 120-minute wall, 20 minutes in a 60-minute wall facing a corridor, 45 minutes in any other 60-minute wall. The protection flag records whether that opening was given its rating; neither is a verified fire-door specification.
A rated assembly is a wall or floor built to a tested design that resists fire for a stated time, 1 hour or 2 hours. That rating gets drawn on a wall, but it's decided by the two rooms the wall separates. That's why a room list alone can't drive the wall specification: you need to know who touches whom.
Rating by room pair
These are the pairs that come up on every apartment floor, with the separations you'll typically see:
| Room pair across the wall | Typical separation | What decides it |
|---|---|---|
| Two rooms inside one dwelling unit | None | Same unit on both sides |
| Dwelling unit and common corridor | 1 hour | Keeping the unit apart from the exit path |
| Dwelling unit and another dwelling unit | 1 or 2 hours | Mostly the building's construction type |
Read that table as typical, not as a lookup. The actual number depends on the occupancy classification on each side, the building's construction type, whether it's sprinklered, and which code edition the jurisdiction adopted, with its local amendments (#67 is about that part). I first titled this thesis “Fire ratings are just edge properties.” The “just” oversold it. The pair tells you which walls need a decision; the project's code basis tells you the number. Determine it for the actual building.
Not every pair is a negotiation, either. Some boundaries are one room talking. In the applet below, a stair is assigned 120 minutes on every wall it touches, whatever is on the other side, so the stair sets the rating and the neighbor only sets where it lands.
What the applet changes, and what it doesn't
The plate is 32 by 24 feet, cut into four 16-by-12 rooms labeled Unit, Corridor, Stair and Service. They share four boundaries: two 12 feet long and two 16 feet long, 56 feet in all, each with one 3-foot door. The plate, the room sizes and the boundaries stay fixed. You only change what each room is, so every change in the readouts comes from the pairs. The ratings are authored example properties (120 minutes beside a stair, 60 beside a unit or between two unlike uses, zero between two identical corridor or service uses), not a code lookup. Each wall is labelled with its rating in minutes, and each door with the rating that follows from its wall (more on that below).
- Different uses. Unit–Corridor (12 ft) and Corridor–Service (16 ft) get 60 minutes: 28 ft. Unit–Stair (16 ft) and Stair–Service (12 ft) get 120: another 28 ft. The two corridor walls carry 20-minute doors; the two stair walls carry 90-minute doors.
- Four separate units. Every boundary becomes a unit-to-unit wall, so all 56 ft get 60 minutes. The applet treats each Unit as its own dwelling and picks the 1-hour end of the 1-or-2 range, so every door is a 45-minute door. If the four rooms were one apartment, the answer would be zero. The applet doesn't draw that case; the all-corridor example is its zero case, where identical uses on both sides leave every boundary unrated.
- Move the stair. This is the one to watch. The stair moves from the lower left to the lower right, replacing the service room, and the lower left becomes a second corridor. To build it by hand, click the lower-left room to Corridor and the lower-right room to Stair. The readouts don't move: still 28 ft at 60 minutes and 28 ft at 120. A quantity takeoff would say nothing happened. But the 16-foot wall under the unit dropped from 120 to 60, and the 16-foot wall under the corridor went from 60 to 120. The doors follow: the 90-minute door is now in the wall under the corridor, and the unit's door is a 20-minute door. Same footage, different walls, different doors.
- Clear an opening's flag. The door in the Stair–Service wall loses its protection flag. Its label still says 90 minutes, now marked unset, and the count of rated boundaries with an unset opening goes from 0 to 1.
- A larger plate. At 46 by 38 feet the boundaries grow to 19 and 23 feet, 84 ft in all, split 42 and 42. No rating changed. Rated length follows boundary length, not room count.
The moved stair is the expensive mistake. If wall types come from a tally of rated footage instead of from the pairs, the 2-hour wall gets built on the wrong side of the plate, and somebody finds out at inspection, right after the GC has framed it and hung the doors.
The door follows the wall
A door in a rated wall is an opening in that edge, so its protection is an edge property one step removed: the opening requirement follows from the wall's rating and what that wall is doing, and the wall's rating follows from the pair. Get the pair right and the door schedule can be read off the plan. Get it wrong and you have the wrong wall and the wrong doors. The door's required rating isn't simply the wall's number copied over. The applet labels each door with the pattern the first version of this thesis stated: a 120-minute wall takes a 90-minute door, not a 120-minute one, because the model code typically accepts a lower rating at the opening; a 60-minute wall takes a 20-minute door where it is a corridor wall and a 45-minute door for other separations. Treat those as typical example values, not a lookup. The real number is its own entry in the adopted code, keyed to what the wall is doing (corridor wall, exit enclosure, occupancy separation), and the applet's flag only records whether the opening was given its rating. Neither is a fire-door specification.
The judgment
Room labels can't drive the whole specification; the adjacency matters. Every shared boundary in a plan should carry the pair it separates, and its rating, along with every opening in it, should be recomputed from that pair whenever a room changes use or a wall moves. The graph tells you which edges need attention. The code analysis for the actual project (occupancy, construction type, sprinklers, adopted edition) tells you the number. Keep those two jobs separate and you catch the moved stair on the plan, not at inspection. The thickness that rating adds, and whose side it comes out of, is #31's problem.