Stairs Connect Floors

Count the stairs on a plan and you learn how many stairs there are. You don’t learn how many ways down each floor has. A stair connects levels and is part of the building’s circulation network. Its location can influence how people move between floors, how spaces relate vertically and how the building organizes its exits. Two stairs that share one shaft, or a second stair that stops short of the top floors, still count as two on the schedule. Whether they are two ways down depends on where they go.

Two stairs are not always two ways down

Each floor has a center node and two stair positions, A and B, each joined to it by a 30 ft horizontal link; each floor-to-floor stair segment is a 12 ft vertical link, and F1 is at grade. A level has only one way down when a single interrupted segment would cut it off; in a shared shaft one interruption breaks both stairs at that level. Stair footprints are a chosen 120 ft² per stair per floor. The model tests connectivity, not code-compliant egress, exit separation, stair width or capacity.

The graph goes vertical

Thesis 41, Egress as a Graph, worked on one floor: rooms and corridors are nodes and edges, and travel distance is the length of a route through them. It ended on two questions a route length can’t answer. Where does a route split into two directions? And are the exits far enough apart that one event can’t block both? Stack floors and both questions move into the section.

Each floor is its own graph. A stair is what joins them: a chain of vertical edges, one segment per floor-to-floor run, from the highest floor it serves down to grade, the level where people walk out of the building. A floor is connected to the ground only if an unbroken chain reaches it. That gives every stair two properties a plan symbol hides. Continuity: break one run and every floor above the break loses that stair. Independence: two chains are two ways down only if no single break interrupts both.

Five floors, two stairs, one break

The drawing is a section through a schematic five-story building. Each floor has a center node and two stair positions, A on the left and B on the right, each 30 ft from the center. Each floor-to-floor run adds 12 ft, and the first floor is at grade. Every number below is a center route: walk from the floor’s center to a stair, then down it to grade.

• Two stairs, separate shafts. All five floors reach grade. F2 is 42 ft out (30 + 12); F5 is 78 ft (30 + 4 × 12). Every floor above the first has two independent ways down, so no single interrupted run cuts anyone off.
• Interrupt stair A between F3 and F4. The first card doesn’t move: 5 of 5 floors still connected, F4 still 66 ft out, F5 still 78 ft. The routes did change. F4 and F5 now go down B, and B is all they have; the drawing turns them gold. A table of route lengths scores this plan exactly like the last one, the same way a straight-line check couldn’t see 41’s blocked corridor.
• Only stair A, same break. F4 and F5 have no path to grade. Three of five floors are connected, and F2 and F3 hang on a single stair.
• Both stairs in one shaft, same break. Two stairs on the schedule and 1,200 ft² of stair footprint, the same as the separated pair (two stairs at the drawing’s chosen 120 ft² each, on five floors). The interruption in the shared shaft takes both runs, and F4 and F5 are stranded exactly as if there were one stair: 3 of 5. Placement turned two stairs into one way down.

Put the two stairs in one shaft and leave every run intact, and the cards say 5 of 5 connected, 78 ft to the top floor, 1,200 ft² — identical to two separate stairs, except that four floors now have only one way down. That last card is the only one that knows where the stairs are.

The cards count floors connected to grade, floors with only one way down (a single interrupted segment would cut them off; in a shared shaft, one interruption takes both stairs), active vertical segments, and the chosen stair footprint. In this drawing A and B both sit 30 ft from the floor’s center, so switching from A to B costs nothing in length. That’s a simplification of the drawing, not a property of stairs: in a real plan the second stair is usually farther from somebody, and the switch shows up as a longer route.

Placement is the decision

Thinking of stairs as vertical connections makes their placement an early planning decision. They occupy space and structure, yet their value is not captured by area efficiency alone. They shape orientation, access and safe movement through the building. Stairs also affect the relationship between floors, so their usefulness includes more than their counted area.

The shared-shaft case is the plainest version. Same two stairs, same area, same route lengths, and one way down instead of two. An area takeoff can’t see that difference. A route table can’t either. The April applet couldn’t: it let you drop stairs into any of five positions across the section and labeled any two of them “IBC COMPLIANT · 2 remote means of egress,” side by side included. Counting stairs is how you get that answer.

What the graph doesn’t settle

The stair’s role depends on the floors it serves and the project’s life-safety design. Width, enclosure, separation and travel distance are among the issues that need review under the applicable code. A diagram of connections can help reveal routes, but it cannot establish compliance without checking those conditions.

Whether two stairs sharing one enclosure can count as separate exits, how far apart exits have to be, how wide a stair must be for the people it carries: those are code questions, and the drawing answers none of them. It answers the one before them — which floors depend on which runs — and that’s the question worth bringing to whoever reviews the stairs.

Count ways down, not stairs

The stairs’ final configuration must be checked against the actual building and its governing life-safety strategy. The judgment I’d make before that check: for every floor, ask how many independent ways down it has and which single break would take one away. Two stairs in one shaft, or a second stair that quits a few floors short, answer that question worse than the stair count suggests. Place stairs as edges in the section before you draw a single tread, because once the core is fixed, so is the answer.