Exits, travel distance and the mezzanine stair

Exit width, from Chapter 5, decides how many people can pass. Travel distance decides how far anyone has to walk before they reach safety, or at least a choice of routes to safety. In a large single-storey factory, travel distance, not width, usually decides how many exits there are and where they go.

This chapter places the exits for the running case workshop and office, checks every part of the floor against the travel distance limits in NCC 2022 Volume One, and then finds that the owner’s preferred mezzanine stair fails. It also covers the door details that turn a hole in the wall into a usable exit.

The rules for Class 5 to 9 buildings

D2D3 sets the number of exits. Every storey needs at least one. In Class 2 to 8 buildings, two are required in some cases, including where the building’s effective height exceeds 25 m. A single-storey factory could therefore, in principle, have one exit. In practice, the travel distance rules force more.

D2D5(3) sets the travel distances for Class 5 to 9 buildings:

• No point on a floor may be more than 20 m from an exit, or from a point from which travel in different directions to two exits is available.
• Where that two-way point is reached within 20 m, the distance to one of those exits must not exceed 40 m.
• In a Class 5 or 6 building, the 20 m may be increased to 30 m where a single exit serves the storey at the level of access to a road or open space (D2D5(3)(b)).

D2D6 then controls the spacing of alternative exits. They must be at least 9 m apart and, in most buildings other than Class 2 and 3 and patient care areas, not more than 60 m apart. Alternative paths of travel must not converge to less than 6 m apart.

Figure 1Travel limits for the running case, NCC 2022 Volume One

The logic of the two limits is worth holding on to. The first 20 m is the distance a person may have to cover with only one way out, where a fire near that exit could block their only route. Once they reach a point with a choice of directions, a fire is unlikely to block both, so a longer total distance of 40 m is tolerated. A layout that gives people an early choice of routes therefore earns more distance than one that funnels everyone down a single aisle.

How distances are measured

The way the distance is measured matters as much as the limit. D2D19 says where the distance ends: at the nearest part of a doorway to a road or open space, the nearest part of the doorway to a fire-isolated stair, or, for a non-fire-isolated stair, the nearest part of the nearest riser.

D2D20 says how to measure. Within a room, measure in a straight line from the point to the room’s doorway, and then on to the exit or the point where travel to two exits diverges. Where walls or built obstructions force a change of direction, measure along the path past them. For a non-fire-isolated stair, measure along the line of the nosings plus the distance across any landing. The NCC’s guidance notes that furniture and cars are generally not counted as obstructions, but walls and built-in elements are.

In a factory, the interesting cases are machines and racking. Heavy machines and floor-fixed racking behave like built obstructions: people must walk around them. The running case agreed with the certifier that fixed racking and machine tools are treated as obstructions and that paths are measured along the aisles.

The running case: the workshop floor

This is an illustration. Take the south-west corner of the workshop as the origin, with x along the 50 m length and y across the 30 m width. The fire wall to the office is at x = 50 m. The workshop has three personnel exits, each a door to open space:

• P1 in the south wall at (25, 0)
• P2 in the north wall at (25, 30)
• P3 in the west wall at (0, 15)

The large roller doors in the south wall are for vehicles and are not counted as exits. Under D3D24(2), a roller shutter can serve as a required exit only for a Class 6, 7 or 8 part of not more than 200 m², and only in other limited conditions. The workshop is 1,500 m².

Check the spacing first. P1 to P2 is 30 m. P1 to P3 and P2 to P3 are each √(25² + 15²) = 29.15 m. All are between 9 m and 60 m.

Then find the worst point on the floor. A grid check across the workshop finds that the point farthest from its nearest exit is at the middle of the fire wall, (50, 15), which is 29.15 m from both P1 and P2. That is more than 20 m. But from that point, the routes to P1 and P2 head in different directions immediately: one south-west, the other north-west. Two-way travel is available at the point itself, so the 40 m limit applies, and 29.15 m is within it. The corners by the fire wall are 25 m from P1 or P2, again with immediate two-way travel. The west half of the floor is within 20 m of P3 or one of the side doors.

The routes around machines add length. The certifier asked for the two worst cases to be drawn along the actual aisles. Both remained under 35 m, inside the 40 m limit.

The office

The office compartment is 10 m by 30 m. It has its own exits: a front door at the middle of the east wall, O1 at (60, 15), and a side door at O2 in the south wall at (55, 0). The farthest point from O1 is a back corner, √(10² + 15²) = 18.0 m away, inside 20 m on its own. The office meets D2D5(3) without relying on two-way travel or on the Class 5 concession. Office occupants never pass through the workshop.

The mezzanine: the owner’s stair fails

The mezzanine is 19 m by 10 m in the north-west corner of the workshop, from x = 0 to 19 m and y = 20 to 30 m, with its deck 3.0 m above the floor. The owner’s preferred stair is at the east end of the mezzanine, with its top riser at (19, 21), descending eastwards to the floor so that its foot sits near the north exit P2.

The stair itself is straightforward. A rise of 3.0 m in 17 risers gives a riser of about 176.5 mm; with a 260 mm going over 16 treads the stair runs 4.16 m horizontally, and its line of nosings is about 5.03 m long. Chapter 9 checks it against the stair rules.

The travel distance is the problem. The farthest point on the mezzanine is its north-west corner, (0, 30). In a straight line, the distance to the top riser is:

√((19 − 0)² + (21 − 30)²) = √(361 + 81) = 21.0 m

That already exceeds 20 m, and there is no two-way travel on the mezzanine because it has only one stair. Worse, the mezzanine will carry floor-fixed shelving in rows, with the main aisle along its south edge. Measured along the aisles, the path from the far corner is 9 m south and then 19 m east: 28 m. The DTS limit is 20 m.

Figure 2Mezzanine stair positions tested against D2D5(3)

• Top riser position Stair at the east end: (19, 21); Stair at the middle of the south edge: (9.5, 20)
• Farthest point, straight line Stair at the east end: 21.0 m; Stair at the middle of the south edge: 13.8 m
• Farthest point, along aisles Stair at the east end: 28 m; Stair at the middle of the south edge: 18.5 m
• Limit, D2D5(3) Stair at the east end: 20 m; Stair at the middle of the south edge: 20 m
• Stair foot to nearest door to open space, D2D14(5) Stair at the east end: 9.2 m to P2; Stair at the middle of the south edge: 9.5 m to P3
• Total to open space, D2D14(3) limit 80 m Stair at the east end: about 42 m along aisles; Stair at the middle of the south edge: about 33 m along aisles

Fixing the mezzanine

The designer tested three options.

1. Move the stair to the middle of the south edge. With the top riser at (9.5, 20), the farthest corners are √(9.5² + 10²) = 13.8 m away in a straight line and about 18.5 m along the aisles. The stair descends southwards and its foot, at about (9.5, 15.8), is 9.5 m from the west exit P3, inside the 20 m limit for stair discharge in D2D14(5). The total to open space is about 18.5 + 5.03 + 9.5 = 33 m, well inside the 80 m limit of D2D14(3). This complies.
2. Add a second stair at the west end. Two stairs would give two-way travel on the mezzanine, but a second stair costs floor space below and money, for a mezzanine assumed to hold seven people.
3. Keep the east stair and prove it with a Performance Solution. The relevant Performance Requirements include D1P4 for exits and D1P6 for paths of travel. A fire engineer would need to show that occupants on the mezzanine can evacuate safely over the longer route, perhaps with smoke detection and an alarm on the mezzanine. It is possible, but it adds fees, time and conditions.

The owner chose option 1. The cost was operational: the stair foot now lands in the fabrication area, so a 2 m by 2 m landing zone at its foot was marked and kept clear of forklift routes. The Performance Solution was considered and set aside because the DTS fix was cheap and permanent.

This is the chapter’s main lesson about detection. The failure was found by the most basic check: measure from the worst point, along the route a person would actually walk. It was missed at sketch stage because the stair was placed for convenience at floor level and nobody measured on the mezzanine itself.

Turning a doorway into an exit

An exit has to work under pressure, in smoke, with people who may never have used it. Several DTS clauses turn the door itself into a working part of the exit:

• Swing direction (D3D25). A swinging door in a required exit must swing in the direction of egress, except in a building or part of not more than 200 m² where the door is the only required exit and is held open while occupied, and for sanitary compartments and airlocks. A door’s swing must not unduly obstruct a stair, ramp or passage, and when fully open it must not encroach more than 100 mm on the required exit width.
• Latch operation (D3D26). A door in a required exit, forming part of one or in the path of travel to one, must be openable from the egress side without a key, by a single-hand downward action on one device between 900 mm and 1.1 m above the floor, or a single-hand pushing action on one device between 900 mm and 1.2 m.
• Discharge (D2D15). An exit must not be blocked at its point of discharge, with barriers such as bollards where vehicles could obstruct it, and the path from an exit across open space to the road must be at least as wide as the exit and never less than 1 m.

The running case met each requirement on paper, then nearly failed two of them in practice. The owner’s security adviser proposed keyed deadlocks on the three workshop doors for night-time security. The certifier pointed out that the doors must open without a key from inside. The solution was hardware that is locked from outside but opens with a single lever action from inside, with an alarm on opening outside working hours. Then the site plan showed the south exit P1 opening directly onto the truck apron. Bollards and a painted walkway now protect the discharge, so a parked truck cannot block the door.

Figure 3Exit record for the running case

Exercise: add a training room on the mezzanine

The owner later proposes turning the eastern 8 m of the mezzanine into a training room, enclosed by walls with a door at its south-west corner at (11, 20.5). The far corner of the training room is (19, 30). Does the travel distance still comply with the stair at (9.5, 20)?

Worked answer. Measure as D2D20 requires: from the far point in a straight line to the room’s doorway, then on to the stair. The first leg is √((19 − 11)² + (30 − 20.5)²) = √(64 + 90.25) = √154.25 = 12.4 m. The second leg, from the door at (11, 20.5) to the top riser at (9.5, 20), is √(1.5² + 0.5²) = 1.6 m. The total is about 14.0 m, inside the 20 m limit.

Travel distance is not the only question. A training room is not storage: at an office-type rate of 10 m² per person, its 80 m² would add 8 people, and if it were treated as a space where people assemble, the count could be much higher and the class could change (Chapter 2). The owner should take the proposal to the certifier before building it, because the use changes more than the distance.