Appendix C Methods of measurement

1. Some form of measurement is an integral part of many of the provisions in this document.
Paragraphs 2 to 5 and Diagrams C1 to C7 show how the various forms of measurement should
be made.

Occupant capacity

2. The occupant capacity of a room, storey, building or part of a building is:

  1. the maximum number of persons it is designed to hold; or
  2. the number calculated by dividing the area of room or storey(s) (m2) by a floor space factor (m2 per person) such as those given in Table C1 for guidance.

Note: ‘area’ excludes stair enclosures, lifts, sanitary accommodation and any other fixed part of the building structure (but counters and display units, etc. should not be excluded).

Travel distance

3. Travel distance is measured by way of the shortest route which if:

  1. there is fixed seating or other fixed obstructions, is along the centre line of the seatways and gangways;
  2. it includes a stair, is along the pitch line on
    the centre line of travel.


4. The width of:

  1. a door (or doorway) is the clear width when the door is open (see Diagram C1);
  2. an escape route is the width at 1500mm above floor level when defined by walls or, elsewhere, the minimum width of passage available between any fixed obstructions;
  3. a stair is the clear width between the walls or balustrades.

Note 1: In the case of escape routes and stairs, handrails and strings which do not intrude more than 100mm into these widths may be ignored (see Diagram C1).

Note 2: The rails used for guiding a stair-lift may be ignored when considering the width of a stair.
However, it is important that the chair or carriage is able to be parked in a position that does not cause an obstruction to either the stair or landing.

Free Area of Smoke Ventilators

5. The free area of a smoke ventilator, specified in this Approved Document, may be measured by either:

  1. the declared aerodynamic free area in accordance with BS EN 12101-2:2003 Smoke and heat control systems. Specification for natural smoke and heat exhaust ventilators; or,
  2. The total unobstructed cross sectional area, measured in the plane where the area is at a minimum and at right angles to the direction of air flow (see diagram C7).

Table C1 Occupancy Load Factors

DIAGRAM C1 Measuring the width of doors N.B. Clear Opening width

Diagram C2 Measuring the Cubic Capacity of a building

DIAGRAM C3 Measuring the area of flat and double pitched roofs, rooflights, rooms

DIAGRAM C4 Height of a Building

Diagram C5 Number of storeys

Diagram C5 Height of a storey in a building is the height from the lowest level the building meets the external ground to the finished floor height of the top floor in the building

Diagram C7 Calculating the free area of smoke ventilators

No diagrams

No tables