There is a definition in the IMC306.5 Equipment and appliances on roofs or elevated structures.
Where equipment and appliances requiring access are
installed on roofs or elevated structures at a height exceeding
16 feet (4877 mm), such access shall be provided by a permanent
approved means of access, the extent of which shall be
from grade or floor level to the equipment and appliances’level
service space. Such access shall not require climbing over
obstructions greater than 30 inches (762 mm) high or walking
on roofs having a slope greater than 4 units vertical in 12 units
horizontal (33-percent slope).
Permanent ladders installed to provide the required access
shall comply with the following minimum design criteria:
1. The side railing shall extend above the parapet or roof
edge not less than 30 inches (762 mm).
2. Ladders shall have rung spacing not to exceed 14 inches
(356 mm) on center.
3. Ladders shall have a toe spacing not less than 6 inches
(152 mm) deep.
4. There shall be a minimum of 18 inches (457 mm)
between rails.
5. Rungs shall have a minimum 0.75-inch (19 mm) diameter
and be capable of withstanding a 300-pound (136.1
kg) load.
6. Ladders over 30 feet (9144 mm) in height shall be provided
with offset sections and landings capable of withstanding
100 pounds (488.2 kg/m2) per square foot.
7. Ladders shall be protected against corrosion by
approved means.
Catwalks installed to provide the required access shall be not
less than 24 inches (610 mm) wide and shall have railings as
required for service platforms.
