Figure 1-17. Location of Air Terminals on Gently Sloping Roofs
On flat roofs position the air terminals around the perimeter in the manner shown in Figure 1-18.
Provide additional air terminals placed at 50-foot intervals over the interior of flat and gently sloping roofs
which exceed 50 feet in width.
Terminals are to be provided within 2 feet of corners, the end of ridges, or edges of main roofs.
Terminals less than 24 inches in height are to be spaced 20 feet or less. Terminals 24 inches or
taller may be placed at intervals not exceeding 25 feet.
Ensure that no part of the structure extends outside the cone of protection established by the air
terminals. Determine the cone of protection by preparing a simple scaled profile drawing of the structure and
then superimposing a 45-degree (a 1:1 cone of protection) triangle on the profile. The apex of the triangle
should coincide with the tip of the air terminal whose protected zone is being verified, as illustrated in Figure
1-19. Alternatively for existing structures, the field expedient method illustrated in Figure l-20 showing a 2:1
cone of protection can be used to determine the coverage of prominent projections. This method is particularly
useful for small structures.
To determine if all parts of a flat roofed structure such as vents, pipes, cabling, or raised extensions are
protected, use the method illustrated in Figure 1-21 to calculate the zone protected by two vertical terminals.
This method can also be used to determine the coverage provided by vertical masts or horizontal wires. In
Figure 1-21 point P represents the point of discrimination. That is, the point of departure of the final stepped
leader of the downward traveling stroke (see Volume I, Section 3.2). To determine if the air terminals are