Smoke alarms need to be located on (or near) the ceiling for optimum detection of smoke in a fire situation with special care taken to avoid dead air spaces.
A dead air space is an area in which trapped hot air will prevent smoke from reaching the alarm. This space generally occurs at the apex of cathedral ceilings, the corner junction of walls and ceilings and between exposed joists etc. (see diagram 1).
Smoke alarms may be located on the wall in accordance with 3.7.5.5(b). Diagram 1 provides location criteria and the dimensional relationship to building elements and the associated dead air spaces.
Where a smoke alarm is located on the ceiling it must be—
a minimum of 300 mm away from the corner junction of the wall and ceiling; and
between 500 mm and 1500 mm away from the high point and apexes of the ceiling, if the room has a sloping ceiling.
Where (a) is not possible, the smoke alarm may be installed on the wall, and located a minimum of 300 mm and a maximum of 500 mm off the ceiling at the junction with the wall.