When mounting a smoke alarm in your home, measure at least 4 inches down from where the wall meets the ceiling and make a mark. That mark signals the minimum distance from the ceiling that the edge of the smoke detector can meet; it is not a mark to signify where to install, since that would put the edge even closer. Space your fire alarm so that it is a maximum of 12 inches from the ceiling. It has to be high enough so that it will detect the rising smoke and heat, but not so close as to limit the stretch of the sensor.
If you are mounting a detector on your ceiling, you know how high it is going to be. However, you must take into account spacing around the detector. The detector should be placed at least 20 inches from the wall. If your ceiling has a pitch or slope, the detector should be placed at the highest point of the ceiling; this is where smoke and heat will travel first. Leave a minimum of 20 inches of space all around any detector, whether it is wall-mounted or ceiling-mounted, to allow the sensor to work unobstructed. The detector should be placed at least 3 feet from an air conditioner unit or vent, to prevent the interior electronics from freezing and therefore not functioning properly.
Mounting a fire alarm in a stairway offers a special challenge. If there are no doors to the stairway, you can mount the alarm anywhere along the wall or floor of the stairway, as long as it is in the path of rising smoke and follows all spacing rules. For a closed stairway, such as those that have a door at the top or bottom, place the detector at the end of the stairway without a door. Air, and therefore smoke, are naturally pushed under a door; a detector mounted high on the wall or ceiling near the door will not be able to sense the low-traveling smoke. Placing the detector at the opposite end of the stairway from the door allows the smoke to travel under the door and rise again on the other side to the level of the detector.
If you know the radius of the sensor's detection capabilities from the alarm itself outwards, you can adjust your detector mounting as necessary. For example, the top of a sloped ceiling is the best place to mount a detector. However, if your ceiling ends in a point, the detector may not fit. If you know the sensor's detecting capabilities, you can place the detector so that you ensure that it will detect any smoke that rises to the apex, without actually placing the detector there. Find out this spacing of detection from your alarm's manufacturer.