Ceilings have a direct impact on detector spacing and location. This impact directly affects smoke and heat detectors, so an understanding of ceilings and their impact is a critical aspect to fire alarm design.

NFPA 72 defines ceilings in Chapter 3 as an upper surface, regardless of height. Further, it defines ceiling as either level or sloping. A level ceiling has a slope of less than or equal 1 in 8, while a sloping ceiling has a slope of greater than 1 in 8.

The expression of slope in this form shows the relationship of amount of rise over a specific travel distance (commonly called “rise over run”). A slope of 1 in 8 means that the ceiling rises 1 foot for every 8 feet of distance. The longer the distance to rise, the shallower (narrower angle the slope). A slope of 1 in 12, for example, would not be considered a sloped ceiling.

A good way to visualize this is to consider a pizza. If you cut into 8 slices, and you get once slice, you have 1 in 8. Now imagine if the pizza were cut into 12 slices – your slice of pizza would be thinner (a narrower angle) than if the pizza were cut into 8 slices. Conversely, if you cut the pizza into 6 slices, you slice would be larger (a wider angle) than an 8 slice pizza.

When determining if a ceiling is sloped, the second number becomes the more critical one. Just like the pizza analog a 1 in 8 pizza slice is narrower than a 1 in 4 pizza slice but is wider than a 1 in 12 pizza slice. So a slope of 1 in 4 is steeper (approximately 14°) than a 1 in 8 (approximately 7°), and a 1 in 12 slope is less steep (approximately 5°) than a 1 in 8 slope.

NFPA 72 further defines sloping ceilings as peaked and shed. A peak ceiling slopes in two directions from the highest point. A shed ceiling slopes from one side towards the opposite side.

NFPA 72 requires that the first row of detector on a slope ceiling be located within 3 feet of the highest point of the ceiling. In the case of a peak ceiling (which slopes in two directions) this can be on either side of the peak. The remaining detectors are spaced based on a horizontal project of the ceiling.

A horizontal projection of the ceiling requires that the spacing measurement be along a level surface (the floor), rather than down the slope itself. Essentially, detector spacing is determined by measuring along the floor, and then drawing an imaginary line up to the ceiling for detector location.

Since heat detector spacing is adjusted based on ceiling height, NFPA 72 has additional rules for heat detector spacing based on the degree of slope of the ceiling. If the ceiling slope is less than 30°, detectors are spaced based on height at the peak. If the ceiling slope s 30° or more, detectors are spacing based overage ceiling height.