Detector layout in ordinary buildings is done from a table: a maximum spacing on a grid, halved near walls, adjusted for sloped ceilings. That table is not arbitrary and it is not fundamental. It is the fossil of a set of full-scale fire tests in rooms with flat, fairly low ceilings, and it encodes a physical picture — hot gas rises from the fire as a plume, strikes the ceiling, and spreads outward as a thin fast layer whose temperature and velocity fall the further it travels, so a device beyond a certain radius sees a signal too weak and too late. Everything that changes that layer breaks the table. A high ceiling lets the plume cool and dilute before it arrives, and can leave it floating below a warm ceiling instead of reaching it at all; deep beams pond the flow into pockets; ventilation blows it sideways. In those spaces the layout must come from a calculation or a model of the flow, and the resulting spacing is usually much tighter. After this Concept you can say whether a given space may be laid out from a spacing table or needs the underlying flow calculated.
Fire and smoke detectors are point detectors that must have combustion products physically reach their sensing element to activate, so their effective coverage is bounded by a detection radius determ…