Abstract Wind tunnel tests were conducted to show that the 2.5 times rule for the determination of a necessary height for a stack close to a building is adequate for a building whose width perpendicular to the wind direction is twice its height, but that it is unnecessarily conservative for a tall thin building. An alternative rule, called Briggs' alternative, is shown to be adequate. The study was undertaken in a meteorological wind tunnel by placing model stacks and buildings in a simulated neutral atmospheric boundary layer. Smoke was used for flow visualization and methane for quantitative concentration measurements downwind of the building. The plumes were neutrally buoyant and the stacks were placed within one building height of the building in all cases.