How commercial roofs shed water
A store, office, church or apartment building usually sends far more water to its gutters than a house does. The roofs are wider, the runs are longer, and flat or low-slope sections push water toward a handful of outlets.
That changes the hardware. Commercial systems often use gutters larger than the 5-inch size most houses have, with bigger downspouts to match. Some buildings have box gutters, square-sided troughs that are often built into the roof edge or set behind a low wall called a parapet. Flat roofs may drain through scuppers, openings in the parapet that feed a collector box and a downspout.
Leaks behave differently too. When a built-in gutter fails, the water can end up inside the walls or ceiling instead of on the ground, so a stain indoors may be a gutter problem.
Heavy rain is part of the design question here. Atlanta’s airport, the nearest station with a standard 30-year NOAA normal, logs an inch or more of rain on close to 15 days in an average year. Ask each company whether it has worked on your type of system, since lining or rebuilding a box gutter is specialized sheet-metal work.