Q: Our ceilings have what look like black soot marks, evenly spaced every 15 inches or so. What is the problem?
A: There are several possible causes to this problem, so you should probably consult a home inspector or energy consultant.
The condition is called ghosting, and the marks are likely caused by particulate matter in the air. In many cases, the source is the burning of candles. Another source of indoor soot is a faulty fireplace, which might be caused by a chimney that needs cleaning (for a wood-burning fireplace) or a poorly functioning valve (for a gas unit.) A faulty furnace or water heater can also be sources of soot, which is basically a by-product of faulty combustion.
Others blame the marks on dust that is drawn by moisture to the ceiling rafters. The marks correspond with the rafters, which are generally 16 inches apart. Where drywall touches the framing, the surface is cooler than where drywall touches insulation. Condensation tends to collect on cooler surfaces, so any dust floating around will stick. The dust could be blowing in from the outside, or it could be generated inside the house, perhaps by a poorly adjusted gas appliance.