Figuring out what caused that stain you noticed in your ceiling starts with dividing the possible culprits into two categories: roof and attic. Although liquid sometimes migrates sideways before dropping onto the ceiling, it doesn’t happen that way too often, so the problem is usually directly above the stain.

ROOF

    Any roof penetration—like a plumbing vent pipe, gas appliance flue, fireplace chimney, or roof exhaust vent for a range or dryer—is where most leaks start. Roof damage from a falling tree branch or damaged shingles is another possible cause.
    There is also windblown rain that comes in through a gable end or box roof vent in a storm. This may be a one-time event caused by high winds. Many roof vents are resistant to blowing rain but, when it is coming straight sideways at 60 mph, some of it is going to get through. 

ATTIC

    The list is longer for stains that start in the attic. If the refrigerant lines to the air handler (indoor unit) of your air conditioning system run through the attic, any areas of missing or damaged insulation will sweat profusely during hot and humid summer months. Sometimes the attic insulation will absorb it and allow the wetness to evaporate before reaching the ceiling except during the hottest time of year. Leakage of cold conditioned air at the seams of duct connections, especially when they are on the bottom of the duct and just above the ceiling, will also cause condensation and a wet stain.  Water overflowing out of a catch pan under an attic air handler is another possibility.

    Plumbing leaks are another possibility. While a leak in a water supply pipe in the attic or floor framing of an upper floor can cause staining, it usually makes a big mess quickly and is easy to identify. But pinhole leaks, especially in older copper pipes can be tiny enough to create a wet stain and some mold growth without flooding an area. Drain piping is the more common type of leak. A stain that is directly below a bathtub or toilet needs further investigation by a plumber.

    An old container of stored liquid in the attic that has corroded through or been knocked over might be the cause or, rarely, rodent urine directly under a nest will be it. 

    A wet stain can be tracked back to the source by a home inspector with any of several different moisture detection tools, such as a infrared camera, electronic moisture meter, or just probing the area with finger tips for wetness. But dry stains can mystify even the best inspector, and may indicate a previous moisture intrusion problem that already been fixed. Sometimes we just have to say “Gosh, don’t know what caused it, but the area is dry now. If the stain comes back after you repair the area, let’s try to troubleshoot it again.” 

    When you repair the stain by painting over it, we recommend starting with a coat of an oil-based stain-blocker paint, like Kilz Original or Kilz Complete, before applying a the matching ceiling color. Most water-based paints will allow the stain to migrate to the surface over time and annoy you all over again.

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Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• What are the building code requirements for notching and boring holes in a wall stud? 

• What causes dark or light "ghost" lines on ceilings and walls?

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code? 

• Why is my garage ceiling sagging? 

• How can I identify what kind of wood flooring I am looking at?

• Why does my concrete floor slab sweat and get slippery?

• What is the minimum ceiling height for rooms in a house? 

• Why are there score line grooves in the concrete floor of the garage?

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

   Visit our STRUCTURE AND ROOMS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.