Black Spots on Ceiling — Articles — Trained Eye Home Inspection

Black Spots on Ceiling


I was wondering if you could help my husband and I. We have found our dream home but in the master bedroom there seems to be little back dots on the wall and on the ceiling. I am wondering if this is mould? If so how did it get there and how do we get rid of it or is it even worth buying the home at all?

Thanks so much for your help.


The black dots on the ceiling and walls in the master bedroom may have one of several sources. These may range from very minor issues, easily repaired, to major concerns with a much more difficult and expensive fix. We will look at several possibilities and suggestions for remediation.

The most commonly seen cause of small black dots in homes is corrosion on fasteners holding the drywall to the wall and ceiling framing. This can normally be identified if the dots are very uniform in size and conformation. If the dots you are seeing are roughly 3/8 of an inch or 10mm in diameter and are very uniform, then this is likely the case. Drywall screws and nails often corrode, producing dark spots in the drywall compound that covers them, if there is a higher than normal relative humidity level or source of moisture in their environment. This may be due to a poorly installed or thin air-vapour barrier, minimal insulation in the exterior walls or ceiling or a large moisture source in the home.

If this first description is accurate for what you are observing and this phenomenon is only seen in this one room, then the cause and repair may be simple. I am assuming that the bedroom has a conventional flat ceiling and not a vaulted ceiling. If the ceiling is the underside of a vaulted or cathedral roof structure, this will not apply. The master bedroom likely had or has an individual source of moisture that is not present in the rest of the home. This may be a portable mist humidifier, en-suite bathroom shower, or other source. The first thing to do is to locate this moisture source and eliminate it. After it is gone, the ceilings and walls can be washed, sealed with an oil or shellac based primer, and repainted to cover the stains. If the corrosion is more advanced, or the drywall compound is damaged and falling off the fasteners, these can be removed and replaced with new drywall screws. The new screw heads and old holes can then be patched, primed and painted.

Other possibilities for the nature of the stains you are seeing may be nicotine or soot. Regular burning of candles or a nearby fireplace will cause a fair amount of soot to be discharged into the air with the smoke, and may be causing the stains. These stains often are more concentrated on drywall screws or along wall studs and ceiling joists, which are slightly colder than the surrounding drywall. Soot stains are normal black in colour and may be a plausible explanation. Nicotine stains, from cigarette smoke, are normally brownish in colour, but may appear darker if they have been present for many years and are mixed with dirt. If either of these two items is the cause of the stains, washing the walls with TSP (trisodium phosphate), will remove the stains before repainting.

The final possibility, that we will discuss, is that the stains are indeed mould. If the spots are uniform in nature, look to the first two explanations. If the stains are uneven in distribution, smaller in size or concentrated in certain areas, they may be mould. This is especially true if they are found in higher concentrations in the corners of the exterior walls or ceilings, in the closet, behind furniture, or under windows. In this case the mould growth may be caused may be air leakage and condensation behind the drywall combined with a lack of air movement. This is quite common in closets, where excess storage combined with a lack of heat or air movement is the ideal environment for mould.

If the dots are small and minimal amounts are present on the drywall surface only, simple cleaning with soap and water may be enough remove the mould. If the mould is embedded in the drywall surface itself, removal and discarding may be required. In this case, a lack of proper air sealing and ventilation in the attic or a roof or wall leak may be the cause of the mould. Professional help will be required to safely remove the contaminated building materials, provide proper remediation techniques and remove the source of the moisture causing the mould growth.

For additional information on how to identify mould and when to call in the professionals check out the CMHC website at They have several articles on indoor air quality, relative humidity and an excellent document on mould in homes entitled Fighting Mould.

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