Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16A Photo Guide to Stains on Indoor Surfaces in buildings

  • POST a QUESTION or READ FAQs about identifying the cause and finding the cure for indoor stains on ceilings, walls, floors, carpeting, furniture, and other indoor surfaces and materials

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Pictures help diagnose types & causes of stains on indoor surfaces & materials:

This article describes, provides photographs, & diagnoses the causes of interior wall, ceiling, flooring or carpeting stains and explains how to recognize their probable cause and source, including soot stains, house dust stains, pet or animal stains, and thermal tracking or thermal bridging stains associated with building air leaks, and building insulation defects.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

A Photographic Guide to Help Diagnose the Causes of Indoor Stains on Carpet, Cabinet Doors, HVAC Registers

Ceiling stains at recessed lights © Daniel FriedmanOften dark indoor ceiling, wall, carpet or floor stains are mistaken for toxic indoor mold when they come from other causes instead.

When investigating a building for a mold problem, you can save mold test costs by learning how to recognize

MOLD APPEARANCE - STUFF THAT IS NOT MOLD or is only Harmless Mold but may be mistaken for more serious contamination - save your money.

Because some clients have on occasion sent samples to our mold test lab that really should not have been collected, much less looked-at, we provide this library of photographs of things that are "not mold" and don't need to be tested.

"Black mold" often mistaken for "toxic fungal growth." Photos of HVAC and carpet stains (excluding the moldy carpet photos) were provided courtesy of Pat Belkin, Charlotte, NC.

Here are some examples of troubling indoor surface stains with some preliminary opinions about what these stains may be about. We emphasize preliminary opinion because here we comment on these photos before an expert diagnostic building inspection has been performed and before any lab samples have been collected and analyzed from these surfaces.

Whether or not such sampling and analysis are justified depends on the experience of the building occupants (health complaints or health vulnerability), the history of the building (exposure to leaks, damage, pets, contaminants), and other site investigative results.

See When to hire a professional to investigate a building for help in deciding when to go further.

How to Recognize & Diagnose Phantom Stains on Indoor Surfaces

Photograph of indoor stains on a cabinet door

Often when people become concerned about indoor air quality, mold, or stains, they begin to study their building surfaces more carefully than ever before.

When this happens we sometimes find people reporting as "new" stains that were there from original construction, but where simply not noticed, or were not so particularly noticed before other concerns increased their level of attention.

This interesting photograph of a large yellowish stain on the surface of what appears to be a birch kitchen cabinet door surface (probably the door's interior surface) could be an example of this phenomenon.

What would support the "phantom stain" hypothesis for an item such as this cabinet door?

Stain pattern: If the stain appears in a regular pattern on companion surfaces such as other cabinet doors it may be an artifact of the door's manufacture

Stain location: If examination of other cabinet surfaces shows similar stains in varying locations

Manufacturing artifacts: If examination of sample cabinets from the same supplier or manufacturer, new, say in a showroom, show similar markings

Ability to remove surface staining material: If a tape sample cannot remove any debris from this are of darkened color

Stain particulate or chemical components: If a tape sample of surface debris removed particles which are determined to be finish coatings or wood fibers without fungal or chemical modification,

Presence of common causes of stains: If there are no moisture, food, air movement, or other suspect sources that have affected some of these cabinets but not others

Time of occurrence of stain: If the stain is under the finish-coating of the wood cabinet surface rather than something which was deposited on top of the coating (though indeed moisture can in some circumstances affect surfaces below their coating)

then this may be the case with this example photo of a yellowish stain on a birch ply cabinet door interior

How to Diagnose Stains & Discoloration on Heating or Cooling Supply or Return Registers or Plenums

Photograph of stains on a heating or cooling supply register

Supply Register debris: The left photo above shows typical house dust deposition on a heating or cooling ceiling air supply register, where you can see brown debris adhered to the metal register surface. A second example is in our photo below.

Photograph of paint overspray and house dust on a heating return plenum

These particles adhere to the register surface due to either moisture from condensation or in this location, more likely due to static electricity as particles are moving across a normally dry surface.

Dust and dirt stains on a ceiling around an air supply register © D Friedman at

See Stains HVAC Supply Registers for more examples.

What to do about supply register debris: House dust, normally composed primarily of human skin cells and fabric fibers, is not usually an environmental or air quality issue, though at high levels on surfaces it can be diagnostic of building conditions such as high moisture or poor HVAC system maintenance.

We can reduce this debris deposition by duct and air handler cleaning and by better and constant maintenance of filters at the return air registers. If other information disclosed by the building investigation warrants, one have this debris screened for mold, allergens or other problematic particles by using a forensic laboratory whose technicians are expert in house dust analysis.

Return register & return air plenum debris: The right hand photo shows a combination of paint overspray (white particles on the black return plenum insulation liner), and house dust (brown debris on the metal frame intended to hold a return air filter).

What to do about return plenum debris: The brown dust and debris indicates that the air filter used at this location has been leaky.

See An Analysis of the Effectiveness of Air Filters on HVAC Systems for further advice on air filters that do not leak in this location.

How to Diagnose Stains & Discoloration on Heating or Air Conditioning Filters

Photograph of indoor stains on a cabinet door

Air filter debris: This home air filter was taken from the central return air plenum and register cover shown in the photograph just above.

The brownish stains on the filter are a typical color (brown to gray) of debris found on any indoor air filter. Such "stains' indicate that the filter is doing its job of reducing the level of airborne debris in the building.

What to do about air filter debris heating and air conditioning filters should be changed at least monthly whenever this equipment is in operation. In a dusty environment more frequent changes may be needed.

Also see An Analysis of the EFFECTIVENESS of AIR FILTERS on HVAC Systems for further advice on air filters that do not leak in this location and

also see WHEN TO HIRE a PROFESSIONAL to investigate a building for help in deciding when to go further in inspecting, diagnosing, and testing particles and debris on a building air filter. In many circumstances further testing would not be justified.

How to Diagnose Mold Stains & Mold Growth on Floor Carpeting

Photograph of stains on floor carpeting

These photographs show two common patterns of mold growth on the upper surface of a carpet that was left wet in a building. In the left photograph a brown mold is growing in a rather uniform pattern without definite edges on carpet in a closet.

The black mold visible in the upper left and right portions of the photograph will probably be a different mold genera and species - this is an example of the dangers of careless sampling of mold in buildings since a lot of what people think is present in a building depends on exactly how mold samples are collected.

Photograph of floor carpet stains

The right hand photo above shows two rather round black patterns of mold growth on floor carpeting in the same building.

Even if no mold was visibleOn other carpet surfaces in this building it is likely that the carpeting, if it was wet, has become mold contaminated - a condition that might be confirmed by inspection of the carpet backing, padding, or other building surfaces.

Carpets or furniture that have been wet or had mold growth: Wall-to-wall carpets and upholstered furniture that have been soaked and/or have had mold growth on their surfaces, most likely cannot be adequately cleaned and should be replaced.




Carpet padding and subfloor: And where carpeting has been wet, don't forget that the padding below the carpeting and even the floor and subfloor below may be damaged or moldy.


Even carpeting which has not been wet may be a significant problem mold reservoir in a building if the carpeting has been exposed to a high level of airborne mold or other allergens. This condition occurs, for example, when a water-damaged moldy building has been remediated without proper dust and debris control.

The difference is that carpeting or upholstered furniture that has never been wet and that has not had mold growth on its surface, that is, it has been subjected to settled dust only, may often be adequately cleaned using HEPA vacuuming methods.


Screening samples of carpeting and other building surfaces taken outside of the remediation work area both before and after a mold remediation project can protect both the remediation company and the building owner from unanticipated additional mold cleanup work after the initial mold remediation project has been completed.

Another case of severe carpet staining due to furniture left on wet carpeting, accompanied by mold growth, is shown in this photograph.

How to Diagnose Stains & Discoloration on Rugs or Wall to Wall Carpeting

Photograph of stains on floor carpeting

Possible thermal tracking stains are shown by the darkened debris on the floor carpet in the left photo above, where a grayish line appears to follow the point where the wall to wall carpeting abuts the building wall or wall baseboard trim.

See THERMAL TRACKING STAINS for detailed discussion of this phenomenon and how to diagnose it.

Photograph of floor carpet stains

Possible furniture footprint stains on carpeting are suggested by the right hand photo above. Sometimes a stain like this, particularly where it follows the same shape as an object which has been placed on the carpet, suggests that the stain was deposited from the object itself, or dirt on its surface, or bleed-out if the object was placed on a carpet left damp after carpet shampooing.

A Severe carpet staining [image] due to furniture left on wet carpeting, accompanied by mold growth, is shown in this photograph.

How to Diagnose "Clean" Areas of Carpet Surrounded by Stains or Discoloration

Photograph of indoor stains on a cabinet door

"Clean" areas of carpet under furniture can also tell us what's been going on in a building. In the right hand photo above, the carpet appears darker inside the stain perimeter which suggests that the mark we see is either from a spill on the carpet or from the footprint of an object which was placed on the carpeting.

But if a mark on carpeting outlines an area of carpet which is lighter or cleaner than the surrounding carpeting, we usually find that an object which had been placed on the carpet was actually protecting that surface from settling dust, soot, or other debris in the building.

In this case we'd look further for an indoor source of high levels of airborne soot or other debris, such as a malfunctioning gas or oil fired heating system.

Because such heating systems could be unsafe (for example, risk of fatal carbon monoxide poisoning), the inspector should be one who is quite familiar with inspection methods and indicators of an unsafe or improperly-functioning heating or cooling system.

Usually soot marks, thermal bridging, or thermal tracking stains appear, if at all, in the building interior locations listed just below discussed in the remaining sections of this article.

Photos & Diagnosing Stains on Building Walls and Ceilings

Common stains on painted inteior walls and ceilings include

Ceiling stains at recessed lights © Daniel Friedman

Red stains on interior wall around picture (C) InspectApedia C.R.

The contributor of this photo, C.R. (1/27/2016) commented:

... [I have ] often seen mold *behind” objects close to cold walls (due to the decreased airflow stopping condensate from evaporating, I assume) but it just seems weird to get it on a surface that should actually be *drier* than behind the pictures. Just goes to show all these different molds have their own specific preferred conditions, eh?

Photos & Diagnosing Stains on Building Floors other than Carpets

Pet urine stains on a hardwood floor © Daniel Friedman

Common sources of stains on building floors include:

Stains on Plumbing Fixtures, Sinks, Showers, Toilets

Enameled cast iron sink photos (C) Daniel Friedman

Stains on plumbing fixtures may be due to simply poor housekeeping or to contaminants in the water supply, or as shown on the porcelain-coated cast iron sink above, a combination of the two.




Continue reading at THERMAL TRACKING BRIDGING GHOSTING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STAINS on INDOOR SURFACES, PHOTO GUIDE FAQs - questions & answers about the cause and cure of various stains on indoor surfaces, posted originally at this page


Suggested citation for this web page

STAINS on INDOOR SURFACES, PHOTO GUIDE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Ask a Question or Search InspectApedia

Questions & answers or comments about identifying the cause and finding the cure for indoor stains on ceilings, walls, floors, carpeting, furniture, and other indoor surfaces and materials

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher - Daniel Friedman