Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16Stains on Building Interiors Diagnostic & Photo Guide to Stains on Carpets, Ceilings, Floors, Walls

  • STAIN DIAGNOSIS on BUILDING INTERIORS - CONTENTS: Interior building stain guide to identifying the cause of stains that appear on building interior surfaces. How to Look for Causes of Indoor Stains. An investigation strategy for indoor stain diagnosis, cause, cure, removal, & prevention
  • POST a QUESTION or READ FAQs about diagnosing the causes of stains found on indoor surfaces

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How to diagnose Indoor stains in buildings:

This article describes & diagnoses the cause of interior stains on buildings: stains of all colors, black or dark, grown, green, red, yellow, tank, or gray can appear on interior wall, ceiling, flooring or carpeting and may be due to a variety 9of sources, some of which are dangerous, others just "cosmetic".

Here we explain 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.

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Guide to Types of Stains on Building Interior Surfaces:

Black mold on drywall © Daniel Friedman

In the following guide we list types of stains found on building interiors, organized by by stain color & appearance, by building location or material, and by stain cause.

Some of these types of roof stains or discoloration are only cosmetic in nature, while others may indicate growths that are likely to reduce the roof covering life.

Black or dark stains on building interiors: such as on carpets, ceilings, walls, floors, drywall, or trim may be due to mold contamination but might also be soot from heating equipment, candles, fireplaces, pets, or simply soiling from use and lack of cleaning.

Dark indoor stains on walls and ceilings are often caused by thermal tracking - a deposition of house dust and debris on cooler surfaces - a stain cause we explain at THERMAL TRACKING & THERMAL BRIDGING.

Mold or fungus growing on building surfaces -
See MOLD in BUILDINGS for a guide to looking for mold contamination in or on buildings,
see WHAT MOLD LOOKS LIKE for a guide to just what mold looks like on building surfaces.

Watch out: don't assume that all black stains are toxic dangerous mold, and don't assume that mold only shows up as "black" on building surfaces. There are harmful molds that are just about any color you can imagine, there are harmless black molds, and there are plenty of building stains that are not mold at all.

Be sure also to see MOLD APPEARANCE - STUFF THAT IS NOT MOLD to avoid becoming excited about harmless substances that people think are toxic mold. If you know your building has a mold problem,

Stains indicate chimney leaks and damage © Daniel Friedman

Brown or other dark chimney stains on interior walls or ceilings near chimneys - See STAINS on/near CHIMNEYS where we discuss stains found on chimney exteriors or stains on building interior walls or ceilings near chimneys.

Also see SOOT STAINING - Black Soot Stains on Roof Shingles Around Chimneys

and see Proximate cause roof shingle stains



as well as STAINS & Thermal Tracking.

See Black stains from animals for details about pet stains on building floors (urine) and walls (various) and see Pet Stains on Walls for diagnosing stains such as the black marks left by pets on walls.

Readers should also
see STAINS on INDOOR SURFACES, PHOTO GUIDE where we illustrate various colors of indoor stains on surfaces and their causes.


We discuss carpet stains related to thermal tracking, air leaks, soot deposition in detail at CARPET STAIN DIAGNOSIS and we discuss carpet and other indoor soot, dust & stain testing procedures at

Often these stains are mistaken for toxic indoor mold. When investigating a building for a mold problem, you can save mold test costs by learning how to recognize 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.

These are substances that you can easily learn to recognize in buildings. Save your mold test money, and increase the accuracy of your mold contamination inspection or test for toxic or allergenic mold in buildings: review these items to learn recognize non-fungal materials or even possibly harmless cosmetic "black mold" often mistaken for "toxic fungal growth."

Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16

Rule out natural color variations: Some apparent building stains may not be an actual stain at all but rather the original color or color variations in natural materials such as the birch plywood door shown at left. Sometimes what you think is a new building stain is actually a color variation that has been in place for some time, but was not previously attended.

Identify thermal tracking stains from normal house dust: If you are able to rule out specific air leaks causing normal deposition of house dust, and if you are quite sure that you have an abnormal level of stains/soot like material showing up on various surfaces then these courses of investigation are suggested as an aid to diagnosing stains on indoor ceilings, walls, floors, carpets, cabinet interiors, closet interiors, or even house contents:

How to Separate normal "thermal tracking" dust deposition patterns from other types of indoor staining

If stains or debris are appearing on surfaces which are not cooled by their location (such as exterior walls, hollow interior walls which are entertaining internal air movement due to convection from below to above, areas near cooling air registers), then it may be possible to state with confidence that the stains appearing are due more to a high level of particulate debris in the building than to the more common thermal tracking phenomenon

How to Look for Causes of Indoor Stains

Black indoor stain is dog dirt not mold

Look for the source or potential sources of abnormal levels of indoor airborne debris, soot, particles, dust, such as a malfunctioning oil or gas fired appliance, any other combustion sources, even a mal-adjusted pilot light on gas stoves or heaters can be a soot source as well as the oft cited candles, fireplaces, and even pets (for example lots of dog traffic between indoors and out brings in high levels of dust).

Photo at left: dark stains low on interior walls were identified as pet stains: a large dog frequently lay against the wall in the areas shown. [Click to enlarge any image]

See Black stains from animals for details about pet stains on building floors (urine) and for black or other pet stains on walls see Pet Stains on Walls for diagnosing stains such as the black marks in our photo at left.

If/when we can identify an unusual source or a source producing an unusual level of particulate debris we have perhaps answered a key part of this question of the probable source of indoor soot, dust, or debris stains.

Measure Indoor Moisture Levels when Diagnosing Indoor Stains and Soot or Debris Deposits

OPINION: even in a relatively clean home, unusually high moisture levels may result in noticeable levels of dust deposition on indoor surfaces, regardless of the dust source, and even including normal types and sources of house dust.

If moisture levels are a factor in the home, say moisture regularly above 55% RH, we would expect to see more-stained surfaces on those building surfaces that are more likely to be a bit higher in moisture, such as cooler surfaces on walls, ceilings, or in closets or cabinets where temperatures are lower and moisture may condense at a slightly higher level.

Indoor Stain Diagnosis Using Air Movement Patterns

Sometimes we can identify particular sources of air movement, directions of air movement, which we can correlate with the areas where we see staining. A simple example is the higher amount of dust deposition that occurs around heating or air conditioning supply registers on ceilings and walls. Relating air movement patterns to dust or soot or other debris stains may be diagnostic.

Also take a look at a short paper we wrote on an analysis of suspect indoor dust, at An Investigation of Indoor House Dust Debris where we determined that indoor dust levels which had been suspected of originating in an HVAC system were actually carpet dust and fibers.

How to Use Particle Identification to Diagnose the Source of Dust, Dirt, Soot, or Debris Stains

It is often possible to collect samples of suspect dust or debris for microscopic analysis in order to suggest a source or type of source of indoor stains.

It is essential that you select a forensic laboratory whose staff includes people experienced and trained in the identification of a wide range of indoor particles. A lab specializing in mold or allergen identification, for example, may not consider much less apply methods used to identify oil burner soot, common components of ordinary house dust, mite fecals, pet dander, human skin ce

Stains Traced to the Building Water Supply

Stains on plumbing fixtures, laundry, appliances, and surfaces sometimes are traced to minerals or other contaminants in the water supply.


Continue reading at CARPET STAIN DIAGNOSIS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see THERMAL TRACKING BRIDGING GHOSTING that explains many dark stains on building indoor surfaces


Or see STAIN & BIODETERIORATION AGENT CATALOG causes and types of damage that raise aesthetic and conservation issues on cultural artifacts, buildings, and art works.

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