What caused the light coloure "Stains" on this rug ? Diagnosis suggestions (C) InspectAPedia BBCarpet Stain Diagnostic FAQs
Questions & Answers on How to Diagnose Indoor Carpeting & Rug Stains

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Carpet stain diagnosis questions & answers:

FAQs about how to determine the cause of various types of stains and stain patterns found on carpets and rugs.

This article series describes how to identify and diagnose carpeting stains at the edge of carpets near walls in buildings focusing on the cause of various interior wall and ceiling stains and explains how to recognize thermal tracking, thermal bridging stains, building air leaks, and building insulation defects.

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Stains & Marks on Carpeting - Diagnostic FAQs

Black stains on carpet at walls (C) Daniel Friedman JB


Sept 10, 2012) MRS. R. said:

LOVE THIS SITE. NEVER knew I could find such an informative site on my problem carpet staining along the wall. It had become an embarrassing problem (housecleaning issues? I hoped not)Now I know-thermal tracking from crawl space! Thank you.

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Question: quarter sized bleach-like spots on the Shaw carpet

(Oct 10, 2014) Ed said:

We had a very nice plush carpet manufactued by Shaw installed in our house about 2 years ago. All was well until about a month ago when we noticed a few small discolored spots show up in the Den. They were the size of a quarter and look like bleach spots. We blew them off as an accidental spill which we will have repaired with some extra carpet we have. A month later, 2 larger bleach colored spots have surfaced. One in the den and one in our living room. One is about 6"x6" and one is about 12" x 3". Our dog does can get to the den but not the living room.

Also we tested the dog's urine on a piece of the same carpet and it will no create this type of bleach discoloration. We do not detect any leaks in our cement slab floor that could be a possible source. Any ideas on the cause? Could the carpet pad or adhesive cause this type of discoloration? I intend to contact the carpet retailer, but would it be best to contact Shaw Carpets and have an Shaw carpet inspector come out? Any advice wold be appreciated. Ed



Dog pee stains are not likely to be quarter-sized unless your dog is an unusual pee-r. I'd start by determining if the stains are coming from above or below. Careully remove carpet from its tack strips at a corner and pull it back far enough to let you see the condition of the padding and the concrete slab below the areas where the stains appear.

If there are absolutely no visual difference on the subfloor surface or padding below the stains from below non-stained areas then we're looking for a source in the occupied space.

Let me know what you find and we'll take it from there.

Watch out: I do not advise testing carpet by putting dog pee on it. The odor is hard to remove beyond the dog's ability to smell, and putting it there is telling the dog that's a good place for his or her urine.

Question: dark areas forming on cheap nylon carpet

(Dec 3, 2014) Anonymous said:
We recently moved into a house with new cheap nylon carpet. in the last month I have noted large dark areas appearing and have tried to clean them, which either makes them worse or doesn't help An ideas?


Usually there has been a spill if the stain is in a specific rounded spot, or if it's along wall/floor edges or at HVAC registers, it's an air leak or airflow problem depositing dust and debris.

Question: big dirty stains on a carpet over an asphalt tile floor

(June 7, 2015) Bill said:
Hi we have a cream carpet on underlay we never have shoes on it but we are getting big dirty looking stains on the carpet. The floor is our lounge and it is an asphalt floor could it be asphalt stains from when it was wet cleaned? it comes back after each cleaning.



It might be worth taking a look under the carpeting to see if you've got water coming up from below.

Question: dark irregular stain on carpet has doubled in size and doesn't clean off

(July 27, 2015) Anonymous said:
I have a large (3x3), irregularly shaped spot in my carpet That is a darker shade than the whole carpet. It has doubled in size in 15 years and doesn't seem to respond to cleaning. What could it be?


Perhaps a fungal or other stain response to an old spill? Spills of organic liquids, juices, milk, etc. can invite a fungal growth that might expand over time, OR the stain itself may become more apparent as dust and dirt particles stick to the area of original spill;

I'm not an expert but I've also noticed that some spills that are acidic can actually damage the carpet fibers.

Perhaps there is an air or water or moisture source under the floor in that location?

Question: stains found under the carpeting

(July 28, 2015) Anonymous said:
We purchased a new home and are in the process of pulling the carpet. There were no obvious stains on top, but the underneath in almost every room so far is almost completely covered with rust blotchy stains. Two of the rooms had small areas on the sub-floor near the wall that looked moisture stained although they did not feel damp or test positive with a moisture meter. We are assuming pet damage since previous owner had cats and dogs, but is there anything else we should be concerned about? Something is definitely giving me a headache when I'm pulling carpet. Thank you.


Usually pet stains are black or brown on wood flooring;

Tiny red stains around tack strips may indicate prior wet carpets or flooring that rusted the tack strip nails.

If you see rippled cupped flooring I'd assume the floor has been soaked and I'd be thinking about what happened and asking if water entered walls, wet drywall or went other naughty places that risk a hidden mold problem.


Question: how to distinguish between carpet perimeter stains caused by rusty nails in tack strips vs. air leaks or other causes

(Aug 26, 2015) Anonymous said:

(Oct 16, 2015) Anonymous said:
How do you know that the dark areas around the perimeter of carpets is not due to oxidization of the nails in the tack strips ?


Good question Anon:

Nail rust or the rust of the nail tips poking up through the tack strip and grabbing the carpet back are in a regular pattern; you can pull up carpet, separating it carefully (so as not to tear the carpet) in an area of staining to look at the back of the carpeting: if the carpet has been wet or the floor below damp you may see rust spots tracking tack strip nails on the bottom of the carpeting.

Look at the carpet stain from the room side to see how that maps to what you find below the carpeting; for example, if the stains appear on the carpet room side in an area where there are no rust stains from tack strips on the carpet back then we figure the stains are not due to a wet floor below but rather some other source such as an air leak or a spill onto carpet from the room side.

Question: musty odor, now orange spots on the carpet

(May 2, 2016) Eprice said:
I have beige carpet in a cottage that we purchased a few years ago. The house did not have a dehumidifier running when we purchased it so it had a musty odor that we are slowly getting rid of. We noticed a couple of small orange spots in various places on the carpet and didn't think much of it. In the last year the orange spots are appearing all over in different rooms. Our plan is to eventually replace the carpet but we we want to do some remodeling too so it may be a year until we get to it. Any idea why these orange spots are appearing?



If there is a musty odor there is an odor source somewhere, perhaps a mold reservoir that you don't see. I'm not sure what explains the orange spots: possibly a yeast growth. I'd worry that there is moisture coming from below, and I'd check the carpet padding, the floor below that, and the under-side of that floor from below.

Question: yellow stain on carpet due to air leaks?

(Sept 1, 2016) Selina said:
I have a yellow stain I'm my daughters bedroom that is growing could this be due to air leakage


I can't guess from just that e-text, Selina. But usually air bypass leak stains are dark in color as they're depositing house dust. Yellow? The location and pattern of the stain may be diagnostic.

Question: mysterious stains on expensive carpet

2016/09/08 Seonad-Siobhan Duggan said:

I have a very good quality wool carpet, which sees hardly any traffic whatsoever, as I don't live in the house where it is laid! It has started disintegrating for absolutely no apparent reason, other than a stain has been spotted by 2 separate experts who think it may be a spillage of some sort, although we have not spilled anything! The experts have ruled out damp, moths or any type of bug infestation. I am wondering if such a problem, which seems to have no explanation or solution, could be fixed - the carpet is very good quality and is discontinued, so I am obviously, very disappointed not to find the route cause or solution ... any ideas ? Thanks



One thing upon which we can agree, a carpet won't deteriorate for absolutely "no reason". There's a reason, even if we don't know what it is.

Start by looking at what sort of "deterioration" is occurring - that's not clear from your note.
Continue by mapping just where the problem is seen.

If the deterioration is in a specific spot, what's the shape and location of that spot? Either something happened there or something spilled on or was placed on the carpet there or there's some other more rare, I'd say very very rare case of a manufacturing defect. I say rare because the way carpeting is manufactured involves large, continuous sheets of material so it'd be odd for a manufacturing defect to show up as a spot.

If the defect is over all of the carpeting then I'd look for an environmental factor: heat, moisture, airborne contaminants.

I'd also want to look at the under-side of the carpet and at the padding and at the flooring below the padding: those will give invaluable clues.

You can use the page top or bottom CONTACT link to send photos of the conditions you find and we may be able to comment further.

Finally, find and follow the live link to stain testing at the end of the article above where you see this line:

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

Carpeting and IAQ Articles


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