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Ugly Basement Stains: What They Are & How to Deal With Them

white basement wall with splotches of black mold

Originally posted 10/15/2018.  Updated 12/24/2020.


Your average unfinished basement is plain and utilitarian. It’s used as storage; this is where you keep your laundry units, workbenches, exercise equipment, furnaces, and water heaters. It’s not a fancy or highly decorated area, but that doesn’t matter as long as it’s clean and dry.

But homeowners should look over their unfinished basements from time to time, just to be sure that space stays clean and dry.  Sometimes a stain will “just show up.”  I know this sounds absurd; it’s reminiscent of a toddler who has no idea where your cookie went, nor does he know why there’s chocolate on his face.

In reality, these stains are a sign of a problem that has been developing within your foundation.  But you only become aware of it when the evidence of the issue comes through the foundation.  And for all of the stains we’re going to review, the issue is water or moisture in your basement walls.  

Acculevel has been repairing foundations and waterproofing basements since 1996.  We’re a family-owned and operated company based out of central Indiana.  We’re familiar with all the ways water can cause problems in a basement, what each of those issues looks like, and how to eliminate them.

In this article, we’re going to review the five main discolorations that you may see in your basement, explain what they are, and how to treat them.  If you currently have water in your basement, or it has recently flooded, I recommend that you check out this article on how to repair a flooded basement first. 



These stains can be produced by iron ore bacteria, a microorganism that feeds on nutrients in the water.  This bacteria is found in groundwater or soil.  Sometimes called “iron algae,” these bacteria are carried into your basement when water seeps into it.  In addition to the extremely persistent stains, you may also notice unpleasant smelling water or discolored laundry.  We explore this topic in greater detail in another article called Rusty Business

Iron ore bacteria is not a health risk unless the level is extremely high; in these cases, you should not allow infants to ingest the water.  If the level is high enough to be a risk, you aren’t going to want to drink it either- it will have any oily sheen on it.  It will also look and taste like rotting plants or sewage.


How to Deal with Rusty Stains

The stains themselves are hard, if not impossible, to get out of concrete.  The best course of action is two-fold:  begin by installing a water drainage system in your basement, so the water is no longer seeping through the walls.  

You will want to make sure the contractor you hire is well-experienced in working with iron ore bacteria.  It’s important to limit how much oxygen gets into the system; oxygen speeds growth and the spread of the bacteria.  The contractor will also need to incorporate clean-out ports into the system, because the bacteria may create buildup that impairs overall water drainage. 

Then, you can paint the walls in a color of your choice, using a paint that is meant for interior use that is not a “waterproofing” variety.  


White Powder

I know whenever you see a white powdery substance on your basement wall, you automatically think eww, that’s mold!  But this isn’t always the case.  If the substance looks like small crystals growing on the concrete, it’s probably efflorescence.  

Efflorescence is a mineral salt that gets left behind when moisture goes through the concrete and evaporates.  It is not a health risk, but it is a clear sign that your basement is excessively damp.

How to Deal with Efflorescence 

Begin with waterproofing, and possibly a dehumidifier.  Once the humidity level is at a healthy rate (50% is ideal), use a coarse wire brush to remove the crystals from the blocks.  If the blocks have been badly discolored, you may need to paint the wall(s).


Black or Fuzzy Splotches

This is where the situation gets unhealthy.  If the splotches on your wall are fuzzy or have a strong odor, that’s mold.  Mold can be dangerous for people with allergies or respiratory ailments.  Even if you are in perfect health and have no allergies, living in a house with mold can have a serious effect on your pulmonary system. 

How to Deal with Mold

Before you worry about the aesthetics of the splotches on your wall, you have important tasks to accomplish.  In addition to getting the basement waterproofed, you also need an experienced contractor to evaluate and treat the mold.  

Please don’t consider mold a DIY project.  The EPA recommends that an affected space larger than 10 square feet should be examined by an expert.  And whatever you do, do not use bleach to clean concrete.  Bleach is highly effective, but only when used properly on non-porous surfaces.  Concrete is porous.  

We have an article that goes into considerable detail about mold treatment options and pricing.  One of the options includes a moisture barrier that can be painted over, if the mold is in a prominent place that you would like to cover. 

basement corner with mold and standing waterThis photo was taken by an Acculevel project manager during a routine free estimate.  Water has been leaking into the basement, causing paint to flake off and mold to grow.


Chipping or Peeling Paint

It’s a good rule of thumb to leave your basement walls bare, until you have the waterproofing done.  But if you bought the house from someone who didn’t know this rule, you may be dealing with the aftermath. 

When water seeps into your foundation, it’s applying a certain amount of force (called hydrostatic pressure).  Over time, the water dislodges anything in its way- this includes paints, sealants, or any other treatment that’s been applied.  

How to Deal with Peeling Paint

This is going to take a lot of elbow grease, and it’s not going to be perfect.  You’ll want to use a scrub brush to remove the flakes or peeling.  And you may have to do this multiple times, until the surface is clear.  Make sure you’re not damaging the concrete itself; the last thing you want is to create another problem!  

In the meantime… you guessed it!  Install a waterproofing system so the water stops making this situation worse.  You may also want to install a dehumidifier, to help dry out the walls more quickly.


Flaking or Deteriorating Concrete

Do not confuse a damaged paint job with flaking or deteriorating concrete. You’ll want to be sure of what you’re seeing- if the wall has been coated with paint or other sealants, only that surface application will be coming off.

If the concrete itself is crumbling, the flakes will not be paint crumbs.  It will be tiny coarse pieces of concrete.  The industry term for this is called spalling, and it occurs when the water seeping into your foundation contains salt or a chemical that reacts with concrete. As this reaction occurs, it can start to break down the concrete itself..

Unfortunately, if your foundation is degrading in this way, the affected sections will need to be replaced.  This is one service Acculevel does not perform.  If you need to find a contractor who can replace foundations, we have a guide on how to find a good general contractor here.


The Next Step

Ready to get your basement waterproofed and want to talk to a contractor right away?  If you live in our service area, you can contact Acculevel.  We will schedule an appointment with one of our knowledgeable project managers, who will provide you with a free estimate.  But if you have more questions, like:

  • What type of waterproofing is best for your home?  
  • How much will waterproofing cost? 
  • Should you choose an interior or exterior method?  

We tackle all of these questions and more, in our Basement Waterproofing Guide. You can learn more about a specific topic, read the whole thing, or go straight to the costs chapter- whatever works for you!

link to our waterproofing guide

And if you’re more of a visual learner, you are welcome to take an online course, taught by Acculevel project manager Greg Backus.



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