Slab Foundations: The #1 Myth in Mold Avoidance

What’s the #1 myth regarding mold avoidance?

Avoid mold problems by living in a home or working in a building on a slab foundation. WRONG!

Regarding slab foundations… know that in most parts of the country and the world, slab foundations have foundation walls going down to undisturbed soil and going below the local frost line. This is basically the same as a basement, except that builders fill holes created to pour the foundation walls back up with dirt. So, a slab foundation consists of a very short basement, filled with dirt.

Some parts of the country that are very warm use perimeter ‘grade beams’, which are just even shallower ‘basements’ with dirt inside. Water settling against a structure can get through slab foundation walls and wick up through the slab itself very easily. The phenomenon actually has a name called ‘rising damp‘. Concrete is very porous and acts like a sponge to wick water into itself, and often into walls sitting on it. Toilets seals and other drains that go through slabs can leak just the same as anywhere else. Ductwork in slabs is highly prone to mold due to condensation.

Know that slab foundation systems can have the same water condensation/seepage/drainage/water accumulation problems that all foundation systems can have, including basements. Building owners just can see the water damage easily. So people mistakenly think there isn’t a problem, just because they can ‘t see it.

Difference between a slab and unfinished basement

The difference in an unfinished basement, is that you can go down there and see or smell any possible moisture issues. You can also check plumbing pipes and connections to toilets above, from below. And in a basement, it’s easy to fix challenges or leaks when they do occur, hopefully in a timely way. Timely action in response to water damage incidents is the key to avoiding costly clean-up in addition to repair costs, and is key to avoiding health issues from long term water accumulation.

With slab foundations, the same leaks, water infiltration, even condensation can be occurring but you can’t see the accumulation or check for it easily. Leaks and accumulation of water often goes unnoticed for a very long time as the problem grows unseen. Fixing water accumulation below slab foundations is very difficult and therefore very expensive too, as it often involves cutting up and removing pieces of concrete slab.

The myth about slab foundations

It seems some doctors and other health advocates have perpetuated the myth that slab foundations are mold resistant. As an architect myself, who has dealt with complicated health issues, I know lots about health remedies, but I’m not the one to answer questions about how various systems in the body work. I do not recommend getting information on building systems and how they work best from a medical doctor.

Attached is a picture of ‘rising damp’ in a solid masonry wall. There is no basement or crawl space, only a ‘slab’ foundation. Note that there are no gutters to carry water away. The very large roof of this building is dumping gallons of water against the foundation, where it is wicking up into the wall. I had a mold reaction myself while visiting this building (a church) recently. The second picture is a university classroom building. Look around and you will see this often, especially after a rain event. Over time, the damage is evident even without the rain.

Church building with no gutters to carry water away from the structure
University classroom building with moisture stains

Get your questions answered!

Cheryl Ciecko, AIA, ALA, LEED AP, is a licensed architect who helps homeowners and building professionals create safe homes through online education to prevent mold, water damage, and poor indoor air quality. You can find her video masterclasses, online programs, and education subscription options at .

For detailed answers on your specific situation or building project, schedule your appointment for an individual consultation.


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The information contained in this presentation is for educational purposes only and is not intended to diagnose, prescribe, treat, or cure any disease, ailment or injury to the body. It is not medical advice. FDA regulations prohibit the use of medical claims in conjunction with the sale of any product not approved by the FDA. Statements made herein have not been evaluated by the FDA. 

Any products, techniques, and/or personal usage tips referred to are not suggested as a replacement for proper treatment from a licensed health care professional. I am not a licensed health care professional and the decision to use or not to use any of this information is the sole responsibility of the listener and/or reader. Everyone is an individual with different body types, different blood types, different body chemistries, and it is important to remember that what works for one person may not work for another person.