Basement/Crawlspace Wall Black Markings?

I apologize up front if this is a dumb question, I am still learning. This is the inside of an exterior wall from within a crawlspace area built in 1963 with active water intrusion issues and efflorescence everywhere. The crawlspace is blocked off from the rest of the basement/garage by the interior block wall on the right. The ground slope on the other side on the exterior wall roughly aligns with the slope of the black area. However, as you can see, the black markings also extends to the interior wall. I am thinking the black marking was a dampproof coating applied to the interior side of the wall that has long since failed? Perhaps the basement was enlarged and dirt excavated at a later point? Anyone have the answer to this puzzle?

I would agree with that assessment.

I am not sure what you mean by the dirt being excavated. Is the dirt on the exterior higher than the dirt on the interior? If so, not unusual but should not happen in modern crawls.

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Yes, the ground level on the exterior roughly corresponds to height of the black coating. I am thinking the interior wall and dampproof coating were original to the house. Then, someone removed dirt from the crawlspace at some point later for some reason.

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Welcome to our forum, Brian!..enjoy participating. :smiley:

Possibly…or because of the dirt back fill against the exterior wall, the CMU block wicked moisture to the exposed interior. The surface coating could have been an impermeable coating intended to mitigate the migrating moisture. Think basement CMU block wall. Something to consider

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It appears to be a damp-proof coating applied to the interior, as you stated. Whether it was applied at the start, or later, it was never going to work. As a matter of fact, it is detrimental to the block wall because it will help trap moisture.

I would note it as “appears to be an improper interior application of a water-proofing coating. This may cause rapid deterioration of the block wall due to trapped moisture. Water-proofing coatings should be installed on the exterior only.”


Okay. I’m not saying interior surface coatings are a long term solution. However, I’ve never witnessed rapid deterioration due to this. In fact, it’s pretty common in my area to help reduce wicking, efflorescence etc. Typically, the result I see is the coating bubbles or blisters and the moisture or efflorescence wins. In fact, these coatings can do a pretty good job for years for minor moisture migration.

Eventually, the problem should be resolved from the exterior. Just my experience.

Tar based coating to attempt to damp proof. Don’t show to @manderson7


I don’t disagree. The water will win as shown. Those block are wet and this home needs exterior excavation and waterproofing.

Sixty year old home, CMU block and that may be red clay. Bad combo.

Ok, maybe rapid was the wrong word. “Accelerated” would probably be better. I would rather have block that dries to the inside than block that never dries at all.

Agreed!! 10 characters

Mystery solved. A tar-based coating was applied to the interior of the foundation, likely original to the house. I would hazard to guess that the same was done to the exterior.

Despite decades of getting soaked and loads of efflorescence, the CMU blocks still appear to be in good shape. I did not see any spalling, cracking, or evidence of foundation movement. Perhaps the saving grace is the fact that we are in an area (Northern CA) that is typically bone-dry eight months out of the year. The crawlspace is well-ventilated, so the CMU blocks are dry most of the year.

The crawlspace faces the large back yard that has about an 5 degree upslope that is the cause of the moisture. The owner is putting in a French drain near the house at the bottom of the slope, upstream of the crawlspace. I recommended that he put in drop inlets along the drain to collect surface water and also install a water barrier between the drain and the crawlspace to help force the water out the drain and away from the foundation. That should at least reduce the water. If that is not enough, I suggested that he might then consider adding a second drain or swale farther up the slope. The reasoning is that the French drains are relatively cheap to install as he has been doing the work himself.

When they didn’t know differently…or didn’t quite care enough to do it correctly.