Home & Garden Columns

About the House: Getting the Real Dirt on Dirt

By Matt Cantor
Friday January 04, 2008

I have preferred over the years to confine my writing to subjects outside of my actual day-to-day vocation, but sometimes a discussion of my work helps a bit to illustrate a point. It’s not very glamorous but I spend a lot of my life in crawlspaces. The cats look at me funny, wondering what I’m doing in their bathroom. People often say, as I suit up to get sub-domestic, “Well, here’s where you earn your money!” It’s really not true, but the comment reveals how unpleasant the average person perceives this to be.  

Sadly, this pedestrian and reasonably safe activity gives rise to more understanding about houses, their life cycles and their maladies than virtually any other single procedure. No effort is made to hide any system in the construction of crawlspaces. Over time, some tasks (e.g., seismic retrofitting) result in hiding certain features, but mostly it’s Open Architecture as the I.T. folks say and one feature that is rarely hidden and so revelatory is the ground itself. 

It may seem silly or obvious to say it, but looking at the dirt below your house can be tremendously revealing and I learn so much of real concern from a simple examination of the soil in a crawlspace that it’s one of the only items that I consider absolutely requisite in the examination of a home. 

So what’s the dirt on dirt? What can we learn from looking at soil? There are at least two major areas of study that can be plumbed from a simple visual examination of the soil under your house and for this reason, it’s well worth your time and the drudge to pull on a coverall and take a crawl. 

First, look for signs of moisture. One sign that may not be obvious but is of great value is the softness of the top layer of soil. Under houses that have remained dry for many years, the soil, even clay, will tend to be broken up and powdery. Over time a range of forces including animal activity will tend to break up the soil into a powdery texture if dry conditions have prevailed. 

If the soil is all hard and caked, this shows that, at some point, and perhaps recently, the soil has been damp or wet. In houses that flood seasonally, the ground may be dry to the touch but will tend to have formed into a mud cake. This will often be cracked like a desert soil (called laterization) if there is sufficient clay content.  

Here in Berkeley, where we live on a clay bed, the soil will commonly be seen in this state when it has been seasonally wetted. If you take a screwdriver and dig down a few inches, you can often see that the soil is slightly damp. Pinch the soil and see if it sticks together. This helps determine if it’s damp. Dry clay will crumble apart. Think about the time of year. If this is August, you may expect things to be really dry and if you discover that things are damp, you may be able to discern the presence of a subterranean source of water.  

Don’t forget to consider plumbing leaks and excessive watering, although these will not have a uniform or homogenous effect. They’ll form a pattern that coincides with the activity or source. A leak around a sewer pipe that might be invisible can produce localized soil conditions such as these but will not do so a few yards away. When the entire crawlspace shows a similar condition, that ain’t no leak. 

All properties are in some sort of drainage plain and the condition of soil relates to those conditions so it’s worthwhile to think about the slope of the ground, the proximity to nearby creeks and the local geography while looking at the soil. 

Look at the color of the soil. Dry soils are lighter in color, as a rule and dark areas may be damp areas. A low-lying portion of the crawlspace that is also darker in color may be a place that is currently damp or at least damper than the rest.  

Both concrete and soil will exhibit an effect called efflorescence in which evaporative salts such as bromide or chloride are driven to the surface along with water as it escapes to the surface seeking equilibrium. The depots are usually white and on soil can leave small white dotted peaks on the surface. Sometimes the whitish depots are more widespread but this is less common. In any event, this is a sure sign of significant moisture in the crawlspace. 

Many houses will display this effect on the surface of concrete in the crawlspace. As water travels through the foundation, it will pull this salts to the surface leaving a fluffy crystalline formation much like sea foam on the surface of the concrete. This can be easily brushed away and is not harmful in and of itself but tells of water flow through these hard but porous structures. Over time, this can weaken concrete but in the overall scheme of things, it’s not significant. What is significant is that water facilitates soil migration and soil migration cracks, rotates and maligns foundations, SO, dry soils below your house are a darned good idea.  

If the soil appears to be puffy and soft on the surface, you may be looking at a very high clay content and possibly a clay with a high expansion potential. 

Clay soils push houses up when they get wet and then drop them back down as they dry. Keeping this kind of soil dry can be the difference between a house that is being slowly misshapen and one that stays in a nice rectilinear shape (assuming it started out that way). 

Also, as we’ve discussed on many other occasions, damp below the house can and does create damp inside the house (even when imperceptible) and this grows tiny forests of fungi (including mold) that can affect our health. Keeping low humidity levels is extremely important and damp crawlspaces are primary culprits in cases of mold. 

Looking at soil may relate to science but you don’t have to be a scientist to do it. I’ve found that taking the time to touch and look and ponder in the monastery of mice can teach quite a lot. That and the use of a really bright flashlight. By the way, don’t use one of those million candlepower torches. They’ll just blind you. A high quality flashlight of 25,000-50,000 candlepower is perfect. An automotive trouble-light works pretty well too but you may want a 100 watt bulb. Be sure to compare the appearance of soil across the entire tire crawlspace so that you can discern patterns of dampness.  

If you choose to explore your crawlspace, the only safety warning I’ll offer is a strong admonition to wear a respirator. Not a dust mask but a real respirator, like painters wear. 

Now, this is a fairly broad look at a complex science but I firmly believe that some simple triage can be incredibly informative and of enormous financial benefit. Just don’t expect your results to make the scientific journals. They may tell you that it’s good garage science but it won’t qualify as “ground-breaking” (Sorry, couldn’t help myself).