In Santa Clara County, as in much of California, we have adobe clay soil and it’s expansive.  That is, when the dirt gets wet, it expands, and when it dries out, it contracts. Hence it’s sometimes referred to as “shrink-swell” soils.  (Every state in the union has areas with this problem – a color-coded map on shows areas with more and less expansive soils.)

Why is expansive soil an issue for homeowners and would-be homeowners in Silicon Valley?

The trouble is that the expanding and contracting soil is far stronger than concrete and the foundations upon which a home sits. A well written and illustrated six page paper can be found online explaining the mechanics involved for those interested in more detail on the hows and whys of expansive soils. (It states that the ground can life as much as 5,500 pounds per square inch!)

What I’d like to focus on here is mitigating the risks and preventing the problems associated with expansive soils.

The trouble is not so much that the soil is wet or dry.  The problem is in the back and forth, the movement. When the soil is kept at an even amount of moisture, it does not expand and contract.

Obviously, rain is seasonal and we cannot control all moisture on or near the house. We can, though, work to move water away from the house and away from the foundation.

Keep rain away from foundations on adobe clay soil!

Winter storms can bring an enormous amount of water onto a home’s roof, and when it channels down gutters and downspouts, there can be a large amount of water exiting in just a few places.  Where does that water go?


downspout no extender near foundation

In 95% of the homes I see in San Jose, Los Gatos and Saratoga, the water simply dumps out at the base of the downspout – right up against the home.  This is typical, but is unwise, as it causes the soils to become excessively damp in one spot.

Ideally, you want to move the water away from the house and foundation, at least by 6 feet or so.  Solutions can be expensive (underground and run all the way to the street) or can be very inexpensive (a plastic extender added to the end of the extra long downspout) as in the photo below.


Downspout with plastic extender on Springer

Leaks that are untended can wreck havoc on your foundation. This photo below displays a foundation crack that includes efflorescence (the white powdery stuff) and rusty discoloration, which is most likely a bad sign for the rebar inside the foundation. The source of the trouble? Probably a combination of a leaky hose bib and water that got under the house from a kitchen faucet leak or from grading issues and heavy rains.




Another main culprit is grading. Too often, a walk around the house in question reveals that the land is sloped toward the house rather than away from it.  When it rains, or when sprinklers run, the water in the yard will come careening toward the house – exactly what you do not want to happen.

Sometimes, it’s inevitable, as in the case with hillside construction. What to do? Specialists find a way to grade the soil closest to the home away from it and to intercept the hill-driven water before it gets close to the home.  Imagine making a tiny valley between the home and the hill, and the small culvert catching the water so it can be diverted. That’s essentially how it works.

High water table areas will require the help of specialists too, as these can cause water to bubble up under or near the home and again create havoc on the home’s stability.

If you’re a current homeowner or home seller in the Bay Area, have a walk around your property and see what’s happening with your downspouts.  In some (few) cases, the downspout may empty onto concrete that is sloped away from the structure, and it may be OK as is.  In many cases, though, that downspout just terminates at the base of the home and empties into soil or landscaping that’s waiting to absorb the moisture and expand.  A home inspector will “call” this.  Are your floors inside out of level? Are the hardwood floors cupped? These may be the result of water not being diverted away from the home – you’ll want to address this immediately, before it worsens.  Having this work done will also give homebuyers confidence that your property has been well maintained when and if you do go to sell it.

If you’re a current homebuyer in the area, watch for this detail. Does the home have downspouts and gutters? If not, you’ll want to add them and then the extenders. If the downspouts are like most I’ve seen and stop at the base of the home, pay particular attention to the concrete nearby, any exposed foundation (watching for cracks), floors inside (looking for “out of level” and if hardwood, any cupping).  Hopefully the floors are level and undamaged.  If undulating, though, you may need to speak with a structural engineer about solutions.





  • Mary Pope-Handy

    Silicon Valley Realtor, selling homes in Los Gatos, Saratoga, San Jose, Silicon Valley, and nearby since 1993. Prolific blogger with a network of sites.

    View all posts