This Old San Carlos House: The Foundation.

April 7, 2011


The Most Important Part.

Welcome to “This Old San Carlos House“, an occasional series on the White Oaks Blog that focuses on the different aspects of purchasing an older home — like many of those you'll find in San Carlos.   I figured I would start this series with arguably the most important – and likely the least understood- part of your old San Carlos house:  The foundation.  Why is it the most important?  Because the entire house sits on the foundation, so if you have big problems with the foundation you'll likely see it throughout the house.   And why is it the least understood?  Because it's the least visible part of the house.   We spend time in our homes or out in our yards, but very few people ever climb under their own house to see what's going on.

Most homes in San Carlos are built on a raised, concrete perimeter foundation, which is also referred to a spread footer foundation.   Basically, as the name implies, a trench is dug around the exterior footprint of the home.  A large concrete footer is then poured creating a perimeter that the house is then built upon.  The diagram below shows the key components of a raised perimeter foundation:

- Drawing courtesy of


There is also a foundation construction technique known as “post and beam” that some houses in San Carlos do utilize, but for the purpose of this discussion we'll focus solely on concrete perimeter since it applies to most homes in San Carlos.

Don't Crack over Cracks.

Reading a property inspection report on a home that you're thinking of buying is not for the faint of heart.  It's not easy to read about everything that's wrong with what may be your very next home.   And if there's one deficiency that's called out in these reports that scares the daylights out potential buyers more than any other, it's this:  Foundation cracks.

Whether it's because of fear of an incredibly expensive repair, or the lack of understanding about foundation cracks, it's the one fault that sends the most buyers scurrying to the next property.   But it's important to know that foundation cracks are very common with homes in this area — especially older homes. And it's also important to know that most cracks can be repaired relatively easily.   The fewer the number of cracks and the smaller the width, the better.  The larger the crack, the more significant the condition contributing to the crack probably is.

What causes foundation cracks?  According to foundation specialists, there are two main causes of foundation cracks in homes in this area:  Seismic events, and soil settlement.    The former is to be expected since we live in earthquake country.    Earthquakes cause ground movement, and ground movement is not good for a foundation.

Soil settlement is also common in San Carlos.  Much of the town sits on clay-based soil, which expands and contracts significantly as the moisture level in the soil changes.  This constant swelling and contracting of the ground subjects your old foundation to uneven stresses, and voila…it eventually pops a crack.  The more moisture the soil has, the more expansion the foundation will experience.   Which brings us to the next topic on foundation health….

Drier is Better.

It's ironic that while water is essential to every form of life, it's easily the biggest enemy of a home.   And that holds especially true with foundations.   In a perfect world, water from outside is channeled away from your home, and the underside (aka crawl space) of your home is dry as a bone.  That minimizes the effects of soil expansion,  and keeps your foundation healthy.   But we don't live in a perfect world, and one thing you'll consistently see in property inspections is that the dirt under many houses is wet.  Sometimes downright muddy — and that's not good.

Even if the crawl space is dry when it is inspected, there are several tell-tale conditions that tip the inspector that water is getting under the house.  You may have seen these terms in an inspection:

  • Efflorescence:  Efflorescence is caused when soluble salts and minerals come to the surface of concrete and mortars after being exposed to moisture.  So if there's standing water or excessive moisture in the foundation area, it will leave a chalky calling card — even if the underside of the home is dry.
  • Spalling.  Spalling is the condition where prolonged exposure to moisture causes the concrete in the foundation to actually start peeling or crumbling, which means the foundation in that area is starting to deteriorate.  Not a good thing.

In my book, moisture under the home is one of the biggest issues you should be looking at before deciding to purchase a home.  Consequently, I'll dedicate an entire post in this series to drainage.

Bolt it Down.

Over the past 50 years, the building code for homes has constantly evolved as we understand more about modern construction materials and building techniques.  One area that this holds especially true is in the seismic worthiness of a home.   The act of physically bolting the home to the foundation has been a common practice ever since homes were built in San Carlos.  But the building code has changed over the years, and what was acceptable in the 40's is no longer deemed sufficient by today's standards.  Generally, the code now recommends larger bolts and more of them.

So you'll often see in a comment in the inspection report that say something to the effect of “The home is bolted to the foundation in a manner that was acceptable at the time of construction, but is outdated by today's standards.”    A foundation expert can easily bring the seismic worthiness of your home up to current standards.    There are even simple methods that increase the rigidity and shear strength of the cripple walls which helps the home withstand earthquakes better.

The Bottom Line.

The foundation is one of the most important parts of your old San Carlos home — your home is only as good as the foundation it rests on.  So it's important to pay particular attention to the foundation and crawl space conditions in the property inspection reports.  It's highly recommended that you get your own inspections done when you're purchasing a home, and consult a foundation expert if you have any concerns.   You'll find that most any anomaly with a foundation can be fixed — it just costs some money.

And don't freak out you see the words “foundation crack” in the inspection.  Smaller cracks are common in this area and can be easily addressed.  Bigger cracks may be indicative of a more significant issue, but again, be sure to consult with a foundation expert.  It may not be as bad as you think.

Moisture is the enemy of all homes, especially when water gets under the house.  Prolonged exposure to moisture can wreak havoc on a foundation — so be sure to pay close attention to that aspect of the property inspection.  We'll discuss drainage in more detail in this series.

Finally, don't forget that you're OLD San Carlos home was built in an era when the building code was less stringent than it is today.  But it's relatively straightforward to bring your foundation's seismic worthiness up to today's standards.

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  1. Sue on April 8, 2011 at 4:31 pm

    A nice post! I have moisture issue in my crawl space. I discovered some standing water in the lowest point after a heavy rain. A plumber told me that some part of Belmont, San carlos and redwood city has high water table. Have you heard anything about that? I had some contractors come over to my house and they suggest I put a vapor barrier and a sump pump in my crawl space. Do you think that would help?

  2. iamnoone on April 8, 2011 at 9:08 pm

    You should consider a french drain around your foundation first. I keeps the water from getting into the crawlspace in the first place, and might be cheaper.

  3. Chuck Gillooley on April 17, 2011 at 2:28 pm


    Thanks for your comment. Yes, I have heard that explanation for water appearing in the crawl spaces of homes that are in the “flats” of San Carlos and Belmont. I know in San Carlos there are a series of underground streams the flow to the bay, and sometimes that water can find its way to the surface underneath your home.

    Depending on whether you live in the hills or the flats, what soil your home sits on, and how well (or not) that your home is designed with drainage in mind, there could be a variety of factors contributing to the moisture under your home. And like the previous commenter said, there are remedies such as a french drain help remedy the situation.

    The next edition of “This Old San Carlos House” next week will specifically address drainage — stay tuned!

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