Ask the Builder

Foundation footer: A critical element on which your building's success will rest

This concrete foundation should last for hundreds of years, at least.
This concrete foundation should last for hundreds of years, at least. (By Tim Carter)
  Enlarge Photo    

Network News

X Profile
View More Activity
By Tim Carter
Saturday, September 25, 2010

DEAR TIM: know very little about home construction. In particular, one of the things I don't fully understand is the foundation footer. I'm also stumped about piers. What's the real purpose of these things? Can you install a foundation without concrete footings? What tips can you share about pouring concrete footings? I'm sure you've sloshed around in wet concrete down in more excavations than you care to remember. -- Brad T., Topeka, Kan.

DEAR BRAD: You bet I can tell stories about concrete footers. Several jobs come to mind, but perhaps the one that stands out the most is the time I had to pour a massive footer for a commercial building with just one helper. Fortunately for me, the job site allowed the concrete trucks access to all sides of the excavation hole, the weather was cool, and my helper that day was lucid. You can't afford to let mistakes happen when you have expensive concrete tumbling in the drums of several 10-yard ready-mix concrete trucks!

A foundation footer is perhaps one of the most critical aspects of the house. It's almost always the first building element that contacts the soil that the house rests upon. In rare cases, a footer may be supported by piers placed in the soil underneath.

A pier is a narrow vertical structural element made from wood, steel or concrete. The best analogy I can come up with is a table leg. Piers are often used to support a footer that's resting on poor soil. Piers extend down through the bad soil until they hit bedrock or great soil, or deep enough to create enough friction to adequately support the building's foundation.

The primary purpose of the footer is to spread out the weight of the structure across a larger footprint than the foundation would if it were in direct contact with the earth. Frequently, a concrete footer is 20, 24 or even 30 inches wide and at least 8 inches thick. More often than not, you'll see them 10 inches thick. The average foundation wall is usually only 8 inches wide.

If you were to calculate the total weight of a completed house plus everything in it, you'd probably be stunned by the total tonnage. Just the interior furnishings and possessions in an average home can weigh tens of thousands of pounds. Add this to the many tons of weight of the building materials and you end up with enormous concentrated loads. Without a footer under a foundation wall, the wall could actually start to slice into soil much like a knife cuts into a stick of butter.

But a footer, because it's wider than the foundation wall, displaces this weight over a much larger area. A footer also creates a nice, level surface for the foundation contractor to set his forms.

Some precast concrete foundation systems are routinely installed without a poured concrete footer. They rest on compacted gravel that acts as a footer. Be sure that this type of footer is approved for your area and that the gravel used is crushed and angular. Rounded gravel might not be a good choice.

Reinforcing steel is a must in concrete footings. Concrete has great compressive strength but little strength when subjected to tension. If the ground moves under the footing, concrete can come under tension, crack and displace. The presence of steel dramatically increases the tensile strength of the footer. When overlapping the steel bars, make sure they overlap at least 18 to 24 inches. Structural engineers will specify the best location of the steel inside the concrete footer.

I like to place a keyway in my foundation footers. This is a channel or groove that's created after the concrete is smoothed over in the forms. You can slide a two-by-four in the concrete to create this feature. When you then pour the concrete foundation, that concrete flows into the groove in the footer. This mechanical connection between the two elements helps keep the foundation wall from sliding across the footer if the foundation wall is subjected to horizontal pressure.

Take the time to ensure the footer is as level as possible. This will make the job of setting the foundation forms much easier. It's also critical that the footer be square so the foundation is centered on the footer.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site, http://www.askthebuilder.com.


© 2010 The Washington Post Company

Network News

X My Profile