As you probably know I’m documenting the construction of a new house with the Total House Construction Series – Foundation to Finish. This series will follow the construction of a new home with lots of energy efficient design features. Yesterday the concrete footings were built that will eventually hold up the concrete foundation walls and the wood framed house.
In this part of the Country full depth basements are the standard when it comes to house foundations. Most houses are built on shallow foundations which are typically called spread footings. Concrete (spread) footings serve two main purposes. First they help “spread” out the load from the structure above. Secondly they create a level stable surface to begin building the foundation above on.
Depending on the type of soils that your new house will sit on the concrete footings will vary in width and depth. Every type of soil has an allowable load that it can take without settling and causing movement in your house. So the width of the footing is determined in order to reduce the pressure on the soil such that it’s below the safe load for the soil. Here’s an example to make it easier to understand. Let’s say a house has a total load of 100 pounds. And let’s say that house is sitting on a footing that is 1 foot wide. The total pressure on the soil would be 100 pounds per square foot. Now let’s take the same 100 pound house and put it on a footing that is 2 feet wide, now the pressure is 50 pounds per square foot. So by simply adjusting the width of the footing we can reduce the pressure exerted on the soil.
For this new house the footings will be 10 inches thick and 16 inches wide. That’s a very standard size spread footing around here with good soils. By the way, the soils here as you can see from the photos are very good. We have sand and gravel throughout this building lot site.
Here in New Hampshire we are required to place the bottom of the footing 4 feet below finished grade in order to protect the footing from frost. If the footings are too close to the surface then frost could potentially get under the footing and lift the house causing severe damage. This house has several footing elevations to accommodate different layouts within the house. For example, the main house has 8 foot high concrete walls. Those wall have almost 7 feet of cover below the finished grade. The garage will be built on 4 foot high frost walls so the footings will be 4 feet higher than the house walls. The basement of this house also has a walk out (walk outs are when the finished grade is at or below the slab grade). So those footings must be 4 feet below finished grade which means they will be 4 feet below the main house footings.
This probably all sounds a bit confusing but if you look at the photos you’ll notice what I’m talking about. Once the concrete walls are poured I’ll show you more photos and it will all start making sense.
The other issue to point out is the coordination with utilities. This house will have a mechanical room in the center of the basement. Therefore we’ll need to bring the electrical conduit and water line up from below the slab. In order to do that we placed sleeves in the concrete footings so we can slide the conduit and water line under the footings before we pour the slab.
So make sure you keep coming back to follow this series.
Previous posts from this series:
Total House Construction Series – Foundation to Finish
First Step In Building A New House – Blue Prints
Getting Bids When You Build A New House
New House Energy Efficient Details
House Building Lot
Ranch Style Home Floor Plan
Initial Site Work for House Building Lot