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Post 55A – Building Foundations – The Footer.

When a house or other small building is built the first thing that must be built is the foundation and the first part of the foundation is the footer.   This is the very bottom part of the building and is the part of the structure everything else rests upon.

There are several critical things about the footer:  Depth, Width, and Thickness.  These are mandated by local building codes, but even if you are in an area where there is little or no code enforcement these are still important considerations.  On this site, I am not going to provide numbers because I have no way of knowing those for you at your location and your situation.  What I will do is talk about why each of the dimensions are important.

The footer in normal situations in conventional buildings here in America is poured concrete.  The goal of the poured concrete is to provide a solid block of material to support the weight of the building.   However, if you think about it, the soil below the footer actually supports everything.  That explains the reason for the width of the footer,  it spreads the load over a greater area of the soil.  This means less force per square inch on the soil and less soil compaction once the building is constructed.

The thickness of the footer is simply to provide enough strength within the footer itself so the footer does not break.  Normally the footer also contains some steel rebar to assist in “hanging together”.

Depending upon the average temperature of an area, there is a depth defined as the frost line.  Below this depth the soil cannot be expected to ever freeze, even during the coldest winter on record.  Depending upon the type of soil, when the water in the soil freezes it can form pockets that will draw in more water eventually forming “ice lenses”.  These can cause the soil he heave upward.  This is bad news if it is below the footer because doing so can warp the building and crack walls.  This is especially the case if the building has brick walls. The building code will require that the footer be below the frost line for this reason.  In Indiana where I grew up the frost line depth was 36 inches, here where I now live in Alabama, the frost line depth is only 6 inches.

So far we have an idea why the footer must be wide & thick and why it must be below the frost line.  Now we still have a few things to think about.   How are we going to make the footer and the rest of the foundation square and level while we keep it below he frost line?   This is especially a problem if the land we are going to build on is sloping.

Gary.

A very good website about concrete footers:

http://www.concretenetwork.com/concrete/footing_fundamentals/introduction.htm

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