Expert: Dan Griffin - 9/22/2009

Question
My husband is pouring footers for a concrete slab that we will eventually turn into a garage.  What should the dimensions be for the actual footers?  (e.g. to ensure that it's below the frost line and wide enough to support the garage walls?)  We want it to be sturdy, but not waste money on unnecessary concrete...

Answer
Erin, I wish this was a simple question.

Footings must hold up the weight imposed on them.  The footing size is subject to the load bearing capacity of the soil at your location.  It is then subject to the total load imposed (how much does your building weigh, which is subject to what it is made of).  Unless you are in a very strange soil, I'm sure normal widths and loads will be adequate.  I would feel fairly safe suggesting 8" deep x 16" wide with 2 #5 rebars properly tied and supported.

Frost depth varies all over the world.  It involves typical depth that moisture freezes in the ground.  Florida will be much different than NY.  Here is one that will get you close, but as you see, there is quite a range in NY state:
http://www.usace.army.mil/usace-docs/eng-manuals/em1110-1-1905/c-2.pdf#search='f...

The ultimate answer and the one that really counts will be your local building inspector.  If you are not subject to local inspection, your county extension may have good information for your location.

There is a good article in a recent issue of  Journal of Light Construction about shallow, above frost line, footings:
http://www.jlconline.com/cgi-bin/jlconline.storefront/4ab9ffbc0826488b27170a3210...
and
http://www.jlconline.com/cgi-bin/jlconline.storefront/4ab9ffbc0826488b27170a3210...