Mark
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Hi, I'm writing with another question about my screened in porch. In New England it is recommended that footings be 4' deep to get below the frost line. I have noticed that, normally, frost doesn't go below 6" on the lawn, but goes deeper in ground that cars are traveling over (the frost is pushed deeper). Do you think it would be necessary to place the footings at a 4' depth since there will be no traffic in this area? The porch will be 10' x 32'. Thanks.
Absolutely. The 4 foot depth would probably be a required part of your design for a building permit.
The 4 foot depth is that depth to which historical records show that frost (frozen ground) can penetrate. In some parts of the country it's less, in some places it's more. Some folks try to get away with not going the distance and installing shallower footings. Luck may be with them and they'll never have a problem. But more often than not, frost will get under the shallow footings and lift...and then drop. And the structure sitting on those footings will move, also, and can sustain damage. If that structure is connected to another, both are damaged.
The question is two-fold. Will the local authorites allow you to show less in your design plans? Will luck be with you if you go shallow?
[This message has been edited by treebeard (edited July 10, 2003).]
It is not only frost getting below a footing that can lift a foundation. There is an ice lens formed at about 8-18" depth that is responsible for more damage. It can lock itself onto the piling or pier and as it expands, it will jack the concrete right up out of the ground if there is not enough resisting loads in the weight of the porch and the friction of the pier to soil below the lens.
