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RE: Frost footings - whats the big deal?

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The City of Chicago (frost depth of 42”) allows minor/secondary one story structures (garages, decks, etc.) to be supported on 12-18”

Deep foundations provided the foundation and superstructure is “engineered” to resist any frost heave movements.


Nice provision, especially since in Chicago, you can’t even plant a geranium (or so it seems) without getting City approval on the excavation.




David L. Fisher SE PE

Senior Director


Cape Cod Grand Cayman Holdings Ltd. - Cayman

Fisher+Partners Structural Engineers Ltd. - Cayman

372 West Ontario Chicago 60610

75 Fort Street Georgetown Grand Cayman BWI

319 A Street Boston 02210




312.573.1726 facsimile

312.622.0409 mobile





From: Jim Wilson [mailto:wilsonengineers(--nospam--at)]
Sent: Monday, November 05, 2007 9:36 AM
To: seaint(--nospam--at)
Subject: Frost footings - whats the big deal?


Frost footing issues are a real hot-button topic with local inspectors.  But in some cases I think they are going overboard in their interpretation and critical review.  Are there any legitimate references or theories on how or when frost depth requirements can perhaps be relaxed?


For example,

-Are frost footings appropriate when adding a small addition to an existing structure not on frost footings?  If a house has worked for 100 years with 18" deep footings, won't adding 48" deep footings next to them cause potential heave differentials?

-If an existing garage slab opening is not on frost footings because the rest of the foundation has 4' of dirt piled around it, and the slab has no cracks after 40 years, isn't it acceptable to leave it alone without underpinning this small area?

-If deck footings go down 24" instead of 48", is that acceptable, even if it is not ideal?


These are similar to situations I have seen where inspectors are drawing a line in the sand.  The costs they are subjecting homeowners to to make repairs seems to exceed the risk of some possibly slight foundation movement.


Local frost depth in northeast PA is 42" or 48".  Ground is often very sandy and rocky with positive drainage. 


Jim Wilson, PE

Stroudsburg, PA