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Re: Deck Design for Wyoming

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Gail,
 
        Not considering local legal requirements whether or not you require frost depth protection for your deck will depend on the answers to three questions: how much uplift can you expect due to frost action?; will this uplift be cumulative over several frost cycles?; and what will be the effect on other parts of the structure?.
 
        Frost action uplift is caused by the volume change as water changes to ice.  If there is no water (you are on bedrock, free draining granular material or other material that will always remain dry) there is no need to have your foundation below "frost depth".
 
        I have heard of frost acting on posts (for fences or communication lines) a few millimeters per year until they actually "pop" out of the ground; but I have not experienced this extreme personally.  My only personal experience comes from the foundation for my own deck for my previous house (which I finally vacated this past week).  My deck was supported by the house (via joist hangers) on one side and on a row of five "piles" (drilled six feet into a salty clay soil with a post hole auger by me personally) on the opposite side.  One of these "piles" lifted about two inches during the first ten years or so but does not appear to have moved at all during the last twenty years.  In my case virtually no one noticed this uplift unless I pointed it out.
 
        I expect the real answer to your question is contained in the answer the question "What are the consequences of frost uplift on the constructed facility?"  An inch or two of uplift for your deck is probably irrelevant providing it goes back down again after the frost.  It is unlikely to even be noticed.  On the other hand, if you enclose it as a porch or sunroom an inch or two of uplift may damage the seals between porch and main residence which, I expect, would be very undesirable.  If you plan to barbeque on the deck and use a gas appliance you should consider flexibility of the utility connection.  If you intend to have a membrane surface which requires controlled drainage uplift could present a problem.
 
        I expect that there is probably no engineering reason to have frost protected foundations for your deck but there could be a need depending upon the specifics.  In Calgary, Alberta, Canada, I expect that less than one deck in ten (or even one in twenty-five) has a properly engineered frost protected foundation and almost all of them are perfectly satisfactory.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Saturday, June 18, 2005 8:18 PM
Subject: Deck Design for Wyoming

I am thinking of redoing the deck on my house in Jackson Hole. 
 
Is it customary for deck supports to be founded below frost depth? 
 
I don't think the supports for the current deck are, and there doesn't seem to have been any problems in the 15 years since the house was built.  The current deck is only about 10' x 10' though and  I am thinking of enlarging it to 15' x 30'. 
 
It is a non-elevated deck, no more than 30 inches off grade.
 
Gail Kelley