How do I find my frost line?

A frost line is the depth in which water within soil is expected to freeze in cold temperatures. Finding your frost line is usually best investigated locally through your municipality's building or zoning departments. Contact your city or county offices. For quick reference, you can review general frost line depths in this map on Decks.com. You can also find the most recently reported current frost line on the National Weather Service (this won't tell you what the maximum depth is, but it will give you an idea what has been reported within the last few days, so if you are building in winter, you will be able to find out how deep the ground has been frozen). 

In general, you will want to set your post depth to the maximum frost line for your area. However, if you have a very deep frostline, you may not be able to get to the line with the fence post because it may not be long enough. In this case, you may want to consider extending your footer with a steel post insert, digging a deeper hole and embedding the Trex post part of the way into the concrete while it is thick but not yet cured, or using a longer Trex post (so, for example, use a 12' Trex post for an 6' tall fence if your frost line is 48", then cut off the excess material). 

Consult with local fence contractors and authorities to determine how to handle setting posts to account for your frost line.