Fence glossary · Installation
Frost line
The maximum depth to which ground can be expected to freeze at a given location. Fence post footings are set below it so that water freezing in the surrounding soil cannot lift the post. In southeastern Pennsylvania the depth enforced is set by the municipality, not statewide.
The mechanism is simple and unforgiving. Water held in soil expands as it freezes, and if a footing sits inside the freezing zone the expanding ground grips its sides and jacks it upward. On thaw the post does not settle all the way back. Repeat that through a winter's worth of cycles and the post walks out of the ground, taking the fence line with it. That is frost heave, and it is the single most common structural failure we see on fences across Chester and Delaware County — almost always on a fence whose posts were set too shallow to begin with.
The soil here makes it worse. The clay-heavy ground draining to the Brandywine and Red Clay watersheds holds water rather than shedding it, so there is more water available to freeze against a footing than there would be in sandy soil. Heave shows up first in the predictable places: the low corner of a yard, the downhill end of a swale, the run beside a downspout, the base of a slope where surface water collects. In known wet ground it is worth going deeper than the minimum and belling the bottom of the hole so the footing has a shoulder to resist against, because neither is possible after the concrete has cured.
The depth itself is a local number. Pennsylvania municipalities adopt and amend the Uniform Construction Code on their own schedule, and the footing depth a code office enforces can differ across a township line. We confirm it with the governing municipality for the parcel rather than working from a regional rule of thumb, and we set posts on one day and hang panels on the next so nothing is loaded onto green concrete.
