How Deep Should Postholes Be Set?

How Deep Should Postholes Be Set?Anyone who has ever entertained the idea of installing a fence on their property knows that fencing contractors need to dig postholes to set the fence in securely.

But most fencing contractors disagree on how deep those holes should be. Although you might find general information on the subject, an online search won’t tell you how deep postholes should be set in Alberta, Canada, specifically.

This is because the ground conditions have a great influence on the ideal posthole depth of any plot of land. And different plots of land, even ones nearby one another, can have different soil conditions and experience different weather on a regular basis.

We use a formula to determine how deep our postholes need to be. Some of the factors we use include the style of the fence, terrain, wind load, and digging conditions. In general, the height of the fence determines how deep the postholes must be: one-third below, and two-thirds above.

In industry standards, this means, for instance, that a six-foot-tall wood fence would have nine-foot-long posts, and three of those feet would be underground. But weather and soil conditions can change from one plot of land to another, even when they’re right next to each other.

In First Class Fencing’s standards, this means, a six-foot tall wood fence would have ten-foot-long posts, and three and a half of those feet would be underground. This is the only way we can ensure the frost line is broken and confidently provide a 5-year warranty on our wood products.

Weather and Soil Conditions are Key

So how can two plots of land right next to one another can have vastly different ground and weather conditions? Here’s a simple example: If they’re on a slope.

But there are many other ways weather can affect posthole depth. For instance, it’s vitally important that the posts go deeper than the frost zone does during the winter. In Alberta, Canada, that can mean going as much as four feet deep.

Many environmental factors also contribute to frost depth. The frequency of snow plowing or shoveling is one factor that pushes frost deeper. This is why paved streets usually have a much deeper frost zone than lawns, yards, and fields.

At the same time, nobody wants to have the water table push their fence posts up, so there is no sense in simply extending the posthole depth indefinitely. There is a sweet spot for posthole depth in Alberta, Canada, and we have mastered the formula for it.

Concrete Quality and Installation Matters

Concrete quality and installation have a powerful effect on fence stability. Installing a fence with the proper posthole depth, filling the hole with dry concrete, and then spraying water in the hole is not a strong long-term construction solution.

At First Class Fencing, we carefully measure posthole depth and mix the concrete outside of the hole before pouring it in. This gives us a smooth, consistent result that benefits from our 5-year warranty.

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