What Is Under Your Yard, and Why Footings Get Dug

In most of the country, deck footing depth has a simple answer: below the frost line. Florida does not have one, which removes the rule and exposes the real question underneath it. What is this footing going to sit on?

The county’s own list

Alachua County’s environmental protection department publishes a fact sheet on sinkholes, and the most useful part of it for anyone building a deck is not about sinkholes at all. It lists common causes of ground subsidence around houses that have nothing to do with karst:

• Decay of land-clearing debris buried when a structure was built
• Decay of tree stumps and large roots
• Leaking water pipes and fittings
• Cracked and leaking swimming pools
• Cracked stormwater piping carrying soil away with the runoff
• Poor compaction of soil around utility lines
• Runoff from roofs, gutters, and pavement

Read that list as a deck builder and it is a list of things a post hole can land on. A footing bearing on a buried stump is a footing that will drop when the stump rots. A footing on loose backfill over a utility trench is a footing that keeps settling as the backfill consolidates.

Which is why the hole gets dug

There is no substitute for excavating until the hole reaches material that was never disturbed. On an established lot with mature planting that is usually straightforward. On a subdivision lot cleared within the last decade it can vary considerably from one corner of the deck to the other.

A contractor who states a uniform footing depth before digging anything is describing an assumption. One who explains that holes go down until they hit undisturbed bearing, and that a post landing over an old stump hole either goes deeper or moves, is describing a method.

The karst context

The wider geology is worth knowing even though it rarely changes a deck. The same county fact sheet records that sinkholes form most commonly in western and central Alachua County, in areas where limestone is exposed or thinly covered by less than 25 feet of permeable sand, and that they are less common where clay-containing materials are over 100 feet thick, such as in eastern Alachua County.

A deck spreads a light load across many footings, so this is background rather than a hazard. What it does justify is paying attention. The county’s own list of sinkhole warning signs includes slanting fence posts, doors and windows that stop closing properly, and rainwater pooling where it has not pooled before. If something like that has started happening on your property, that is worth a professional look independent of any deck.

The cheap thing you can do yourself

Watch where water runs and pools in the yard during a heavy summer storm before the posts go in. Ground that holds water is ground where post bases sit damp for a large part of the year, and correcting the grade beforehand costs a fraction of what it costs afterwards.

The framing and footings guide covers how the load travels from the boards down to the ground.