Building on North Florida Sand: Base Prep That Holds

The first slab I ever poured was a 10 by 12 shed pad in Mandarin, and I skipped the compactor on purpose. My reasoning, which I stand by as reasoning and not as engineering, was that the ground there is sand, sand is already loose, so what am I supposed to be compacting? I paid for that opinion fourteen months later, when one corner had dropped enough that the shed door would not latch.

This is the article I would hand to the version of me standing in that backyard with a rented mixer and a bad plan. I am a homeowner, not a contractor, and most of what follows I learned by doing it wrong.

Sand does not compact itself

North Florida sand is clean and poorly graded — the grains are mostly one size, with little fine material between them to lock things together. Pour a bucket out and it finds an angle and stops. That looks stable. It is not dense.

What densifies sand is vibration, not impact, and it works best slightly damp. That cost me a day: I rented a jumping jack rammer, the tall one that hops, because it looked more serious. Rammers are for cohesive soil — clay and silt. Sand wants a vibratory plate compactor, the flat one that shuffles along, at $70 to $110 per day, run over damp sand in lifts of 6 inches or less, four or five passes in overlapping lanes and then again crosswise.

Contractors specify compaction as a percentage of Proctor density — typically 95 percent — and hire a testing outfit to verify it. A homeowner cannot measure that. My rough proxies: my boot leaves no print, the surface does not ripple ahead of the plate, and a piece of #4 rebar pushed in by hand stops in the first few inches.

Water is the other half of the problem

Mandarin sits near the St. Johns, and in the wet season the water table can be within three to five feet of the surface, less after a week of afternoon storms. That matters in ways I had not considered:

You cannot compact soup. If the excavation is holding water, nothing you do with a plate compactor helps. Wait it out or pump it down.
Sand walls collapse. A vertical cut in sand does not stay vertical, especially wet. My forms needed twice the stakes I planned for, and I still lost a section of edge.
A storm undoes prep overnight. I compacted Thursday, got four inches of rain Friday, redid it Saturday. Now I would cover the base or pour the next morning.
Where the water goes afterward. Slope the finished slab 1/8 to 1/4 inch per foot away from the house and crown the subgrade the same way so nothing ponds underneath.

What goes between the sand and the concrete

Well-compacted native sand can serve as subgrade for a light pad. For anything a vehicle drives on, the common build here is 4 to 6 inches of compacted crushed base or limerock over the sand, which spreads wheel loads and gives you a stable surface to screed off — figure $1 to $3 per square foot. Separation fabric between sand and base is cheap insurance where it stays wet.

The good news about Jacksonville is what you do not need. There is essentially no frost, so nobody digs footings 30 inches down the way they do where I live now. A driveway at 4 inches, 3,500 to 4,000 psi handles cars; go to 5 or 6 inches with #4 rebar at 18 inches on center, held at mid-depth, wherever a truck, RV or dumpster will sit. A deeper lip around the edge, roughly 12 by 12 inches, keeps the perimeter from breaking off.

Permit requirements for driveways and the connection to the street are published by the City of Jacksonville, worth reading before you buy lumber. I did not pull one for a shed pad. I would for a driveway.

The step I would pay someone else for

Base prep is DIY-able. It is dull, it is a rental and a weekend, and taken seriously the result is fine. Placing and finishing is where I would spend money now. Once a truck starts discharging you have roughly 45 minutes to get that load down, screeded and floated, and Florida heat shrinks the window. A crew of four does what took me and two friends most of a day. When I priced my last project, having a Jacksonville concrete contractor pour while I did my own demo and hauling landed within a few hundred dollars of the all-DIY number, and I got a flat slab out of it. The wider case for this step is in compaction, the step that decides everything.

Frequently asked questions

Do I really need to compact sand before pouring? Yes. Loose sand can settle several percent of its depth under load, which on 8 inches of fill leaves a slab bridging a void. Compact it damp, in lifts of 6 inches or less, with a vibratory plate rather than a rammer.

How long should I wait after heavy rain to pour? Long enough that the base is damp rather than saturated and nothing is standing in the excavation — in summer, often one to two dry days. Concrete placed on a soaked base loses support as the water drains away later.

Is a vapor barrier needed under an outdoor slab? Under a driveway or shed pad, generally no, and plastic there can make finishing harder by trapping bleed water at the surface. Under a conditioned interior floor it is a different conversation.