Under and Through the Slab

The Foundations Series, Lesson 2 · Taught by Professor Elias Walls · Researched and reviewed by Grant Doutt, Certified Master Inspector®

🎓 The Professor's Quick Answer

Under a Central Florida slab are layers you'll never see again: compacted sand, buried plumbing, and a 6-mil plastic vapor retarder. On many homes there's a fourth: termite-treated soil. Florida also allows a different route, a borate spray on the bottom 2 feet of the framing and the inside of the block walls, applied after the house is framed instead of under the slab. Each has its own rule, and most leave a paper trail you can ask for.

Plumbing is normally the only system that passes all the way through the slab, and the code is strict about how those pipes are sleeved and sealed. After the pour, concrete doesn't dry so much as cure, a chemical reaction that needs moisture and time. With good curing, typical concrete reaches about 70% of its specified strength in roughly 7 days. Block walls often start going up well before that.

This lesson covers how the pad is built and tested, how the plumbing goes in, how the two main termite options work, what the plastic sheet is for, why pipes are the only things that pass through, how concrete cures, and how fast the walls go up.

🏫 Welcome Back to the Knowledge Center

Last class we built a slab from the footer up and stood back to admire it on pour day. I also left you a sandwich diagram on the board: subgrade, fill, plumbing, termite treatment, vapor retarder, steel, concrete. I promised we'd come back and take a bite out of each layer.

Today we keep that promise. We go under the slab, then through it, then forward to the days after the trucks leave, when the concrete is still hardening and the block crew is already unloading pallets. Everything in today's lesson gets buried. That's precisely why it's worth knowing.

🏗️ The Pad: Building Ground You Can Trust

First the builder makes a pad: the flat, firm ground the house will sit on. Around Clermont, Minneola, and Groveland, that usually means scraping, filling, and compacting. The 2023 Florida Building Code, Residential, sets the rules:

• Strip it clean. Vegetation, topsoil, and foreign material are removed from the area inside the foundation walls. The termite section of the code adds stumps, dead roots, cardboard, and trash, within 1 foot of the foundation too.
• Clean fill only. Fill brought in must be free of vegetation and foreign material, and compacted for uniform support of the slab.
• Limits on depth. Unless the building official approves more, fill under a slab stops at 24 inches of clean sand or gravel, or 8 inches of earth.
• The gravel question. The code calls for a 4-inch base course of sand, gravel, or crushed stone where a slab is below grade, but not where the slab sits on well-drained sand or sand-gravel soils. That's why, on much of the Lake Wales Ridge, you'll see the slab placed on compacted sand rather than a layer of stone.

Compaction is where the work hides. Loose sand settles under load, so the crew compacts fill in layers, called lifts, and on many jobs a testing lab checks the result. Polk County's residential inspection checklist, for example, looks for compacted soil tested to 95% of the laboratory maximum density (the modified Proctor test, ASTM D1557), in lifts no thicker than 12 inches. Neighboring Citrus County's 2024 inspection guide uses the same 95% benchmark and wants the compaction report on site for the slab inspection.

Your county and your plans may word it differently, and a soil engineer's report governs. The idea doesn't change: no slab should go on sand that hasn't been packed and, where required, proven.

🚰 Plumbing Goes In First

Next comes the plumber. In Lake County's published sequence, rough plumbing is the first inspection, and nothing proceeds until it passes. Drain lines are laid in sloped trenches, with stubs sticking up wherever a toilet, tub, shower, or sink will be. Some builders run water lines under the slab too; many run them overhead instead. The code is particular about the trench:

• Full support. Buried pipe is supported along its entire length, never propped on blocks.
• Clean backfill. Free of debris and rocks, placed in 6-inch layers, and tamped until 12 inches covers the pipe.
• Stay out of the footer's way. A trench alongside a footing can't cut into the soil below a line sloping down at 45 degrees from the footing's bottom edge.
• Prove it's tight. Drains are filled with water and held at least 15 minutes, with no section tested under less than a 5-foot head of water. Water lines get their own pressure test.

Where a tub drain needs open space in the slab, builders use a permanent plastic or metal box, not a wooden form. Wooden boxes have to come out on every home, and on soil-treated homes the code requires the permanent form outright. Buried wood is an invitation, and you're about to meet the guests.

🐜 Termite Protection: Soil or Wood

Florida takes termites seriously, and for good reason. A University of Florida study published in 2026 found that two invasive species, the Formosan subterranean termite and the Asian subterranean termite, keep spreading beyond South Florida. The Formosan termite is now established in most coastal counties and most large urban centers in the state.

So the code requires termite protection on every new home. The 2023 Florida Building Code lists the choices: a soil-applied termiticide, a baiting system, a termiticide applied to wood, or another approved method labeled for new construction. A licensed pest control company applies it, following the product label. Bait stations go in around the outside after final grading, so this lesson focuses on the two that happen during construction.

Option 1: Treated Soil Under the Slab

Every soil rule in the code begins with "if soil treatment is used." Here's what they require:

• Treat last, not first. The soil inside the foundation is treated only after all excavation, backfilling, and compaction are done.
• Disturb it, re-treat it. Any treated soil that gets dug up or disturbed afterward must be treated again.
• Cover it fast. Treated soil is protected with at least a 6-mil vapor retarder so rain doesn't dilute it. If it rains before the plastic goes down, the code requires re-treatment.
• Step carefully. Work done between treatment and pour, including setting steel, must avoid penetrating the treated soil.
• Finish outside. Soil within 1 foot of the house under exterior concrete gets treated too, and a final vertical barrier goes around the outside after construction and landscaping are done.

Option 2: Borate on the Walls

The other common choice is a borate wood treatment, such as Bora-Care or Tim-Bor. Nothing goes in the dirt. Bora-Care's label says plainly that it is not a soil termiticide. Instead, the pest control company comes back once the house is framed and dried in, and sprays the bottom of the walls. On a slab home, Bora-Care's label calls for:

• The bottom 2 feet. Every bottom plate, plus the bottom 2 feet of the studs, wood or metal, on all walls that sit on the slab.
• The block walls, too. On a concrete-block house, a continuous 2-foot band on the inside face of every exterior block wall, measured up from the slab. In a typical Central Florida home, that means the spray goes on the block itself, plus the bottom of the framed interior walls.
• Pipes and tub boxes. Wood around plumbing, conduit, and duct penetrations gets a 2-foot-high barrier, and the slab is sprayed at least 6 inches out from each pipe and 1 foot out from each tub box.
• A strip of slab. The concrete is sprayed 2 to 8 inches out from the walls, and along any visible cracks.

When a wood treatment is used, the code says the soil-treatment rules don't apply. The treatment follows the label and must be finished before final building approval. Because borate solution is clear, many companies add a green or blue dye, so a colored band along the bottom of the walls before drywall is a good sign it was done.

Each option has trade-offs. Treated soil puts a barrier in the ground that termites meet before they reach the house, but rain on the pad or digging afterward means re-treating. Borate stays put on dry wood and keeps chemicals out of the soil, but termites have to feed on treated wood to be affected. University of Florida researchers also warn that framing moved or added after the spray isn't protected unless the builder calls the company back.

What about groundwater? You may hear that builders switched to borates because soil treatments were leaching into groundwater. There's real history behind that worry. Chlordane, the long-lasting termiticide used under homes for decades, lost its last U.S. uses in 1988 and still turns up in some soil and groundwater today. Today's soil termiticides break down much faster, and labels such as Termidor's restrict treatment near wells and cisterns. Research on a modern product under simulated slabs found most of it stayed close to where it was applied. Florida's code treats soil, bait, and wood treatments as equal options and doesn't favor one for groundwater. Builders choose for many reasons, including cost and scheduling.

Is borate allowed where you live? Yes. It's written into the statewide Florida Building Code that Lake, Polk, Orange, and Citrus counties all enforce, so it isn't a special local exception.

The paper trail. Whichever method is used, each treatment gets a certificate on the job-site board listing the product, applicator, date, and area treated. When protection is complete, the licensed pest control company issues a Certificate of Compliance to the building department. Inside the finished home, a permanent sign near the water heater or electric panel names the provider and notes the need for reinspection and contract renewal.

Our pre-drywall lesson picks up the termite story from there, once the walls are framed.

🛡️ The Vapor Retarder: Plastic With a Purpose

Now the most underrated layer: a sheet of plastic. The code calls for 6-mil polyethylene (or another approved vapor retarder), joints lapped at least 6 inches, between the slab and the ground beneath it. It has two jobs:

1. Slow the moisture. Concrete is porous. Without a retarder, ground moisture can wick up into the slab and your flooring.
2. Shield treated soil. On soil-treated homes, the sheet also keeps rain off the termiticide until the pour.

It's required whichever termite option is used. The code skips it in garages, carports, small unheated storage rooms, driveways, walks, and patios. Under living space it's required unless the building official approves otherwise.

What can go wrong. Tears from boots and wheelbarrows, unlapped seams, and gaps around pipes all let moisture through. Some plans call for heavier sheets made to ASTM E1745, which starts at 10 mils. Either way, the slab inspection is the last time anyone sees it.

🔌 Why Plumbing Is the Only Thing That Passes Through

Here's a question for the class: what comes up through the floor of your house?

Your walls stand on the slab, your wires come down from the attic, your ductwork lives overhead. In most Central Florida homes, the only thing that passes all the way through the slab is plumbing. Now and then an electrical conduit runs under it, to a kitchen island for example, and that gets its own underground inspection.

Builders keep it that way on purpose. Every penetration is a weak spot in the concrete, the vapor retarder, and any treated soil, which is also why the borate label calls for extra spray around every pipe. So the code is careful:

• Sleeved, not bare. Pipe through concrete gets a sleeve or wrap that protects it from the concrete and lets it expand and contract. (CPVC is exempt.)
• No paper or wood. Slab sleeves can't contain cellulose, so no cardboard tubes.
• Sealed against termites. With soil treatment, the sleeve is no thicker than 0.010 inch and clamped tight to the pipe within the slab.
• Clean up the scraps. Wooden grade stakes, forms, tub trap boxes, and plumbing supports must come out from under and around the building.

One wire you might spot. A copper wire coming out of the foundation near the electric meter is often the grounding connection to footer steel. The National Electrical Code accepts at least 20 feet of ½-inch rebar encased in a footing as a grounding electrode.

⏳ How Concrete Cures

Within hours of pouring, the slab is hard enough to walk on. It isn't finished. It's curing.

Here's what most people get backwards: concrete doesn't harden because it dries. It hardens because water reacts chemically with cement, called hydration. Lose the water too early, and the reaction slows near the surface. Curing means keeping concrete moist and at a reasonable temperature long enough. The American Concrete Institute's curing guide puts numbers on it:

• About a week. A 7-day minimum curing period will often be enough for concrete to reach about 70% of its specified compressive strength.
• The 70% marker. Curing is commonly kept up until concrete reaches at least 70% of its specified 28-day strength.
• Heat changes the clock. High temperatures can shorten the time to early strength, while cooler concrete stretches it.

On a home slab, curing usually means a sprayed-on curing compound or keeping the surface wet. In Central Florida the enemy isn't cold. It's sun, wind, and heat pulling water out of the top too soon, which is where some fine cracks we'll study next class are born.

Cured isn't dry. A slab can be strong enough to build on long before it's dry enough for some flooring, which is why flooring installers often test slab moisture first.

🧱 How Fast the Block Walls Go Up

This surprises buyers driving past their lot: the slab gets poured, and within days there are walls.

Is there a required wait? The residential code doesn't set one number. Timing depends on the mix, weather, design, and wall weight. Concrete Construction magazine's answer: a properly designed slab can usually begin taking load within a few days, and if you're unsure of the concrete's strength, wait five to seven days. The builder and plans set the schedule. A typical Central Florida block home rises like this:

1. Layout. Chalk lines on the slab mark walls and openings.
2. Block, course by course. Block goes up on mortar, with the footer dowels inside the cells that will carry steel.
3. Clean the cells. Mortar sticking more than ½ inch into a cell to be grouted comes out. Where the building official requires it, cleanouts in the bottom course let debris be removed when a grout pour exceeds 64 inches.
4. Lintel inspection. In Lake County's sequence, it comes after the block walls are up and before the lintels are poured.
5. Fill the cells and pour the beam. Grout, placed in lifts no taller than 8 feet, locks steel, block, and foundation together.
6. Trusses. Then the roof framing goes up.

So yes, walls rise on concrete that's still gaining strength. That's normal and planned for. What matters is that the steel lines up, the cells are clean, and the inspections pass in order.

🧭 What We Check During the Inspection

Let's be precise. Four Corners' new-construction inspections start at pre-drywall, when the pad, trenches, plastic, and any soil treatment are already under concrete. We don't test soil, measure compaction, verify the termite application, or test concrete strength; the building department and the licensed pest control company handle those steps.

What we can do is read what's still visible:

• At pre-drywall, we look at every place plumbing comes up through the slab, before tile and cabinets hide it: whether a sleeve or wrap is visible, and whether there are gaps around the pipe.
• Along the slab and walls, we look for wood scraps, stakes, or form boards left behind, and for cracks or damage in the exposed slab.
• Outside, we look at grade, mulch or soil against the house, and whether downspouts and AC condensate lines discharge at least 1 foot from the wall, as the code requires.
• On borate-treated homes, the dyed band may still be visible along the bottom of the walls at pre-drywall. We note what we see, but the pest control company's certificate is the record.
• At final, we note whether the termite treatment notice is posted near the water heater or electric panel.

When something needs a specialist, we say so plainly and recommend the right licensed trade.

❓ Questions From the Back Row

"Why is there a plastic box under my tub drain?"

That's the code at work. Wooden tub boxes have to be removed, so no buried wood is left to feed termites. On soil-treated homes, the code requires a permanent plastic or metal form, so the treated soil never has to be dug up later.

"Was my house treated in the soil or on the walls?"

Ask the builder for the termite Certificate of Compliance, or call the company named on the notice near your water heater or electric panel. They can tell you which method was used and what the warranty covers.

"Does the termite pretreatment protect my house forever?"

No. The notice near your water heater or electric panel names the company and the need for reinspection and contract renewal. Call that company and ask exactly what's covered and when it renews.

"My slab was poured Monday and block went up Thursday. Was that too fast?"

Not necessarily. Industry guidance says a properly designed slab can usually take load within a few days. The mix, weather, and plans decide, and the lintel inspection checks the walls before the cells are filled.

"What happens if a pipe under my slab leaks?"

A licensed plumber locates it and either repairs it through the slab or reroutes the line, often through walls or the attic.

"Is the plastic under the slab the same as a waterproofing membrane?"

No. It's a vapor retarder: it slows ground moisture and, on soil-treated homes, shields the termiticide. It isn't designed to stop liquid water under pressure.

🚌 Field Trip — Class Dismissed

Your homework is a scavenger hunt. Find the termite protection notice near your water heater or electric panel. Write down the company name, then call and ask: was it a soil or borate treatment, is my protection still active, and when does it renew? If your home is newer, search your city or county's permit records for the dates the slab and lintel inspections passed, and count the days between them. Now you know how fast your walls went up.

Next class: slab cracks — shrinkage vs. structural, and what we look for.

Sources: 2023 Florida Building Code, Residential (8th Edition), Sections R318 (including R318.1 and R318.1.8), R506.1 and R506.2 through R506.2.3, R606.3.5.1 and R606.3.5.2; 2023 Florida Building Code, Building, Sections 105.10, 105.11 and 1816.1.8; Bora-Care label, Nisus Corporation (EPA Reg. No. 64405-1); Florida Department of Agriculture and Consumer Services, Proper Treatment for the Prevention of Subterranean Termites in New Construction (rev. 2020); UF/IFAS EDIS IN1277, Termite Prevention and Control; Termidor SC label, BASF (EPA Reg. No. 7969-210); U.S. EPA, Chlordane technical fact sheet; Ying and Kookana, Environmental Toxicology and Chemistry (2006), fipronil under-slab study; 2023 Florida Building Code, Plumbing, Sections 305.1, 306, 307.5, 312.2 and 312.5; NFPA 70 § 250.52(A)(3); Lake County Office of Building Services, BF300 Typical Inspections (April 2024); Polk County Building Division, Residential Inspection Checklist; Citrus County Inspection Guide Sheet (2024); UF/IFAS invasive termite study (February 2026); ACI 308R, Guide to Curing Concrete; ASTM E1745; Concrete Construction, "When to Begin Block Construction on a Slab Edge." Local requirements vary; any specific home should be evaluated on its own plans and condition.

Buying a home in Clermont, Lake County, or anywhere in Central Florida? Call or text (352) 272-1322 or visit fourcornershomeinspections.com.

Taught by Professor Elias Walls · Researched and reviewed by Grant Doutt, Certified Master Inspector® · Last reviewed: November 2026. Professor Elias Walls is the teaching persona of the Four Corners Knowledge Center. Every lesson is researched, written, and fact-checked by the licensed, certified inspectors of Four Corners Inspection Services.