Garage, shed, boat and RV slabs sized for the real load

Start with what the slab will carry

A garden shed full of tools, a detached garage with two cars, a workshop with heavy machines, a boat on a trailer and a large motorhome put very different demands on concrete. The weight matters, but so does how it is delivered. A trailer jack or a camper stabilizer puts a concentrated load on a small point, and a truck turning its wheels while parked scrubs the surface.

Residential slabs are commonly about four inches thick for light use. Pads for heavy vehicles, boats and RVs are often thicker, and the perimeter under a building wall is usually thickened into an edge that acts like a shallow footing. Tell the contractor about every heavy item, including ones you expect to add later, because it is far easier to build for them now than to reinforce later.

Ground and base on low, sandy lots

Much of the ground around Charleston is sand or sandy loam over layers of clay, with pockets of soft organic soil in low areas and fill on many building sites. Sand compacts well when it is confined and dry enough, but it moves when water runs through it. Clay and organic soils soften when wet and compress under load.

For any slab carrying real weight, the base is where most of the value lies. That means removing topsoil and roots, digging out any soft material, and placing crushed stone in layers compacted with proper equipment. On sites with standing water or very soft soil, a contractor may suggest a thicker stone layer, a fabric to keep soil from mixing into the stone, or raising the pad above the surrounding grade so it drains.

Moisture under enclosed buildings

In a humid climate with a shallow water table, moisture moves upward through the soil and into concrete. Under an open carport that is rarely a problem. Under a closed garage, a finished workshop or a storage building, it can mean damp floors, musty air, rusting tools and flooring that will not stay down.

A vapor retarder, a heavy plastic sheet placed under the slab, slows that movement. Whether one is needed, and where it goes in relation to the stone base, depends on how the building will be used. If you plan to put down epoxy, tile, rubber mats or any sealed floor covering later, mention that before the pour so the slab can be planned for it.

Wind, flooding and anchoring

Hurricanes and strong tropical storms are part of life on this coast. Sheds, carports and small buildings can be lifted or shifted by high wind if they are not tied down. Anchors or embedded hardware are usually set into the concrete while it is fresh or installed at the edges afterward, depending on the building.

Anchoring requirements, setbacks and whether a building permit applies are set locally and may depend on the size of the structure and whether your lot sits in a flood zone. Check with the county or town building office, and with your HOA if you have one, before ordering a building or scheduling the slab. The building maker usually specifies how its product should be anchored as well.

Reinforcement, joints and edges

Wire mesh, rebar and fibers all help hold a cracked slab together, and heavier pads often use rebar in a grid. As with any slab, the steel only helps if it sits within the concrete, so ask what chairs or blocks will keep it lifted while the concrete goes in. Near salt water, adequate concrete cover over the steel slows the rusting that comes with salty air.

On a thicker pad the joints can sit farther apart, since the usual spacing grows with the depth of the concrete. In a garage they are often laid out so that each bay forms its own panel. For a shed, the building footprint and the door location influence where joints go. The edges should be thickened where walls sit or where a heavy vehicle drives on, since that is where the most load is applied.

Access for trucks and equipment

A slab in the back of a lot or behind a fence can be harder to reach than it looks. A concrete truck is heavy and needs a firm path, and it can damage soft lawns, septic areas and irrigation lines. Where a truck cannot reach, a contractor may use a pump or other equipment to move concrete.

Point out gates, septic tanks, drain fields, underground utilities and sprinkler lines before the visit. Utility lines should be located and marked before any digging. The contractor can plan the approach around what they see, and access often affects the schedule and the scope as much as the size of the slab does.

Finish and slope for working surfaces

Garage and workshop slabs are typically finished smooth or lightly textured so they are easy to sweep. Outdoor pads for boats and RVs usually get a broom finish for grip. The surface should fall toward the door or an open edge so water from a wet vehicle or a washdown runs out rather than pooling beneath it.

If you plan to wash a boat on the pad, think about where that water and any salt will go. Directing it toward a lawn or a drain away from the house keeps salty rinse water from collecting against a foundation.

What a slab estimate should list

• Dimensions, overall thickness and thickened edges or heavy areas.
• Site preparation, soft soil removal and the stone base depth.
• Vapor retarder, if one is included, and where it goes.
• Reinforcement type and support.
• Joint layout.
• Anchors or embedded hardware for a building.
• Finish and slope direction.
• Access plan and any pump or equipment needed.

Garage, shed, boat and RV slabs sized for the real load — common questions

Related work

Where the work happens