Slabs for garages, sheds and RVs, sized to the load

Describe the load before anyone sizes the slab

The weight a slab has to carry, and where that weight lands, drives nearly every decision about it. A garden shed full of lawn tools is light and spread out. A detached garage carries cars on four small tire patches plus the weight of the walls along the edges. A travel trailer or motorhome puts heavy loads on its tires and its tongue jack or leveling pads. A boat on a trailer is similar. A workshop may hold a heavy table saw, a lift or a welding bench in one corner.

Tell the contractor what you plan to put on the slab now and what you might add later. It is far easier to pour a thicker section where a future lift or heavy machine will go than to reinforce an existing slab afterward.

Thickness and thickened edges

Light-duty pads for sheds and patios are commonly about four inches thick. Slabs that carry vehicles, especially heavy pickups, campers and boats, are often poured thicker. Where a building will sit on the slab, the edges are frequently thickened into a turned-down footing that carries the weight of the walls and resists the soil moving out from under the perimeter.

The right combination depends on the building, the load and the ground. Structures such as garages and larger workshops may have specific foundation requirements set by local building rules, so ask the local building department which permits and inspections apply before the pour is scheduled. A shed kit or garage package may also come with its own foundation recommendations from the manufacturer, which the contractor should see.

Ground, base and drainage under a heavy pad

A heavy slab is only as good as the ground carrying it. On the dense clay common north of the city and in Harris County, the concern is movement as the clay wets and dries, and soft spots that stay saturated after long rains. On the sandier coastal plain soils found through much of Russell County and south Columbus, the concern is erosion: runoff carrying sand from beneath an edge until the slab cantilevers over a hollow.

Either way, the fix is the same in principle: strip organic soil, compact the subgrade, build a crushed-stone base, and make sure water flows away from all sides of the pad. A pad set into a slope needs particular care on the uphill side, where runoff arrives, and the downhill side, where fill is often added to make the site level. Fill should be placed and compacted in layers rather than dumped and leveled in one pass.

Keeping moisture out of enclosed buildings

In a humid climate, a slab under a closed building can pass a surprising amount of ground moisture up into the space. That shows up as damp floors, rust on tools, mildew on stored boxes and flooring that will not stay stuck down. A plastic vapor retarder placed under the slab is a common measure for garages, workshops and any building that will be enclosed, insulated or finished. It is less important under an open carport or an RV pad, where air moves freely.

Ask the contractor whether a vapor retarder is included and how it will be placed and protected during the pour. Its seams should be overlapped and it should not be torn up by foot traffic and wheelbarrows before the concrete goes in.

Level versus sloped, anchors and openings

Shed floors and garage floors are usually expected to be close to flat so walls sit square and doors close properly, with a slight fall toward the door so water blown in can escape. RV and boat pads, by contrast, should slope gently so rain runs off rather than collecting under the vehicle. Tell the contractor which kind of slab you need.

Buildings on slabs are usually fastened down with anchor bolts set into the fresh concrete or with anchors drilled in later. Setting bolts in the pour requires knowing the exact wall layout ahead of time. Door openings, a ramp or a lip at the door, and any conduit for power to the building are all easier to form before the pour than to add afterward.

Getting the concrete truck to the site

Many sheds and workshops sit at the back of a lot, beyond fences, septic fields, soft lawn or tight gaps between houses. A loaded concrete truck is very heavy and can rut a lawn, crack an existing driveway or damage a septic system or buried lines if it drives over them. When the truck cannot reach, contractors use pump trucks, buggies or wheelbarrows, and that changes the plan and the effort involved. Mark the septic tank and drain field if you have one, and point out any buried irrigation, gas or electric lines you know about before the visit.

What a slab quote should include

• Exact dimensions, and the building or vehicle the slab is designed for.
• Thickness across the slab and at any thickened edges or heavy-load areas.
• Base material and depth, subgrade compaction and handling of fill.
• Reinforcement type and how it will be supported.
• Vapor retarder, if the building will be enclosed.
• Flat or sloped surface, and the direction water will drain.
• Anchor bolts, conduit, door openings or ramps.
• Site access, protection of lawns and drives, and any pump or buggy work.

Slabs for garages, sheds and RVs, sized to the load — common questions

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