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Garage, shed, boat and RV slabs sized to their weight
A slab for a garage, shed, workshop or boat is judged by what sits on it, not by how it looks. Around New Orleans, that often means a fishing boat on a trailer, a camper, a work truck or a building full of tools, all resting on ground that is soft and slow to drain. The weight concentrates at tires, trailer jacks and wall edges, and that is where a thin slab on a weak base gives way first. This page explains how to size and build a slab for the load it will carry and how to compare quotes that describe it.
Begin with what the slab has to hold
Before anything else, list what will sit on the slab now and what might in the future. A garden shed with a mower is a very different load from a garage for two trucks, and both are different from a pad that holds a boat on a tandem-axle trailer or a travel trailer that will stand in one place for months.
Concentrated loads matter more than total weight. A trailer tongue jack, camper stabilizers or a car lift post press into a small area. Tell the contractor about these, including where they will sit, so the slab can be thickened or reinforced where the weight actually lands rather than evenly everywhere.
Soft soil, fill and a firm base
Much of the south shore sits on soft clay and organic soils that compress under steady weight. A slab that holds a heavy load in one spot for months can press into weak ground and settle unevenly. Removing topsoil and any soft or spongy material, then building a compacted granular base, is the foundation of a slab that stays flat.
Many back yards have been built up over the years with imported fill, old building debris or dredged material. The contractor may probe the ground or dig test spots to see what is there. On very soft sites they may suggest a thicker base, a separation fabric or, for larger buildings, a design reviewed by an engineer. Ask what they expect to find and how they will handle surprises.
Thickness and thickened edges
A simple shed pad or patio-style slab is often about four inches thick. Garage and RV slabs that carry vehicles are commonly thicker, and many are poured with a thickened perimeter under the walls so the edge acts as a shallow footing. Where the slab will support a building, the edge detail should match the structure that sits on it.
Local building rules may set requirements for slabs that carry a garage or a permanent structure, and those rules are the building department's to state. Check with your parish or city before designing a slab for anything with walls and a roof, and ask the contractor whether the building supplier has its own foundation requirements.
Reinforcement placed where it works
Rebar grids, welded wire and fibers are all used in slabs. Steel helps hold the slab together once cracks form and helps a heavily loaded slab spread weight across soft ground. Fibers help with fine shrinkage cracking near the surface. For a vehicle or equipment slab, steel is a common choice, sometimes combined with fibers.
Placement is the part that gets skipped. Steel lying on the stone does little. It should be held up on chairs or blocks so it ends up in the slab during the pour. Where a new slab meets an existing driveway or older pad, dowels can help keep the two surfaces level with each other.
Height, drainage and flood questions
In a low, rainy region, where the slab sits relative to the ground around it matters a great deal. A shed or garage floor set too low collects runoff under the doors in every heavy storm. Most owners want the finished slab a little above the surrounding yard, with the ground sloping away on every side and a slight fall toward the door so washing water drains out.
Many properties in Greater New Orleans lie in mapped flood zones, and some parishes set requirements for the elevation of new structures or accessory buildings. Those rules vary and change, so ask your parish floodplain or permit office what applies to your lot before finalizing the slab height. Your HOA may also have rules about outbuildings and their placement.
Joints and embedded items
A large slab needs control joints to manage shrinkage cracking. Deeper slabs can take joints a little farther apart than thin ones, and panels should stay close to square. In a garage, the joint layout can be arranged so that cracks fall between parking spaces rather than under a workbench or a lift.
Think ahead about anything that must pass through or sit in the slab: floor drains, electrical conduit for outlets or a subpanel, water lines for a wash area, anchor bolts for walls, and sleeves for future connections. Setting them before the pour is simple. Cutting them in later is disruptive.
Access for the truck and the pour day
Back-yard slabs raise a practical question: how does the concrete get there? Narrow side yards, fences, overhead lines, septic areas and soft lawns all affect access. Options include a pump truck, wheelbarrows or a chute from the driveway. Ask the contractor how they will reach the spot and whether anything will need to be moved.
Heat and afternoon storms shape the schedule. Large slabs are often poured early in the morning, and curing measures help keep the surface from drying too quickly. Ask how long before walls can be built, before a vehicle can park on it and before a loaded trailer can be set in place.
Comparing slab quotes line by line
- Dimensions, thickness and any thickened edges or load areas.
- Excavation, removal of soft material and base depth and compaction.
- Type and placement of reinforcement.
- Finished height relative to the yard, and fall toward the door.
- Joint layout and embedded items such as conduit, drains and anchor bolts.
- Access method, removal of old material and who handles any required approval.
Garage, shed, boat and RV slabs sized to their weight — common questions
What depth of concrete suits a boat or camper pad?
Slabs for heavy trailers, boats and campers are commonly thicker than a typical four-inch patio slab, and the areas under tires, jacks and stabilizers may be reinforced or thickened further. On the soft soil common around New Orleans the base under the slab is just as important. Tell the contractor the weight and where it will sit so they can plan the right thickness.
Will the parish want a permit for my outbuilding pad?
It often depends on the size and use of the building, where it sits on the lot and whether the property is in a flood zone. Rules vary between parishes, cities and HOAs. Check with your parish or city permit office and floodplain staff, and with your HOA, before ordering materials. The contractor can tell you what information those offices usually ask for.
How high should a shed slab sit above the yard?
High enough that runoff from a heavy storm does not flow in under the door, with the ground sloping away on all sides. How high is best depends on the yard, the drainage and any elevation requirements where you live. Some parishes set rules for accessory buildings in flood zones, so check with the floodplain or permit office before deciding with the contractor.
Why do garage slabs crack even with rebar?
Concrete shrinks as it cures and moves with heat, so cracking is expected. Rebar holds cracked pieces together and helps keep them level, but it does not stop cracks from forming. Control joints give cracks a straight path. Wide or growing cracks usually point to settlement from a weak base or soft ground, which is why preparation below the slab deserves close attention.
Can a new slab be attached to my existing driveway?
Often it can. Contractors frequently bore holes into the side of the old concrete and set smooth steel bars so the two sections stay level while allowing them to move slightly. The new concrete will look lighter at first. If the old driveway is badly settled or cracked, replacing a larger section may give a better result. A contractor can assess the old slab.
How soon can I build a shed on a new slab?
Light framing can often start within a few days, but heavy loads should wait longer while the concrete continues to gain strength over several weeks. Temperature and the mix affect the timing. Parking a vehicle or setting a loaded trailer should wait longer than walking or framing. Ask the contractor for specific dates for each stage based on your pour and the weather.
Talk through your project
Start with the address, what you want poured or replaced, rough dimensions, and a few photos of the area and any existing concrete.
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