Concrete Slabs

A slab is a simple pour with an unforgiving spec, because the load it carries dictates almost every decision. A shed base carries distributed load and can be four inches on a compacted base. A generator or standby power pad carries concentrated point loads and vibration, and wants more thickness plus reinforcement. A hot tub or spa base is the extreme case in a residential setting — water weighs roughly 8.3 pounds per gallon, and a filled 400-gallon spa with occupants is comfortably over two tons concentrated on a small footprint. Getting the thickness right is cheaper than getting it wrong.

Every exterior slab needs somewhere for water to go. We pitch slabs away from structures at roughly a quarter inch per foot, which is enough to shed water without being noticeable underfoot. A dead-level slab holds water at the surface, and standing water on concrete accelerates surface deterioration and, in winter, freeze damage. Where a slab abuts a foundation wall the detail matters more, because water held at that junction ends up against the structure rather than the slab.

Where a new slab meets an existing structure — a house wall, a step, an older slab — it needs an isolation joint so the two can move independently. Concrete expands and contracts with temperature and moisture, and a slab rigidly tied to a foundation will crack somewhere to relieve that stress. Control joints then divide the slab itself into panels sized so that shrinkage cracking lands in the joint rather than across the field.

The same principle that governs driveways governs slabs: the base does the work. Four to six inches of compacted aggregate over undisturbed subgrade, compacted in lifts. On Charlotte's clay soils, and particularly on lots that were cut and filled during construction, we check what is actually under the topsoil before committing to a base depth. Fill that was never properly compacted will consolidate under a new slab regardless of how good the pour is.

Most utility slabs get a broom finish, which is durable, slip-resistant and does not show wear. Where a slab will be walked on in bare feet or wants to look more finished, a light broom or a float finish is an option. We do not recommend a smooth steel-trowel finish on any exterior surface — it looks excellent for about a week and is dangerously slick as soon as it rains.