Laser Grading for Concrete Slab Prep

Every concrete contractor has the same complaint: the dirt was off. Either it was high in one corner and the slab came out thin, or it was low in another and they had to chase the screed with extra mud, or the slope was wrong and now water sits on the finished floor. Slab problems almost always trace back to grading problems. Laser grading is how you stop them from starting.

What a slab needs from the subgrade

A concrete slab has four requirements from the dirt underneath it:

Correct elevation. The top of the slab has to match the surrounding hardscape — door thresholds, sidewalks, driveway. The subgrade has to be exactly one slab-thickness below.
Consistent thickness. A 4-inch slab needs to be 4 inches everywhere, not 3.5 in some spots and 5 in others. Thin sections crack early. Thick sections waste money.
Correct slope. Slabs that drain — patios, washdown areas, garage floors near doors — need a measurable pitch, typically 1% to 2%.
Compacted, uniform support. Soft pockets settle. Hard pockets force cracks. The whole subgrade has to behave the same way.

Laser controls handle the first three. Compaction is a separate step.

What "off by an inch" actually costs

On a 24-foot by 24-foot garage slab (576 square feet) at 4 inches thick, you're pouring about 7.1 cubic yards of concrete. If the subgrade is high in spots, you'll save a little material but the slab will be thin where it can't afford to be. If it's low by an average of one inch across the area, you're pouring an extra 1.8 yards — call it roughly $300 to $400 in extra material plus the labor to spread it.

Multiply that across a shop slab or a commercial pour and the math gets ugly. Laser grading the subgrade pays for itself on the concrete bill alone.

The slab prep sequence

Here's how a typical residential slab prep runs:

Excavate to subgrade depth. Cut the dirt to the elevation that puts the top of the slab where you want it. For a 4-inch slab over a 4-inch gravel base, that's 8 inches below finished floor.
Strip soft material. Remove organic topsoil and any soft pockets. Replace with structural fill.
Compact the subgrade. Pass a plate compactor or roller until the dirt is consistent.
Set the laser. Establish the target elevation for the top of the gravel base.
Place and grade the base. Crushed stone in lifts, finished to grade with laser controls.
Final check. Verify slope, thickness, and edge elevations before forming and pouring.

Steps 4 and 5 are where laser grading earns its keep. A box blade or grader attachment with a receiver hits the target elevation consistently across the whole pad.

Slope targets for different slabs

Different slabs need different slopes:

Interior garage floor: Slope 1% to 2% toward the overhead door so water from a wet vehicle drains out.
Patio: 2% away from the house. No exceptions.
Shop floor with floor drain: 1% to 2% pitch toward the drain.
Pole barn slab: Often flat, but with a slight crown or pitch to keep water from pooling.

The slope has to be in the dirt before it's in the concrete. You can't fix slope at the finish stage.

Why this matters in Greater Chattanooga

Slabs in our area face a few specific issues:

Heavy clay subgrades. Clay shrinks and swells with moisture. A poorly compacted clay subgrade causes slab cracks within a few years.
Sloped sites. Slabs cut into hillsides (common in Hixson, Soddy-Daisy, and parts of Fort Oglethorpe) need careful drainage planning. Uphill water has to be diverted before it reaches the slab edge.
Freeze-thaw cycles. Frost depths run roughly 6 to 12 inches locally. Slabs that hold water freeze in winter and spall.

For more on related work, see the concrete forming and pouring pillar and the building pads pillar.

What to ask your contractor

Before any slab pour, the grading sub should be able to tell you:

The target subgrade elevation (in feet, or relative to a benchmark)
The slope direction and percentage
The tolerance — typically under half an inch across the area for a residential slab
Whether the subgrade has been compacted and proof-rolled

If the answer is "yeah, we'll get it close," walk away. Pour day is the wrong time to discover the dirt was off.

FAQ

How thick should the base under a slab be?

For most residential slabs, four inches of compacted crushed stone over a prepared subgrade is standard. Heavier-load slabs may need more.

Can the concrete contractor do the grading too?

Some can, some can't. If your concrete crew doesn't run laser-controlled grading equipment, bring in a grader for the subgrade work.

How flat does a slab subgrade need to be?

Tolerances vary, but a typical residential slab benefits from a subgrade within half an inch of target across the area. Tighter tolerances may apply for polished or epoxy floors.

Does the slab slope have to be in the dirt or can it be in the concrete?

In the dirt. Trying to build slope at the finish stage means the slab is thicker on one side, which wastes concrete and creates cracking risk.