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Build a Concrete Slab

Construction How-To, Decks, Decks, Landscaping, Outdoor Living, Patios, Stone and Concrete February 10, 2014 Sonia


slab LEAD

By Matt Weber

A concrete slab is one of the basic components of modern construction throughout the world. Whether used as a building foundation, sidewalk, patio or a mounting pad for your AC condenser, the method of construction is largely the same.

Building codes generally require that exterior stairways have a concrete footing or foundation. Without the foundation the steps will sink, heave or tilt to one side. This article shows how to build a 4-in. thick slab that will serve as a foundation for a set of deck steps.

Be prepared for a workout if you’re constructing a concrete slab. It can be tough work, requiring a lot of digging, mixing and other sorts of manual labor.

Step by Step

First, mark the perimeter of the slab location with stakes and string. Make sure the slab area is large enough to completely support the bottom of your stair stringers—or whatever else you plan to use it for.

First step is to excavate the slab location.

First step is to excavate the slab location.

Calculate the depth of the finished slab, including the concrete and the gravel base. Concrete is typically applied at a minimum of two inches, but I recommend a minimum thickness of 4 inches for a durable stair footing. With this type of application, both a fine and coarse aggregate are required. (I used ready-mix bags of Sakrete.)

Add a gravel base for the slab.

Add a gravel base for the slab.

Next, dig a level excavation to the dimensions of your slab, plus room for forming boards.

Tamp down and level the gravel bed.

Tamp down and level the gravel bed.

The first layer of my slab consisted of 5 inches of gravel. I shoveled in the stone and compacted this gravel bed down to 4 inches with a hand tamper.

Dig deep enough to account for the gravel base (4” after compaction) and concrete pour (4”).

Dig deep enough to account for the gravel base (4” after compaction) and concrete pour (4”).

Next, I built a rectangular concrete form using 2-by-4s fastened with some heavy-duty R4 screws from GRK Fasteners. Pros often use duplex nails to build the form, which are easy to pry out after the slab cures, but for a low-profile form like this I find using screws to be even faster. The forms frame the perimeter you marked with the stake and strings.

The concrete form was made from treated 2x4s.

The concrete form was made from treated 2x4s.

I leveled the form on the gravel bed and anchored it in place every 18 inches by screwing in stakes driven in the ground outside the frame. The stakes hold the forms securely so they don’t bow or blow out when filled with the heavy concrete. Once the stakes were in place, I cut them flush with the top of the form so I could use a screed board once the concrete was added.

I used a cutoff saw to trim the rebar to size for the reinforcement grid.

I used a cutoff saw to trim the rebar to size for the reinforcement grid.

There was a slight grade in the soil I had to overcome to ensure that my stair landing was built level. To accommodate the slope I partially buried one end of the form below grade, leaving the top inch of the form above ground to shed water. The opposite end of the slab was set above grade, with the edge of the form exposed. Removing the soil allowed me to level the form from left to right and front to back, and stake it securely. Be sure the form is flat enough to provide an even landing for the stair stringers but slightly sloped to shed water away from the home.

Secure the rebar grid with wire tires.

Secure the rebar grid with wire tires.