Pouring The Perfect Concrete Slab For Construction

If you are using a concrete slab as the base for your construction project, you want it to be perfect. You need the proper depth, a well prepared site, and the proper preparation and curing in order to get the results you want in a building surface.

These complex instructions are intended for a slab that will be used for construction of a home, garage or similar heavy structure. To pour a slab for a garden shed, for example, would take less preparation and material.

Materials Needed

•Metal stakes
•2 x 10 forms
•Support stakes
•2 x 4s

Step One – Mark the Site

Determine the size and location of the size and location needed. If you have architectural drawings, this will be determined for you.

Use three foot metal stakes to mark the corners of the site. Add two feet for drainage on each side of the structure. You should check local building and zoning regulations and code prior to starting any major construction.

Step Two – Determine Top Level of Slab

You want to know the height of the top of your slab, since everything works down from that measurement. Using the stakes, put string around the four edges at the height of the proposed finished slab. Keep the strings level and even by using a transit or laser level.

Step Three – Measure Down

There is much more to a foundation slab than simply concrete. You will need 6” of concrete for the slab, 2” of solid foam insulation, 2 or 3 feet of sand, and 3” of base, usually gravel or larger rock.  This means you need to dig down either 3’11” or 2’11”, depending on the amount of sand used.

Step Four – Prepare the Site

Dig down or bring in fill to reach the necessary depth. Add the rock (3” out to the posts), two to three feet of sand, and compact it tightly with a mechanical compactor. The top of the sand needs to rest 8” below the height of the finished slab.

Dismantle the stakes and move them in two feet so that they now represent that actual size of the finished final slab. Put the strings at approximately six inches off the ground.

Step Five – Install the Concrete Forms

Use 2 x 10s to build forms around the slab area. Pound the forms into the sand so that their top edges are at the height of the finished slab. Use a transit to even the edges and make them the proper height. Use support stakes to help with that.

Measure carefully to ensure that the forms are level, that they are square and that the corners are square.  

Put support stakes every four fee around the forms, then backfill dirt around the forms. Remove the guide strings and the metal guide posts.

Step Six – Trench for Footer

Dig a 16 inch wide trench that is 18 inches deep on the inside of the forms. This creates a footer for more stability.  Any drain pipes or plumbing that needs to go into the slab should be placed at this point, and the sand compacted again.

Step Seven – Insulation and Water Barrier

Over the sand, install 2” of solid foam insulating foam and a plastic, 6 mm vapor barrier on top of the foam.  Cut foam and plastic to fit; tape any seams to complete the seal.

Step Eight- Rebar

Using rebar supports to elevate it 2 inches above plastic surface. Make a grid of 3/8” rebar in 18 inch squares. Use connector wires to join the junction points on the grid. Metal mesh is another option. Put 5/8” rebar in the trenches.

Step Nine – Pour the Concrete

Contact a contractor to pour the 6” of concrete. He or she will calculate the amount of material needed in order to fill a space of your size. Smooth with a 2 x 4 as the concrete is filled. Cover with dampened cloths as it cures.