Video: How to Pour a Concrete Slab
Project overview and what you can save
Forming and pouring a concrete slab can be intimidating. Your heart races because you know that any mistake, even a little one, can quickly turn your slab into a big mess, a mistake literally cast in stone.
In this article, we’ll walk you through the slab-pouring process so you get it right the first time. We’ll pay particular attention to the tough parts where you’re most likely to goof.
Still, pouring a large concrete slab isn’t a job for a beginner. If you haven’t worked with concrete, start with a small sidewalk or garden shed floor before attempting a garage-size slab like this. Even if you’ve got a few small jobs under your belt, it’s a good idea to find an experienced helper. In addition to standard carpentry tools, you’ll need a number of special tools to finish a large slab (see the Tool List below).
The bulk of the work for a new slab is in the excavation and form building. If you have to level a sloped site or bring in a lot of fill, hire an excavator for a day to help prepare the site. Then figure on spending a day building the forms and another pouring the slab.
In our area, hiring a concrete contractor to pour a 16 x 20-ft. slab like this one would cost $3,000 to $4,000. The amount of money you’ll save by doing the work yourself depends mostly on whether you have to hire an excavator. In most cases, you’ll save 30 to 50 percent by doing your own work.
Step 1: Prepare the site
Before you get started, contact your local building department to see whether a permit is required and how close to the lot lines you can build. In most cases, you’ll measure from the lot line to position the slab parallel to it. Then drive four stakes to roughly indicate the corners of the new slab. With the approximate size and location marked, use a line level and string or builder’s level to see how much the ground slopes. Flattening a sloped site means moving tons of soil. You can build up the low side as we did, or dig the high side into the slope and add a low retaining wall to hold back the soil.
Your concrete slab will last longer, with less cracking and movement, if it’s built on solid, well-drained soil. If you have sandy soil, you’re in luck. Just scrape off the sod and topsoil and add gravel fill if needed. If you have clay or loam soil, you should remove enough to allow a 6- to 8-in. layer of compacted gravel under the new concrete.
If you have to remove more than a few inches of dirt, consider renting a skid loader or hiring an excavator. An excavator can also help you get rid of excess soil.
Note: Before you do any digging, call 811 or visit call811.com to arrange to have your local utilities locate and mark buried pipes and wires.
Figure A: Thickened Slab Detail
A typical concrete slab for a garage has thickened edges to support the weight of the structure.
Note: Figure A can be downloaded and printed from Additional Information below.
Step 2: Build strong, level forms for a perfect slab
Photo 1: Set and level one side
Cut a 2×12 to length (or splice it with a cleat) for one side and nail it to a corner stake. Level the 2×12 and nail it to the second corner stake.