Pouring a concrete slab is a complex enough home improvement that most do-it-yourselfers will back off and seek the help of professional tradesmen or concrete contractors. Many homeowners just do not want to risk making a mistake with their home improvements and having it set in stone.
Like most things, pouring a concrete slab is actually not too difficult. You just need to break the job down into smaller parts. That said, if the job is large and you have not done some work with concrete recently, consider hiring a handyman or tradesman who has recently poured concrete slabs. Most contractors with home repair, remodeling or renovation experience have poured concrete slabs for their home remodel clients.
The going rate for a straightforward 16×20 concrete slab five inches thick is $3000 to $4000. About half your costs are going to be in materials. Most home services materials suppliers will have all the tools and equipment you will need. If there is a significant slope or there are drainage issues, that estimate should go up. Those situations will require excavation equipment.
If all goes well, the work should take about three and a half days. The job will not be finished for five to seven days due to curing. The first day is to prep the site. The second day is to create the forms. The third day you pour the slab. The last day is to confirm that the slab has cured properly, and for cleanup. Again, if you have a slope to deal with, or any other complexities, the work can reach into five days. Consider telling your client that it will take a full week to finish the job. Small slabs can be done in a day or two, plus time for curing.
Step 1… Prepare the Site…
Call your local building department and confirm that you do not need a permit. If you are anywhere near another owner’s lot line, also ask the building department about how close you can build to lot lines. Call 811 to have them come and mark any buried utility lines.
Next, set stakes in the ground to define where the slab is going to be. Have your client around while you do this. Now, get out your line level and mason’s string and find out how much of a slope you have to deal with. If you have a significant slope, you have two options: build up the low side, or excavate into the high side and add a retaining wall.
Consider the drainage. Does water sit in this area after rain? If it does, consider digging an extra six inches down and adding gravel backfill. If you are building on a site that is practically a pond after a rainfall, you will need a more elaborate drainage system.
Most concrete contractors use 2×12 straight form boards to build forms for large jobs. You will also need 16d duplex nails. If you need to splice a few boards together to cover the length of the slab, get a couple of four foot 2×12 cleats for the seams. Also get enough 2×4 stakes and 2×4 kickers to place one every two feet along the edge of the form. Get enough mason’s line to go around the outside of the forms at least twice.
If you are working with a small slab, consider making the form away from where the slab will be. This will give you room to maneuver, and it is much easier to build a form on solid, even ground than in the dirt. You will still have to do your leveling and staking in place, of course. Large forms, like for a garage, have to be built in place.
Cut the forms so that there is at least a three inch overhang on every corner. Add the stakes first, pounding them at least eight inches into the ground, and then drive the nails through the stakes and the form boards. Put the kickers in last – these keep the stakes in place.
Consider adding 1/2 rebar to the edges of your slab. Most home remodeling jobs do not need this much reinforcement, but if the slab is large or if it will bear weight, putting in some rebar is a good idea. It takes less time than you would think, and it will make the job a true home improvement.
For little jobs, the 40 pound bags of concrete mix are fine. A three foot square slab will use about five 40 lb bags. For larger jobs, calculate how many yards of concrete you will need. Do to this, figure out how many cubic feet of concrete you will need, then divide by 27 and add five percent. A typical garage requires about seven yards. If you can get it, air-entrained concrete is worth the extra price. It is less likely to crack as it ages.
If you are mixing small amounts of concrete in a bucket, get a mixer extension for a power drill to make the job easier. Upgrade to a concrete mixer if you are mixing much more than half a yard of concrete.
Get one or two screed boards to spread the concrete out after it has been poured. A screed board about two feet wider than the short edge of your form is good. You will also need a floater for finishing work, and an edge tool if you want a nice curved edge on the lip of the slab.
Let the slab cure for at least five days. Seven days is better. If your client is willing, ask them to lightly spray the slab a couple of times a day. Spraying the concrete morning, noon, mid-afternoon and evening is good enough. If they are not willing to spray this often, it is worth having an assistant go over and spray for you. The slower concrete cures, the stronger it is. Concrete also cures best when the temperature never falls below 50 degrees.
There are curing products available as well; see what is available and how much it will add to your costs. Proper curing is critical to how the concrete will look and hold up over the years. Do not think that the job is done just because the concrete is poured and set.