How to Pour a Cement Floating Slab


The term "floating concrete slab" can be a little misleading, as the slab does not actually float in the air but instead rests on a bed of sand, which helps absorb and cushion shock to the slab. Someone who is fairly handy and is willing to put in some hard work can probably install a floating slab themselves. The key to successfully pouring a concrete slab is the preparation, so be sure to take the time to do this properly or else you could end up with cracking, which can be very expensive to fix.

Things You'll Need

  • Measuring tape
  • Hammer
  • 4 3-inch metal stakes
  • String
  • Carpenter's level
  • Shovel, Bobcat or backhoe
  • Pea rock
  • Soil compactor
  • Sand
  • 2 x 10 boards
  • Nails
  • Plastic sheeting
  • Tape
  • Metal mesh
  • Rebar or steel spikes
  • Cement
  • Cement mixer or wheel barrel
  • Gravel
  • 1/2 gallon water
  • Concrete vibrator
  • 2 x 4
  • Concrete float
  • Determine how big your slab will be. Hammer in one 3-inch metal stake where you want one corner of the slab to be located, and use this as your reference point to measure and locate the other corners. Install the 3-inch stakes in the other three corners where the slab will be, assuming the structure is square or rectangular. Tie a string on the ground between the stakes to determine the outline of the slab. The string will be the top of the slab, so use a carpenter's level to make sure the strings are level and adjust the stakes as necessary.

  • Dig a hole 3 feet below the top of the string. You will need to dig out the entire area inside of the stakes and string where the slab will be located. If you are doing it yourself, you may want to rent a Bobcat or backhoe if the slab is large, as this will be hard work if digging by hand.

  • Layer 3 inches of pea rock evenly across the bottom of the entire hole. Use a soil compactor to compact the gravel base. It is very important to have this base to ensure proper drainage. Standing water under a slab can result in damage to the concrete and water leakage.

  • Lay 2 feet of sand on top of the gravel. You may want to do this in layers to ensure proper compaction. After each layer, use the soil compactor to make sure you have a solid base. The top of the sand should be 8 inches below the top of the strings.

  • Install cement forms using 2 x 10 boards as the forms. Start at one corner and set two boards on edge and with the butt joint of the two boards at the corner. You will need to dig a trench a few inches deep where the forms will sit. Use stakes on the outside of the boards to hold them vertical and to provide strength. Attach the boards to the stakes using nails, but do not drive the nails all the way in as you will want to easily remove them later. The top of the forms will be equal to the top surface of the slab, so use a carpenter's level to make sure the forms are level. Use the strings to guide placement of the forms. The area within the forms will be the size of the slab. Backfill on the outside of the forms and remove the metal stakes and string.

  • Lay a sheet of plastic over the entire area. Use tape to seal any seams. This is the vapor barrier and will help prevent any moisture from entering the slab from the ground.

  • Lay out the metal mesh on top of the sand. You will want to cover most of the area where the slab will be located. Insert the steel spikes into the sand in a line every 3 feet. The spikes should sit about 3 inches above the top of the sand. Lift the metal mesh up and attach to the top of the spikes. The mesh should now be "floating" above the sand.

  • Mix the concrete according to the specifications listed on the outside of the cement bag in a cement mixer. Usually the mixture is one part cement, two parts sand and three parts gravel. Start by shoveling one shovel-full of cement, then two shovels-full of sand, and three shovels-full of gravel into the cement mixer and mix together. Add about 1/2 gallon of water to the mixture and let the mixer thoroughly combine the ingredients together. The mixture should be the consistency of peanut butter, so you may need to add a bit more water. If you don't have access to a cement mixer, you can simply mix the ingredients in a wheel barrel with a shovel. It is significantly more work this way but can be just as effective.

  • Pour the concrete into the forms. Ideally you would want to pour the entire slab at one time, but if you are mixing the concrete yourself you will need to mix several batches very quickly as you do not want the concrete to set up before finishing. Use a concrete vibrator to remove any air pockets in the concrete. Once you have filled the forms, use the 2 x 4 to screen across the top of the forms to remove any excess concrete. Finish by smoothing the concrete with the concrete floats.

  • Allow the concrete to cure for at least two days. Pull the nails out of the stakes and remove them from the ground. Pull the 2 x 10s off of the concrete. You now have a floating concrete slab.

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