How to Build a Mono-Slab Foundation

Layout

• 1

  Set wooden stakes in the ground at the corners of the slab's perimeter. Tie two strings to each stake. Run the two strings to the adjacent stakes, pull them taut and tie. Avoid running strings between the two stakes next to the house.

• 2

  Check the layout for square. Tie a string on one stake and connect it to the stake diagonally across from it. Repeat for the remaining two stakes. Measure the length of the two string lines. The layout is square if the two measurements match.

• 3

  Measure 3 inches outside of the perimeter, and set wooden stakes at the corners. Run string lines between adjacent stakes. Avoid running a string line between the two stakes closest to the house.

• 4

  Mark the perimeter of the outside string lines with flour. Remove the inside and outside string lines.

Excavation and Forms

• 5

  Excavate the area inside the flour lines with a shovel to a depth of 4 inches. Start at a corner and work down to the next corner. Continue working corner to corner until the area is excavated.

• 6

  Replace the interior string lines.

• 7

  Set 2-by-4 form boards under the string lines. Ensure the inside edge of the board lies directly under the string. Avoid setting up boards along the length of the house.

• 8

  Set the form boards so that they slope slightly away from the house to improve drainage. Place the form board closest to the house at level. Use a spirit level, which measures level with a bubble positioned between two lines inside of a vial, to place the next form board about half a bubble off of level. Continue down the sides of the perimeter in this way.

• 9

  Put 2-by-4 wooden stakes behind the form boards at 3- to 4-foot intervals. Nail the wooden stakes to the form boards.

Finishing

• 10

  Lay a 1-inch bed of pea gravel evenly throughout the excavation. Smooth the gravel with a screed, a wooden board attached to a long handle.

• 11

  Roll a wire mesh over the gravel. Cut the mesh with a pair of snips to fit. Ensure you leave a 2-inch gap between the mesh and the form boards.

• 12

  Have a ready-mix truck pour the concrete into the forms from a chute. Work the concrete with a shovel or hoe to remove any air pockets.

• 13

  Have a helper take one end of a long board. Take the other end and slide the board over the concrete to level it off.

• 14

  Continue smoothing the concrete with a bull float, a tool with a long tool handle attached to a flat board. Stand at the edge of the slab and push the bull float away from your body.

• 15

  Edge the concrete when it's firm enough to hold its shape. Run a trowel along the edge of the slab to separate the top inch of concrete from the wood.

• 16

  Run a stiff-bristled broom over the surface of the concrete. Create shallow straight lines in the slab that lay at a 90-degree angle to the form boards.

• 17

  Cover the concrete with plastic sheeting. Lay boards on the sheeting to hold it down.

• 18

  Wait one week for the concrete to cure. Remove the sheeting and the form boards.