How to Build a Concrete Slab Form

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Forms hold concrete in place as the concrete cures. Slabs intended as a base for garages and storage buildings need thicker walls and edges, which require wider boards for form construction. Thinner slabs are needed for sidewalks and driveways, so boards can be about half the width of those used for bigger projects. Whether thick or thin, long and narrow, or wide and long, the basic construction of concrete forms is the same.

Things You'll Need

  • Stakes
  • Sledge hammer
  • Circular saw
  • Framing lumber, 2-by-12-inches or 2-by-6-inches
  • Hammer
  • Double-headed nails, 16d
  • Carpenter's level
  • Masonry string
  • Framing lumber, 2-by-4-inches
  • Pound stakes into the ground with a sledge hammer near each corner where the slab will be located.

  • Cut framing lumber with a circular saw so each side board is 3 inches longer than the finished length of the slab. Cut each end board to the exact finished width of the slab. The end boards rest inside the side boards. Use 2-by-12-inch boards for garage and storage shed slabs, and 2-by-6-inch boards for walks and driveways. If the framing lumber is not long enough for the slab size you're building, splice two boards together with a 4-foot span of framing lumber of the same width. Nail the cleat in place across the seam between the two boards.

  • Hold a side board against one of the corner stakes at close to the correct height of the slab. Nail the board by hammering double-headed nails through the stake and into the framing lumber. Err on the high side when approximating the height of the slab. You can adjust the height of the form by tapping the stake in a little deeper.

  • Place a carpenter's level on top of the form board, adjust the board so it is level, then nail the board to the other corner stake. Stretch a length of masonry string across the top of the leveled form board. Keep the string just above the top of the board as you tie it off. The string acts as a guide to help you maintain a straight form board.

  • Cut lengths of 2-by-4-inch lumber long enough to go from the top of the form board to about 8 inches below the ground. Cut enough lumber to brace the form board every 2 feet along its length. Cut the end of each brace on an angle to make it easier to tap into the ground.

  • Drive each brace into the ground along and against the outside face of the form board, making sure the board stays straight and level. Pound the braces into the ground so the top of the brace rests just below the top of the form board. Nail the braces in place with double-headed nails driven through the brace and into the form board. Double-headed nails hold tightly, but make removal easier once the concrete has cured.

  • Cut additional lengths of 2-by-4-inch lumber to act as kickers for each brace. Set kickers set at an angle to each brace running from near the top of the brace to the ground. Tap the kickers into the ground at each brace and nail the top of each to its corresponding brace.

  • Attach an edge board to the braced form board by nailing through the end of the anchored form board and into the edge board. Level the board with a carpenter's level and add a temporary brace to hold the board while you calculate for square.

  • Make sure the two form boards meet at a right angle with the 3-4-5 rule. Make a mark on the shorter board in a multiple of three. If the board is 8 feet long, make a mark 6 feet from the corner where the two boards are joined. Make a mark on the longer board in a multiple of four. If the longer board is 9 feet long, make a mark 8 feet from the corner. The measurement across the open span between the two boards must be a multiple of five. That way, you'll be certain the two boards meet at a 90 degree angle. In this case, the measurement must be 10 feet. If the measurement is more or less than it should be, adjust the second board -- the edge board -- until the measurement equals a right triangle, then anchor the second board in place by nailing it to the corner stake. Brace the second board in the same manner as the first one and add kickers.

  • Attach the other edge board, nail it off and brace it as you did with the first edge board. Attach the final side board for the form after you have added any dirt or gravel fill to the inside of the form.

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References

  • Photo Credit Comstock Images/Comstock/Getty Images
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