How to Add on a Foundation to a House

• 1

  Measure along the length of the house where the slab will abut the existing foundation. Make two marks at the corners with a pencil. Place two wooden stakes in the ground next to the marks.

• 2

  Place two more wooden stakes in the yard at the opposite corners of the slab layout.

• 3

  Tie a string tight around one of the stakes. Take the string and wrap it around a adjacent stake. Pull the string taught before securing it to the stake. Connect all the stakes with this method. Extend the string only between adjacent stakes.

• 4

  Measure between the two diagonals in the rectangle formed by the strings. The measurements should match if the layout is square.

• 5

  Mark the perimeter by sprinkling sand under the three of the four string lines.

• 6

  Remove the strings but leave the stakes in place. Dig a 1 foot wide trench around three sides of the perimeter with a shovel. Ensure that the trench extends about 3 inches beyond the sand lines. Avoid digging between the corner stakes next to the house.

• 7

  Mark the slab height on the existing foundation. Ensure that it's at least 1 inch below the patio doors or back door. Extend the string from a chalk line tool across the length of the marks. Snap the line, which will leave a line of chalk on the existing foundation where it will intersect the new foundation.

Reset Lines

• 8

  Place a wooden stake about 2 feet beyond each outside corner stake.

• 9

  Make a mark on a stake next to the house at the slab height. Measure down one-quarter of an inch for each foot that the slab extends into the yard. Tie the string at that line. Repeat for the other stake abutting the house.

• 10

  Tie each string line to a wooden stake placed 2 feet beyond the outside slab perimeter stake. Ensure that the strings are level.

• 11

  Reset the strings between the outside perimeter stakes. Tie the string to stakes placed 2 feet beyond the perimeter stakes. Ensure that the string passes through the corners of the slab layout. Check for level.

Forms and Excavation

• 12

  Place wooden stakes to use as form stakes near each corner. Hammer them in slightly below the length of the strings with a mallet. Place them on the outside of the string by the width of the dimensional lumber you will use as a form board.

• 13

  Remove the remaining soil from the perimeter with a square shovel. Dig to the depth indicated by your plans, usually about 4 inches for the typical concrete slab.

• 14

  Ensure that you create an even base as you dig.

• 15

  Nail form boards to the form stakes. Use 2-by-4 boards, if you have a slab depth of 4 inches. Drive two nails through the back of the stakes and into the form boards. Place more form stakes every three feet and fasten them to the boards with nails. Avoid placing form boards between the stakes at the existing house foundation.

• 16

  Check the boards for level and square.

Concrete

• 17

  Unroll a wire mesh over the slab perimeter created by the form boards. Place bricks under the mesh every 3 or 4 feet to keep it off the ground. Cut the mesh with wire cutters when you near a border. Keep the mesh at least 1 inch away from the form boards.

• 18

  Have a ready mix truck come to the job site. Place the concrete in a wheelbarrow and transfer it to the new foundation.

• 19

  Pour the concrete between the forms . Have a helper even out the concrete over the mesh as you continue to bring more concrete from the truck.

• 20

  Level the concrete by passing a long 2-by-4 over it. Have a helper hold one end as the two of you guide the board over the surface. Run a bull float, a tool with a long handle and a flat steel base, over the slab to finish smoothing out the surface.