How to Build Garden Shed Foundations

Updated February 21, 2017

If you want to build a garden shed for potting or storage in your backyard, you may find that a professional contractor will charge you a lot of money for performing simple tasks such as preparing a concrete slab and assembling a modular shed kit. These are processes that you can easily complete yourself as a simple weekend project. The first steps to having your own garden shed are clearing the area, building a foundation form and pouring a concrete slab.

Go to a home improvement store or hardware store and choose the concrete that is right for your project. Consult the packaging to find out how much area one package of that brand of concrete will fill, since this will vary. You will need enough concrete to pour and fill a standard slab that is 6 by 6 feet and 4 inches thick. Have the concrete premixed in the store to save yourself this step.

Make sure that the area where you intend to have your garden shed is level and free of debris, plants and grass. You will pour the slab on-site.

Use lower-quality pieces of 2-by-4 lumber to assemble your concrete pour frame. Place the short edge of one of the 6-foot boards one foot in from the end of one of the 8-foot boards and clamp these two boards together. Nail these boards together by hammering a line of three to four nails through the flat of the longer board into the edge of the shorter board. Attach the second shorter board one foot in from the other end of the attached 8-foot board in this same manner. Line up the remaining longer board with the free ends of the shorter boards so that it is perfectly centred. Clamp this board into place and use a measuring tape to ensure that each short board is one foot in from its respective end. Nail this last board into place. Measure your frame to check that it is a perfect 6-by-6-foot square.

Install three 2-by-2-inch wooden pegs against each side of the frame by hammering them halfway into the ground. Each peg should be approximately 2 feet apart. Cut off the top of each peg with a hacksaw so that they are all even with the top of the frame.

Dig a trench along each inside edge of the frame that is 4 inches deep and approximately as wide as the spade of your shovel. This will make your foundation stronger, because it will be heavier at the edges. Lightly spray the soil inside of the frame with a garden hose and tamp it down with a long-handled tamp to make the surface evenly level.

Since small specks of wet concrete may splash up and burn bare skin, make sure that you are dressed in long sleeves, long trousers and shoes that completely cover your feet. Also wear work gloves and safety goggles.

Pour all of your concrete into a wheelbarrow that you can move easily. Transfer the wheelbarrow to the area near the concrete pour frame. Make sure that the lip of the wheelbarrow is hanging over the frame. Lift the back wheel of the wheelbarrow so that the front tips and concrete pours into the frame. Fill the concrete pour frame with an even 1-inch layer of concrete. Carefully level the concrete with your shovel. Find the approximate centre of the frame and place a 5-foot-long reinforcing rod atop the wet concrete. Line the inside perimeter of the frame, approximately 2 inches from each side, with #3 rebar. Bend the rebar gently around at the corners and overlap the reinforcing rod with the rebar where they cross, securing them together with tie wire.

Pour the remainder of the concrete into the frame so that the pour frame is filled nearly to the top. Walk around the outside of the frame and tap each side three to four times with a hammer to break up any air pockets and settle the concrete.

Drag a levelling screed along the top of the frame; work it back and forth in a see-sawing motion to make the surface even. With wide, sweeping motions, smooth the surface of the slab with a rubber float. Cure the concrete for half an hour, then smooth the surface again in this same way. Cure the concrete for another half hour and then smooth the surface one last time. Let the concrete slab sit for several hours, until all of the surface moisture evaporates.

Once the surface has dried, carefully run the smooth face of a metal trowel over it to shape and smooth it to your satisfaction. Cure the concrete slab for two to three days, spraying it lightly each day with a garden hose to maintain an even cure. After the foundation is completely dry, remove the pour frame.

Things You'll Need

  • Concrete
  • 2 6-foot-long, 2-by-4-inch boards
  • 2 8-foot-long, 2-by-4-inch boards
  • Hammer
  • Carpentry nails
  • Measuring tape
  • 12 2-by-2-inch wooden pegs
  • Hacksaw
  • Shovel
  • Garden hose with fine mister attachment
  • Long-handled tamp
  • Safety goggles
  • Work gloves
  • Wheelbarrow
  • Reinforcing rod
  • #3 rebar
  • Tie wire
  • Screed that is longer than your frame
  • Finishing trowel or rubber float
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About the Author

Jourdan Townsend has been writing since childhood. Her articles appear in a collection of student works at the University of Oklahoma as well as in the school's "Honors College Journal." Townsend also composes poetry, some of which can be found in an edition of the "Anthology of Poetry by Young Americans." Townsend holds a Bachelor of Arts in communication.