How to Build a Sturdy Shed Foundation

Garden shed with lounge chairs, a dining table, and lots of green shrubs.

 

oceane2508 / Getty Images 

Overview
  • Working Time: 6 hrs
  • Total Time: 7 hrs
  • Yield: 8-foot square foundation
  • Skill Level: Intermediate
  • Estimated Cost: $200 to $400

When building a shed, few components are more important than the shed foundation. Because the foundation carries the weight of many heavy items—lawnmowers, pressure washers, bags of seed and fertilizer, paint cans, wood, and tools—it has to be sturdy, weathertight, and reliable.

Building a shed foundation is a job made easier with precast concrete blocks. While you can pour a concrete slab for your shed if you wish, precast blocks reduce weight, eliminate concrete drying time, and make it unnecessary to rent a concrete mixer. This floating shed foundation rests close to ground-level, so no need to dig extremely deep holes.

Codes and Permits

You may need to secure a building permit for your shed—often called an accessory structure by permitting departments. Conditions that may trigger permitting:

  • Footprint larger than a certain size (120 square feet, for example)
  • Greater than one story
  • Foundation rests on other than a concrete slab, pier blocks, or soil
  • Attaches to the house or another structure
  • Near an environmentally critical area such as a stream or wetlands

Even if a permit is not required, you must comply with the current building code minimum standards and zoning requirements. If you live in an area that regularly freezes, you may be required to place the blocks in holes that allow them to rest below the frost line.

When to Build Your Shed Foundation

Wet and cold conditions might make the shed foundation-building process uncomfortable but not impossible. Aim for warmer, drier months, from late spring to early fall. If the ground is frozen, you will not be able to break through the frozen layer.

What You'll Need

Equipment / Tools

  • Hammer
  • Cordless drill and driver bits
  • Level
  • Twine and stakes
  • Flat shovel

Materials

  • 12 Metal post bases
  • 12 Concrete deck blocks
  • 4 Bags of drainage gravel, 0.8 cubic foot each
  • 4 Four-by-fours, each 8-foot
  • 9 Two-by-sixes
  • 2 OSB sheets, each 4-foot by 8-foot
  • 9 Hurricane ties

Instructions

  1. Locate Your Shed Foundation

    Locate an ideal place for your shed. Not only should the location be flat but it should have good drainage. Situate the shed so that it is facing a clear area of at least 10 feet so that you can bring large items like wheelbarrows and long dimensional lumber in and out without interference.

  2. Mark the Shed Dimensions and Block Locations

    Use the hammer to pound in four stakes around the perimeter of the shed. Wrap twine around the stakes to create the perimeter. Mark the 12 locations for each of the concrete deck blocks: three rows, each row containing four blocks. The four blocks should be 2 feet apart for a total span of 6 feet (each beam will overhang the outermost blocks by 1-foot).

  3. Dig and Fill Holes For the Blocks

    Dig each of the 12 holes to 8 inches deep or the frost-level depth required in your area. Use a flat-bladed shovel to maintain straight sides on each hole. Fill each hole with 6 inches of drainage gravel. Rest the three four-by-fours across the blocks and check for level. Add or remove gravel underneath blocks to adjust the level.

    Tip

    Make sure that you check for level in the other (90-degree) direction, as well.

  4. Cut and Build Risers

    From the remaining four-by-four, cut 12 pieces of wood, each 4 inches long. Nail the metal post bases onto the end of each of the pieces. Fit the assembly onto the top of each concrete deck blocks with the metal part facing upward. Check for level again. If any part is out of level, you can cut down the four-by-four or put another, longer one in its place, as needed. Fit the three four-by-four beams on the top and screw them into the post bases.

  5. Build the Joist Framework

    Build the joist framework separately. Seven of the two-by-sixes will be spaced 16 inches apart, with the two remaining two-by-sixes being nailed into the ends, two nails per end. With an assistant, lift the joist framework onto the beams. Nail the joist framework to the beams with the hurricane ties.

  6. Attach the OSB Sheets

    Lay the OSB sheets over the joist framework at a 90-degree angle to the joists. Nail the OSB sheets into place.

Tips For Building a Shed Foundation

  • Pressure-treated (PT) wood for the joists and beams is more expensive than non-PT wood but will significantly deter wood rot.
  • You can leave the OSB floor as-is but it will wear out faster than if you have a floor covering. One durable, attractive, and inexpensive floor covering is rustic-grade unfinished hardwood.
  • If you anticipate that the shed will hold only light items, you can space the joists at 24 inches instead of 16 inches.