How to Build a Concrete Slab Foundation For Your Shed

If a wood skid or pier block foundation is not suitable for you, then the best type of foundation may be more to your like.  A concrete slab is generally the best type of foundation for you to build your shed on, but it is also the most work!  Well, you get what you pay for I guess!  If you are willing to undertake such a project, then hopefully this guide can help you understand the steps required to build a concrete slab foundation.

A concrete slab foundation is a great foundation for any of our shed plans that we offer here.  Our Complete Shed Plans or Gazebo Plans Package comes with full details and dimensions that will allow you to build a concrete slab foundation quickly and easily, although it is a lot of work!

Building a Concrete Slab Foundation.

A concrete slab is the most permanent and durable method of constructing a foundation for your shed.  However, slab construction requires greater preparation and expense than wood floor construction using skids or concrete piers.

Site Preparation.   Prepare the site by scraping away all grass or weed material covering the shed area.  Stake out the area for the slab.  Be certain that all corners are square.  If you are using a shed plan for slab construction, remember that all dimensions on the plan are to the outside of the concrete.  Excavate 4" of soil over the shed area and replace with 4" of gravel to ensure proper draining under the slab (see pic above).  Level the gravel fill.

Digging the footing.  Dig a trench for the slab footing approximately 8" wide at the bottom and tapering inward to approximately 16" wide at top.  The footing should extend down about 12" or at least 6" below the local frost line.

Building the forms.  Use 2" scrap lumber to build the forms for the slab.  Set the top of the 2" form board to the desired floor height and level.  The inside face of the form boards must line up exactly with the "string lines" set at the proper building dimensions.  Brace your forms securely since you don't want them to shift or breaker when the concrete is poured.

Preparing to pour the concrete.  Place a 6 mil plastic vapor barrier over the gravel bed before you pour.  Overlap the plastic sheets by at least 12" and do not puncture the plastic.  If you want to insulate your slab from the earth, place 1" of rigid foam insulation over the plastic provided that you have allowed for the additional height.  Add two levels of 1/2" reinforcing bar (rebar) to the top and bottom of the footing and secure the rebar with tie wire held by nails in the forms.  Finally place 6"x6" reinforcing wire mesh over the slab area and support he mesh with small wooden or masonry blocks so that it rests 2" above the vapor barrier.

Estimating the concrete.  This table will help you estimate the approximate amount of concrete require to creat a 4" thick slab with 18" deep footings:

Home