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How to Build a Concrete Slab Foundation For Your Shed

What about a concrete slab foundation? Our free article center was formed to help you build, plan, and execute your wood working project as smoothly as possible.


How to Build a Concrete Slab Foundation
Written by Frank McGill.

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!

concrete slab foundation shed plans

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:

Slab Size Concrete Required
8' x 12' 2.5 cubic yards
12' x 12' 3.5 cubic yards
12' x 16' 5.0 cubic yards
12' x 20' 6.0 cubic yards

pouring a concrete foundation shed plans

Pouring the Concrete Slab

If necessary, have your local building inspector approve the forms before you pour.  If your shed will utilize electrical service or plumbing, place the electrical conduit or plumbing in the proper location before you pour.

Placing.  Be prepared for the arrival of the ready-mix truck or you could be charged a wait time fee.  Have extra helpers, wheelbarrow, and concrete finishing tools at the ready.  When the truck arrives, pour the area farthest from the truck and fill the footing trench making sure that the concrete does not push the forms or rebar out of alignment.  For larger areas, break the work into smaller sections by installing temporary screeding guides.

When one section is poured move to the next section while the helpers screed off the first.  Ask a helper to knock the sides of the forms with a hammer in order to force air pockets out of the concrete.  Be sure that all voids are filled with concrete.  Pay special attention to the perimeter area of the form boards.  Remove the temporary screed guides when you fill in these voids.

Finishing.  Once the concrete has lost its initial shine, begin finishing it with a bull float.  Larger floats have a convenient handle like a broom.  If you are using smaller hand floats, use toe and knee boards placed on the concrete so that you can kneel on the concrete without leaving a deep impression.  Move the float in long sweeping motions.

Anchor bolts should be placed after the concrete has been screeding and bull floated (see pic below).  Place the bolts 1-3/4" away from the edge of the slab.  Double check the bolt spacing and alignment.

For a coarser finish, bull floating is all that is required.  For a slicker, smoother finish, use a steel trowel to go over the work once bull floating is complete.  Use a light touch so that you don't gouge the concrete surface.  Before the concrete hardens completely, take a trowel and cut between the edge of the concrete and the form.

Curing.  Once all the finishing is completed, mist down the slab with water and cover it with a layer of plastic, burlap, or straw.  Keep the surface moist for two to four days as the concrete cures.

anchor bolts concrete foundation shed plans

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