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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!
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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 |
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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.
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Make sure you take a look at
our
shed plans pages and browse through our store. All of
our shed plans are available for immediate download. Here
you can buy our whole package, or just one of our plans as a
sampler. Our categories are always available on the left of
your screen on any page of our site.
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