Constructing a Concrete Slab Foundation
Though it isn't strictly necessary, a concrete slab makes the best foundation for your shed. Exactly how you build it varies. If your soil is firm, remove the grass and pour the slab on grade. If your soil is unstable, excavate and pour a gravel bed, compact it, and build the slab on top. A heavy-duty shed may require a footing below the slab. You will need a building permit for your shed.
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LAY OUT THE SHED
Put batterboards 3 feet outside each corner, stretch mason's line, and square up the lines. Sprinkle flour or powdered chalk along the lines to mark the layout on the ground. Remove the mason's line but not the batterboards. Dig out an area that extends 6 inches beyond the foundation on all sides. Remove the sod, then dig 4 inches deeper for the gravel bed. (If a footing is required, dig out a footing the size and depth required by code.)
LAY A GRAVEL BED
Pour 2 inches of gravel in the footings, dampen with water to help compact it, then compact with a hand tamper. Repeat. Lay about 2 inches of gravel in the rest of the excavation. Dampen and compact with a power tamper. (Rent, don't buy.) Add, dampen, and compact gravel until the surface is at ground level.
BUILD THE FORMS
Nail together a 2x4 box that has an inside dimension the size of the pad. Restring the layout lines. Align the box with the lines and square it by adjusting until the diagonal distances are equal. Use a level to make sure the form doesn't slope. Drive 2x6 stakes at the corners and nail them in place with duplex nails. Drive and nail additional stakes every 2 to 3 feet. For extra support, add kickers made with 2x2s and 2x4s.
REINFORCE THE FOOTING
Put two runs of #4 rebar about 4 inches apart around the trench and support them with commercially sold wire frames called chairs. Tie the rebar to the chairs with tie wire. Overlap the ends of rebar at joints and corners and wire them together. Lay down 6x6-10/10 wire mesh to reinforce the slab, keeping it 1 1/2 inches or so from the edges. Prop the mesh on chairs so it runs through the middle of the form. Overlap sections by 4 to 6 inches and wire them together.
BRUSH ON A RELEASE AGENT
Apply a commercial release agent or vegetable oil to the forms so concrete won’t bond to them. Though once a popular substitute for a release agent, motor oil leaches into the soil and causes damage. Use a release agent instead. Hang two runs of rebar at the top of the trench about 2 inches below the wire mesh. Suspend the rebar from the mesh with wire and tie the ends at joints and corners so the runs are continuous.
POUR THE CONCRETE
Build a sturdy ramp over the form so the wheelbarrow won’t move the forms. Start in a corner and dump each load of concrete against the on before it. If you spread the concrete too much with a shove or rake, the ingredients separate. Work a shovel or rake slowly up and down the concrete to rid if of air pockets. Tap the outside of the forms wit ha hammer so the edges of the concrete don’t become honeycombed by trapped air.
LEVEL THE TOP
Enlist a helper to level the concrete with the top forms using a 2x4 as a screed. Rest the screed on the forms and pull it forward with a side-to-side sawing motion. Make a second pass at 90 degrees to the first. Smooth the surface with a bull float if the slab is large, moving it in broad, sweeping arcs wit ha slightly side-to-side sawing motion. Float with a derby if the slab is small and easy to work from the edges. Stop floating when water appears on the surface.
BROOM THE SURFACE
Push a broom made for concrete work across the surface. The resulting surface will be nonslip, but brooming will not cover any flaws, so be sure th use a float or darby first, as described in step 4. With a trowel, cut carefully along the inside edges of the forms to separate the concrete from them. This makes the forms easier to remove after the concrete cures.
ROUND OVER THE EDGES
If you leave the edges square, they will chip. Round them over wit ha tool called an edger, guiding it along between the form and the pad. If your pad is more than 8 feet long in any direction, cut control joints with a control jointer to help prevent cracking. Get a jointer that will cut one-quarter of the way through the pad. Put a 2x across the pad, resting it on the forms. Guide the control jointer against the 2x to cut the joint. Cover the slab with 6-mil poly that does not touch the concrete until the slab has cured.
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Skill Level: Intermediate
Due to differing conditions, tools, and individual skills, The Home Depot® assumes no responsibility for any damages, injuries suffered, or losses incurred as a result of attempting to replicate any of the home improvement ideas portrayed in this website Before beginning any home improvement project, review it thoroughly to ensure you or your contractor can finish the project and if any doubts or questions remain, consult local experts or authorities. Because codes and regulations vary greatly, you always should check with authorities to ensure that your project complies with all applicable local codes and regulations. Always read and observe all of the safety precautions provided by any tool or equipment manufacturer, and follow all accepted safety procedures.