How to Pour a Concrete Slab Correctly
Concrete slabs are used frequently as foundations for houses and sheds. Concrete makes a sturdy base and also forms a basic floor, eliminating the need for wood floor framing. Check local building codes before starting any concrete slab project; regulations about depth, type of underlying base and other factors will vary with locale. Plan to use wooden forms in most cases, but investigate rental of metal or prepared plywood forms for larger projects. This will add expense but may save labor. Also decide whether to use a concrete supplier or rent a mixer to prepare your own. Does this Spark an idea?
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Things You'll Need
- Stakes
- Builder's twine
- Tape measure
- Excavator
- Gravel
- Compactor, hand or mechanical
- Forms, 2-by-4-inch or 2-by-6-inch
- Hammer
- Framing nails
- 1-by-2-inch or 2-by-4-inch stakes
- Concrete
- Wheelbarrow (optional)
- Rake
- Rubber boots
- Screed, metal or long board
- Trowel or finishing tool
Instructions
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Start any concrete slab project by clearing the site to remove trees and roots, brush, large rocks or other impediments. Check for any underground utilities; locating services will do this. Get the site basically level and down to solid earth, without any fill or debris which might collapse later.
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Lay out the basic slab with stakes at each corner and builder's twine stretched in between. Square the corners of this outline by measuring corner to corner with a tape measure; once the diagonals are equal, the site will be square. Excavate the site to the required depth; 8 inches will suffice for a 4-inch shed slab, but deeper excavations will be needed for house slabs. Check frost line requirements; some jurisdictions require slab sub-bases to go below the "frost line," the depth at which the ground freezes, which may be 12 inches or more.
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Fill the excavated area with gravel. Use medium gravel (1/2-inch or so) for a shed or small house slab, up to 3/4-inch for larger and deeper foundations. Compact the gravel with a hand tamper for a shed or a mechanical device for a house slab. Do this in layers for a deeper base; put down 4 inches of gravel and compact it, for instance, then add four more inches and compact that. Get the base as firm and level as possible. Have any water or gas lines and drain pipes installed.
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Outline the slab with forms. Build forms of 2-by-4-inch or 2-by-6-inch lumber, depending on the depth of the slab. Nail boards together at corners with framing nails and a hammer; square the basic form with diagonal measurements. Brace the form with 1-by-2-inch or 2-by-4-inch stakes outside the form boards, with those boards perfectly vertical. Add "kicker" braces diagonally outside each form brace to further support the form.
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Level the tops of the forms with a level, along the form and across the outline side to side with a long board. Dig a 4-inch trench in the gravel at the perimeter on a house slab; check building regulations first, but generally they allow or require the outer edges of a house foundation to have thicker concrete.
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Pour the slab. Fill it with concrete from a mixer truck or by ferrying wheelbarrow loads from a rented mixer for a shed slab. Wear rubber boots and wade into the concrete to push it with a rake solidly into the forms on all sides; compact it as firmly as possible. Fill the slab to the tops of the forms.
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Smooth the slab with a screed, a board or metal form dragged across the surface on top of the forms. Jiggle the screed as you pull it along the slab to further compress and even the concrete. Screed it until the slab fills the form area and is level with the tops of the forms all the way around.
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Let the concrete set until water starts to appear on the surface, then finish it with a mason's trowel or large finishing tool for big slabs; this tool is like a big push broom but with a wide, flat board or plate at the bottom. Finish the concrete until the surface is flat, level, clear of water bubbles or depressions. Cure the concrete at least a week; cover it with a tarpaulin and mist it slightly with water during the curing process. Remove the forms once the concrete is set.
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