Compared to the task of laying bricks or other paving materials on a sand base, constructing a concrete patio is a more demanding project. First, you must excavate and build forms to contain concrete. Then you must pour the concrete, strike off the excess, level, and trowel a smooth surface, all within the several hours it takes for concrete to set up. For all your pains, however, you'll be rewarded with an absolutely flat, durable surface that will last for decades with little or no maintenance.

Tools for concrete work

Besides ordinary carpentry and garden tools, you will need to improvise, rent, or purchase several specialized items: a tamper for compacting earth, a darby or bull float to smooth wet concrete, an edging trowel to round off edges and separate them from the forms, a jointer to groove control joints into the slab, and a rectangular concrete trowel to finish the surface.

Once concrete starts to flow, it's too late to alter forms, run for tools, or look for more help. So, before you start mixing or the ready-mix truck arrives, make extra sure you have everything and everyone you need on hand.

Building forms

Patio slabs typically measure 4 inches thick, so construct forms with smooth, straight 2x4s, setting their tops about an inch above lawn level. Wet concrete exerts tremendous pressure, so brace forms well. If you're in doubt about whether your forms are rigid enough, drive a few extra stakes and install added braces.

For proper drainage, slope your patio 1/4 inch per foot away from the house.

If you want to edge your patio with wood, construct permanent forms with redwood, cedar, or pressure-treated lumber. Apply a coat of sealer to further enhance the wood's natural rot resistance, then put masking or duct tape on the top edges to keep cement from staining or scratching the wood.

1. Use stakes, string, and powdered chalk to mark your patio's outline. Be sure to place strings about 3 inches outside the slab's perimeter to allow for forms. To mark curves, wind garden hose between stakes. Strip sod; excavate. Tamp the ground well, then saturate with vegetation killer.

2. Install asphalt-impregnated expansion strips wherever the patio will abut a foundation, wall or other structure. Expansion joints accommodate expansion and contraction caused by temperature changes.

3. Adjust strings to reflect proper slope. To begin building forms, pound stakes until their tops are at the same level as the planned top level of the forms. Pound stakes every 3-4 feet. If you like, pound stakes at any height, attach form boards at proper level, then saw stake tops.

4. Using the stakes as guides, hold form boards at correct level, then nail through stakes into boards with doubleheaded nails. Leave strings up during this step, if you prefer, so you can easily double-check the level of the form boards.

5. Make a bed for concrete by spreading gravel, cinders, or crushed rock. The bed's thickness will depend on how well the soil drains. If drainage is poor, provide 3-4 inches; if soil drains well, 1-2 inches will be sufficient.

6. Roll out wire reinforcing mesh, then flatten. Slip rocks or other shims under the mesh, lifting the mesh so it will be suspended about midpoint in the slab's thickness. If you like, skip the shimming and plan to pull the mesh up with a rake during pouring.

7. Pour the concrete; then, with help from a friend, strike it off with a straight 2x4 long enough to ride atop forms on two sides. Begin striking as soon as you've poured the first 3 or 4 feet; this will show whether you're putting in enough material.

8. When you're done pouring and striking the concrete, use bull and darby floats (the darby float is shown) to further smooth the surface and embed aggregate beneath it. Stop floating when water starts to bleed onto the surface of the slab.

9. After the water disappears from the surface, begin edging the slab. Hold the edger flat on the surface with its front tilted up slightly when moving ahead with it. Raise the rear slightly when moving backward. Repeat the edging process after each of the other finishing tasks.

 10. To control cracking, tool grooves (control joints) at distances equal to the slab's width. For patios wider than 10 feet, also run a joint down the center. Control joints should have a depth equal to one-fourth of the slab's thickness. Use a straight board as a guide.

11. Trowel and finish the patio surface. Two passes with a wood trowel leave a rough, skidproof surface; for a slick finish (not advised for patios), make three passes with a metal trowel.

12. Once you've achieved the surface texture you want, cover the slab with sheets of polyethylene to hold in moisture. This retards the curing process, allowing the concrete time to bond properly. After a week, remove the plastic and forms, then backfill against the patio's edges.