install forms

Setting the Forms

Because most concrete slabs are about 4 inches thick, smooth, straight 2x4s make ideal forming materials. When anchoring the forms, drive two double-headed nails through the stakes and into the 2x4s. Place your foot (inset) or hold a sledgehammer (shown below left) against the opposite side of the forms to make nailing easier. Be sure the tops of the forms are level with or above the tops of the stakes, or you will have trouble screeding later (see Pouring, Screeding and Floating, Related Projects). Buttress each form with foot-long 1x4, 2x2, or 2x4 stakes every 3 to 4 feet. Use a string to make sure the forms are straight and level.

curve

Where your plans call for a curve, substitute 3-1/2-inch-wide strips of 1/4-inch hardboard or plywood instead of lumber. For strength, use 2 or 3 plies of hardboard. If you use plywood, cut the strips perpendicular to the wood grain of the surface plies so the curved strips will be easier to bend.

Don't try to measure the length of the pieces to be bent. Tack one end temporarily with two 4-penny nails through the thin material and into the stake. Spring the material into the shape you want, mark the point where you'll cut it, and make the cut. Then nail it in place.

extra support

Don't skimp on bracing your forms. Nothing is quite so disastrous as having forms collapse in the middle of a concrete pour. If that happens, all you can do is frantically pound the forms back together, brace them, and shovel the concrete back in place.

When bracing the forms, pay particular attention to the places where two forms meet. If the forms butt end to end, drive in stakes to lap the joint. At corners, drive stakes near the end of each form. To strengthen curved forms, drive stakes every 1 to 2 feet along the outside radius. Fill gaps beneath the forms with rubble or scraps of wood. Resulting irregularities will be buried later.

divide

Adding dividers on a large project allows you to pour a manageable amount of concrete at one time. If the dividers will be temporary, use any straight length of lumber. If you plan to leave the divider in as part of the slab, however, use redwood or pressure-treated lumber. Two 2x2s sandwich the reinforcing mesh, keeping it at the right level for maximum effectiveness.

protect dividers

Brush on a coat of wood sealer to enhance rot resistance of any wood forms that will remain a permanent part of the slab. Put masking tape on the top surfaces to keep wet concrete from staining the wood and to avoid scratching the forms when you screed. Drive interior stakes 1 inch below the top of the permanent dividers so they will not be visible once the concrete is poured.