Preparing Sites for Wall Footings
Because the weight of a concrete slab is distributed over many square feet, it usually does not need footings. Slabs “float” (ride up and down) 1 to 2 inches as the ground freezes and thaws. But a footing is required to support a wall of any sort (a dry stone wall is one of the few exceptions). A footing spreads the weight so the wall doesn’t sink, so it’s usually twice the width of the structure it supports. The footing must extend below the frost line to avoid damage from frost heave. Check local codes to see how deep the footing must be.
To do its job without sinking or shifting, a footing must rest on stable, undisturbed soil. A footing beneath a foundation wall also must have adequate drainage to avoid damage from hydraulic pressure. To be sure you’ve planned for the demands of your particular climate and soil conditions, consult with your local building department.
Tools: Shovel or digging machinery, screwdriver, mason’s line, line level, carpenter’s level, plumb bob, hammer.
■ In most situations, a footing 22 inches wide and 12 inches deep, with two pieces of 1/2-inch reinforcing bar running through it, is strong enough to support a one-story wall. But if the soil in your area is soft, your local building codes may require a larger footing.
■ Formed or unformed, a footing must rest on undisturbed soil. If the soil is black, it almost certainly is topsoil and you must dig deeper. In most areas, undisturbed soil will be clay. In some areas, you’ll hit bedrock only a few feet below grade. A building inspector should approve pouring directly on top of bedrock, even if you haven’t dug below the frost line.
1. Install batter boards. Lay out the job, driving in stakes to mark the wall’s outside corners. Place batter boards about 3 feet beyond these stakes by driving in 1x4 or 2x4 stakes and attaching 3- to 5-foot-long horizontal pieces to them. Fasten with drywall screws. (Nailing will loosen the stakes.)
2. Lay out the site. Attach mason’s lines to the batter boards and the house. Transfer the line marking the outside of the wall to the batter boards by having someone dangle a plumb bob over the outer edge of each perimeter stake while you stretch the mason’s line. When the lines intersect over the stake, mark the points where the mason’s lines cross the batter boards. Now that you have marked for the outside of the wall, measure over on the batter board and mark for the inside edge of the footing, the outside edge of the footing, and the outer excavation line. The dimensions are for an 8-inch-wide wall and, thus, a 16-inch-wide footing.
3. Dig trench and lay out footing. It may not be necessary to build forms for your footings. Depending on your local codes and the nature of your project, simply digging a trench for the footing may suffice. Check with your local building department. Digging a hole as big and as deep as needed for a wall footing can be a slow, back-breaking job, so consider hiring an excavator or renting a small backhoe or trench digger. Dig the trench to the top of the footing or to about 3 inches below the top of the footing if you will be building a form. Use a plumb bob to locate the outside corners of the footing. Partially drive in the stakes that line up with the outside edges of your forming lumber. Follow the same procedure to position the form stakes for the inside edge of the footing form.
4. Level tops of stakes. If there is a footing on the existing structure, drive in form stakes so the tops of the stakes are at the same level as that footing. Or, drive a stake so its top is at the correct depth for the footing top. Then drive the outside corner stakes so they are level with those next to the building. Check for level with a line level or carpenter’s level resting on a straight board.
Building Forms for Slabs
Once you pour concrete, your slab will be impossible to change. As wet concrete flows into a form, it fills the niches and faithfully reproduces every detail of the mold you provide. If you build strong forms that are straight where they should be and curved correctly according to your plan, the final product will look professional. But if you put up forms that bulge, tilt, or have loose-fitting joints, the finished product will have embarrassing flaws for years to come. Take the time to be fussy when building forms. If the formed surfaces will be visible, inspect your forming lumber for knotholes, cracks, and other defects that would show in the poured concrete. Concrete forms must be sturdy, straight, and plumb. If you’re in doubt about whether the forms are rigid enough, drive in an extra stake or two and add braces.
Tools: Sledgehammer, hammer, circular saw, carpenter’s level, line or water level.
Install straight 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 or hold a sledgehammer 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. 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.
Form a 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.
Add extra support where needed. 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 a large slab. 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. Use 2x2s to sandwich the reinforcing mesh, keeping it at the right level for maximum effectiveness.
Protect permanent 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.