Building Forms for Wall Footings
Footings must be located on undisturbed soil below the frost line. Check your local building codes for requirements regarding size, depth, and metal reinforcement. You may not need to build forms; in some situations a simple trench is enough. Make footings wide enough to distribute the weight of the wall adequately. Usually this means making the footing twice as wide as the wall is thick. A footing also must be thick enough so it will not crack. As a rule, the thickness of a footing should equal the width of the wall, but it should be no less than 8 inches thick. To protect against structural damage that could result when a footing cracks and shifts because of unstable soil conditions, install two or three 1/2-inch reinforcing rods the full length of the footing. Also, be sure to provide for drainage.
Tools: Level, hammer, circular saw, baby sledge or maul, spade or square shovel.
1. Build and install forms... Except for stepped footings, the only forming materials you’ll need are 2x4s for the footing rails and 1x4s for stakes. Position the stakes, then secure the rails to them, making sure the top of each rail is even with or slightly above the top of the stakes.
…or trench and stake. If the soil is firm enough to hold its shape when filled with wet concrete, simply dig a trench footing. Keep the sides of the trench even to avoid wasting concrete. For screeding guides, center a row of stakes about 4 feet apart; check the height of the stakes with a line level.
2. Secure and level the forms. Every few feet along the length of the form, use a carpenter’s level to make sure parallel forms are the same height. Also, check to see that the forms are level lengthwise. Drive stakes every 4 feet to anchor the forms securely. Make sure the stakes penetrate at least 6 inches below the bottom of the footing trench that you dig to ensure that the form boards will be secure.
3. Dig the trench. Once you are satisfied that the forms are level and secure, excavate an additional 5 to 6 inches of earth; the total depth of the footing should be no less than 8 inches. Keep the sides of the trench even with the form as you dig. Check the forms again to see that they are level and aligned properly.
Step down with footings. If your site is sloped, you can step the footings down to save concrete. If you have wood forms, use 2x8s and extra stakes. For an earth form, make a wood riser that wedges securely between the two levels. Stepped forms should rise no more than 2 feet per step; the upper and lower forms should overlap by at least 2 feet.
Building Forms for Poured Walls
Forms for poured walls must be stronger than those for most other concrete projects. This is because they must contend with two substantial forces: the weight of the material itself and the hydrostatic pressure created as wet concrete pours into the wall. If you mix your own concrete, you can pour it into the form slowly and use somewhat less massive forms. But for ready-mix concrete poured directly down the chute of a truck, use 3/4-inch plywood backed by 2x4 studs at least every 24 inches.
Tools: Circular saw, tape measure, square, hammer or drill with screwdriver bit.
1. Construct the forms. Sidewall forms are basically plywood-faced 2x4 stud walls. Working on a flat surface, nail the 2x4 frame together with two 16d common nails or 3-inch screws at each joint. Fasten the plywood sheathing around the perimeter with a few 8d nails or 2-inch screws. Coat the inside form surface with new or used motor oil before setting the form in place. Nail or screw the end pieces securely.
2. Install whalers and spreaders. To strengthen and support the outside of the form, make whalers out of two 2x4s fastened together. Secure these whalers to each sidewall at vertical intervals of no more than 3 feet. To attach them, drill holes through the plywood near each vertical 2x4, and thread No. 8 or No. 9 steel tie wires through the holes and around the whalers. Cut some spreaders— pieces of 1x4 that are exactly as long as the wall will be thick. Have a helper hold a spreader between the vertical 2x4s while you twist the tie wire to secure the whalers in place and tie the whole form together.
3. Check for level and brace. Shore up the end pieces by nailing 2x4 cross pieces to the ends of the whalers. Check the entire structure for level and plumb. Anchor it with braces nailed to 2x4 stakes every 6 to 8 feet along each side of the form.
Using Rented Forms
- Building concrete forms can be time-consuming, and it can be difficult to build forms that are precisely square and straight. Although it will cost you a bit more money, an easier, less time-consuming approach is to rent ready-made concrete forms.
- Rented forms are available in standard sizes, usually 3, 4, and 8 feet long, and a variety of widths. To get the exact wall size you want, you may have to use a combination of widths. There are special forms for corners. Readymade forms come with smooth plywood or plastic-coated surfaces so you’ll have a smooth-looking finish on the wall.
- Forms are assembled side by side with a fastening system that uses wedges; just tap the pieces in with a hammer for a tight fit. Metal whalers take the place of doubled 2x4s. Form ties hold the two sides of the wall form firmly apart at the correct thickness. After the concrete is set, you remove the forms and cut off the ends of the form ties that poke through either side—most of the form tie remains inside the wall.
- In addition to renting the forms, you will have to pay for oil to spray on the forms and the form ties themselves.
- You may have to search to find a form supplier. Most tool rental businesses usually do not carry concrete forms. Some concrete delivery companies do have them or look under “Concrete Forms” in the Yellow Pages of the phone book for a company specializing in rented forms.
Caution! Take No Chances When Preparing to Pour - Weak forms lead to disastrous results. Once a form bursts or bulges, you can try to shore it up. But often there is nothing you can do except let the concrete set, break the wall into rubble, throw the pieces away, and start all over again. The loss in the cost of materials and your time can be enormous. So always err on the side of caution; overbuild, rather than building weak concrete forms.