A wise builder once told me, a basement is a well we pray water will never enter. Those with basements will say, amen. According to the National Association of Home Builders, the #1 reason for builder callbacks is foundations. A properly designed and constructed foundation will never cause a problem, improperly constructed, it will never cease to be a problem. A concrete footing spreads the weight so the wall doesn’t sink, and it’s usually twice the width of the structure it supports. The concrete footing must extend below the frost line to avoid damage from frost heave. Check local codes to see how deep the concrete footing must be. To do its job without sinking or shifting, a footing must rest on stable, undisturbed soil. A concrete footing beneath a foundation wall also much have adequate drainage to avoid damage from hydraulic pressure. To make sure you’ve planned for the demands of your particular climate and soil conditions, consult with your local building department.
In most situations, a concrete footing 22 inches wide and 12 inches deep, with three pieces of ½ 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 concrete 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. Installing Batter Boards
Lay out the job by driving in stakes to mark the wall’s outside corners. Place batter boards about 3 feet beyond these stakes by driving in 1+4 or 2+4 stakes and attaching 3 to 5 foot long horizontal pieces to them. Fasten with drywall screws. ( Nailing will weaken the stakes).
2. Laying out the site for concrete footing
Attach mason’s lines to the batter boards. Transfer the lines 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.
3. Dig trench and lay out for concrete footing
It may not be necessary to build forms for your concrete footings. Depending on your local codes and the nature of your project, digging a trench for the concrete footing may suffice. Digging a hole as big and as deep as needed for a concrete 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 a depth equal 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 corner 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.
4. Level tops of the 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 of 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.
Tips:
Using a water level for building concrete forms
With a water level, you easily can find the same level for two places far from each other, even if you have to turn a corner or two, step down an inclines, or climb a couple of stairs. This simple tool consists of two calibrated plastic tubes attached to a garden hose. Drain spigots and fill spigots let you fill the tubes and hose with water to the desired level. It works on the principle that water seeks its own level.
Using a transit for building concrete forms.
For small jobs, using a line level or water level to lay out a level or accurately sloped slab is most likely sufficient, but if you’d like to perform more efficiently and have a lesser chance for human error, consider renting a transit, or better yet a laser level transit. Transits are a small telescope like device that sits on a tripod. By sighting through it, you can determine level points across a distance. A laser level automatically finds level and shoots a red dot or line up to 200 hundred feet away or more. Combine a laser with a transit and you have the ultimate tool for finding level, ideal for large jobs or if you’re working along.
Checking for square using the 3-4-5 method.
Drive in a stake at one corner of the slab, such as against the foundation, and attach a mason’s line to it. Working from that fixed point, check for square as you drive stakes for the other corners. Use the 3-4-5 method. Measure 3 feet from the stake along one side, mark that point with a stake or tape. Pull the line taut, then measure 4 feet along the line and mark the spot with a piece of tape. Make sure both measurements begin at exactly the same point. Measure between the 3 and 4 foot marks, moving the line until this distance is exactly 5 feet. You’ll then have a square corner. Drive a stake and attach the line. If you have room or have a large slab, use multiples of 3-4-5, such as 6-8-10 or 9-12-15.
1. Building and installing concrete forms.
The only forming materials you’ll need are 2+4s for the footing rails and 1+4s 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. If the soil is firm enough to hold its shape when filled with wet concrete, 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 boards.
Every few feet along the length of the form boards, use a carpenter’s level to make sure parallel forms are the same height. Also check that the forms are level length wise. 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 will dig to ensure that the form boards will be secure.
3. Dig the trench inside the form boards.
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 boards as you dig.
4. Step down concrete footing.
If your site is slop, you can step down the footings to save concrete. Use 2+8s and additional stakes. For an earth form make a wood riser that wedges securely between the two levels. Stepped concrete forms should rise no more than 2 feet per step, the upper and lower forms should overlap by at least 2 feet.
Weak concrete forms lead to disaster. Once a form bursts or bulges, usually all you can do is let the concrete set, break it into rubble, throw the pieces away, and start over. So always think on the side of caution, overbuild, rather than construct weak concrete forms.
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