Digging Foundation Footings
Digging proper foundation footings depends on several different things.
Foundation footings designs depend on the type ground you have and the load you intend to place on it. Check with your local building official or geotechnical engineer and get their recommendation. For this example I will use a 12 inch wide by 12 inch deep footing, with three bars of steel. Again, your ground or local code may require something different – be sure to check. If I am laying a block foundation (and I almost always am, either for a crawl space or raised slab) I make the footings 18” wide. This is to be sure that I end up with footing under the block wall. If you have level ground and are pouring a slab on grade, you can usually use a 12” wide footing and be fine.
There are two basic methods of digging foundation footings – pick & shovel, or power equipment. If you get a backhoe, get one with a trenching bucket the width you want your trench. The buckets are interchangeable. Position the bucket so that it is on the ground line with one third outside and two thirds inside. You must keep about two thirds of the width inside the line, so your load will rest properly on the footing.
STEP 1: First you will need to accurately position the foundation footings stakes on the ground to indicate all the corners. Use a four foot level and plumb down from the string intersection to the ground. This does not have to be perfect by any means, within an inch is fine. Be sure to check plumb in both directions for the stake location. Tap a short stake in the ground showing the corner location, then paint it or mark it and the ground around it to make it easy to see. Twenty penny nails, tent pegs or screw drivers work great for these stakes.
STEP 2: OK, now you need to get the strings out of the way. Loosen the string loop on one end of every string, it doesn't matter which end. Then, go to the other end and pull the string to you, laying it over the batter board in sections as you pull it in. The more diligent you are to keep it untangled, the more chance you have that it might actually remain untangled. Alternatively, you can roll the strings up on a piece of 2x4 and lay the rolls at the base of the batter boards. Leave one end attached to the nail or screw so you know how it runs.
STEP 3: With the strings out of the way, stretch another piece of string from one corner stake (on the ground) to the next corner stake. This does not need to be wrapped around the stake, just be sure that the string lies straight between the two stakes. Now, mark the ground where the string lays. You can use spray paint or marking chalk. I always use a couple of 5 lb bags of corn meal. Just dribble enough corn meal along the top of the string to mark the ground clearly. When you reach the second stake, move the string to the next corner stake and go again. Continue until you have the outside walls of the house completed in your foundation footing.
STEP 4: Use the backhoe to rough in your depth and then finish the foundation footing with a shovel. Do not try to dig the footings completely with the backhoe as this will end up over-sizing the footings and cost you time and money. You will need to re-position the backhoe several times to get all the trenches roughed in. Keep the trench walls as vertical and square as possible. If you err on the depth, make them deeper than your target, rather than too shallow. If your ground slopes you will need to stair step your footings. At the places where these steps are placed, you will need to deepen the footing to 20 inches.
STEP 5: If your ground is level you may skip this step. Once you have the footing roughed in, you will need to determine how much slope you are dealing with. This is easy – just measure the shortest batter board you have to the ground and the tallest one to the ground. The difference between the two is how much slope you have. Next divide the total slope by 8 to see how many steps you will need to place. (each step is 8”) If your total slope is 16 inches, you will need to place four steps – two going down and two coming up. The steps are incremented in 8” steps to accommodate a concrete block foundation wall.
STEP 6: Footing trenches that are not sloping are easy to set up. After you have the loose dirt removed and the trench to depth, pull your layout string back into place, running down the middle of the footing.
STEP 7: Cut 50 or 60 pieces of 1/4” or 3/8” rebar to 18 to 24 inch lengths. Use a cheap chop saw with an abrasive blade to cut rebar. Wear those safety glasses!
WARNING: Cutting rebar produces sparks which can ignite dry grass or debris. Also, a cut piece of rebar will have very sharp edges and often a metal sliver or tab that can cut like a razor blade. Take the proper care when working with it. Consider buying rebar caps to protect the tops of the standing rebar.
STEP 8: About every four feet along the footing trench, hammer a rebar stake in place, directly under the string. Set the height of the rebar stakes by measuring down from the string to the rebar. You must set this height in 8 inch increments. If your height is 20” and you do not have 12 inches deep in the trench, you can either raise the stake four inches – if the concrete will not be above the ground level and run everywhere – or lower the trench bottom so that hammering the stake down to 24 inches (from the string to the top of the rebar) still gives you 12 inches of footing depth. The trench is supposed to contain the footing concrete, so as the trench runs, when a depth of twelve inches means the concrete is spilling out of the trench, it's time to place a step.
If the top of the rebar is below ground level a few inches, it is no problem. The primary concern is to be certain you maintain the target depth of concrete – as a minimum. If you end up with 16 inch deep footings the only result is that you buy more concrete and spend more money, and have a stronger house. These rebar stakes are to tell you how much concrete to put in the footings.
STEP 9: OK, now let's set a step bulkhead. You will need a 2x8 and some smaller stakes. You can cut these stakes yourself, or you can buy “survey” stakes which are pre-cut and available in varying lengths. Get 18” long ones if you buy them. Cut the 2x8 an inch or two longer than the width of the footing where the step is going.
STEP 10: Tap the board in place on edge, across the footing. Measure down from the string to the top of the board and set it to the same height as the rebar stake before it. Check how much room remains from the bottom of the board to the footing bottom. You must maintain the target depth at all places. If your depth will be too shallow, remove the board and dig the footing deeper, gradually gaining the target depth at the step bulkhead. Do not just dig a small area below the board down to the target depth. The footing must be the minimum thickness everywhere.
Some local ordinances require that the bottom of the footing trench step rather than slope. Your engineer or inspector can advise you on this.
STEP 11: Once the footing depth is right, place the bulkhead board in place again. Drive a survey stake on each side into the footing wall behind the 2x8, immediately beside the 2x8 on the downhill side. Once the survey stakes are solid, screw them to the 2x8 several times to hold it in place.
OK, this 2x8 will hold the footing concrete to one level on one side of it, and another level, 8 inches lower on the other side. The concrete will be poured to the top edge of the board on the “up hill” side and to the bottom edge of the board on the “down hill” side. If you put the surveyor stakes on the uphill side of the 2x8, they will be covered with concrete when you pour the footing. Once you get the step in place, it will be obvious which side is which. Refer to Figure # 7 below.`
STEP 13: After you have all the step bulkheads in place in the foundation footings, lay the rebar in the trenches. We are using three #4 bars (1/2”). All the bars need to over-lap at least 12 inches on the ends.
STEP 14: Use a plastic or metal wire stand, called a “three bar rod chair” to hold the rebar in place, up off the trench floor. Space these out every four or five feet and on either side of corners. Overlap the rebar and tie it together with wire ties. These are 6 or 8 inches long pieces of wire with a loop on either end. Wrap the wire around the rebar and use a tie tool to hook the loops and twirl the wire around several times until it tightens on the rebar. The tie tool is available where you get the wire ties. Sometimes the ties are called “bar ties”. Wrap each junction and joint at least twice. In corners, run the rebar long enough into the corner to reach the opposite rebar on the other side of the corner. This does not have to be precise in the foundation footings.
STEP15: After all the rebar is placed, cut four foot pieces of rebar, three for each corner. Bend these pieces 90 degrees, as sharp a bend as you can reasonably make. A trailer hitch receiver makes a fairly good holder in a pinch. They also make rebar bending tools that help, but don't do any better job than the trailer receiver.
STEP 16: Place the corner pieces in the corners, one to each run of rebar and tie them in place, at least two ties on each side of the corner. All the rebar should be in the “top of the bottom third” of the height of the trench. So, a 12 inch deep trench means the rebar should be about 4 inches off the bottom.
Word to the Wise: Do not use bricks or anything similar to hold the rebar up in the foundation footings. Doing so can seriously weaken the foundation footings because it causes voids in the concrete. Rod chairs are cheap, so use them. I suggest using the plastic ones as the wire ones tend to collapse very easily.
STEP 17: Before you are ready to pour your foundation footings you need to make provision for your water line coming in under the house. This can be done by installing a 2” PVC pipe in the footing. Set this sleeve to a minimum depth to be below the frost line in your area. Mark the location of the pipe, by dimension from a corner and make a note of it. Cut the pipe at least six inches longer than the trench width, and hollow out the trench to permit the pipe to fit. Back-fill around the pipe to prevent the concrete from setting near the end of the pipe and giving you fits finding it.
STEP18: The electric service to the house must be grounded in the foundation footings. Different localities accomplish this in different ways. Check with your electrician or building inspector. You may need to clamp a bare copper wire to the rebar before you pour your foundation footings. If so, be sure to leave enough length of wire to reach the service entrance box - at least 8 feet. You will also need to determine where the service will enter, and protect the wire during the construction process. Be sure to take clear pictures of this in your foundation footings as you may need to prove it after the concrete is in place.