Building the Sun Cottage
(Page 2 of 4)
September/October 1983
By Angus W. Macdonald, M.Arch.
FOOTINGS
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If your excavation was fairly level to start with, you may be able to cut the trench for the footings right into the exposed subsoil. Trenches should be cut with smooth vertical sides, and you should check for the correct depth from the lines. The bottom is normally 16 inches below floor level, but it may be deeper under door thresholds or where there is no berming (the bottom of the footing must be below the frost line). When digging, you'll have to remove the original stakes, so the lines stretched between the batter boards will end up marking the building's boundaries.
If, however, trenching isn't appropriate for your soil type, you'll have to form footings on the excavation floor with braced and anchored 2 X 8's. (Once the concrete has set, the boards will be removed.) In this case, your floor level will be 16 inches above the excavation bottom, and your base course (we'll get into that in a few more paragraphs) will be 12 "deep.
The reinforcement for the footings consists of steel "ladders" made from wire and 1/2" steel reinforcing rod, as shown in Fig. 2. You can make a jig to bend the steel by pounding nails into the top of a bench or a large, well-anchored sawhorse (see Fig. 3).
At this point, you'll also need to wire 3/4 " starter bars (to connect the walls to the footings) vertically every 16 inches. Make certain that these rods will fall in the block cores by measuring from the corners or laying a temporary row of blocks next to the hole. Then clean the trench out thoroughly and set the ladders three inches off the bottom. This can be done either by making legs from short pieces of rod or by placing rock or brick fragments under the steel.
Most counties require that the local building official inspect your job site before concrete is poured or trenches are backfilled. Check with the authorities to find out what examinations are necessary, and be sure to give the inspector several days' notice.
Next, measure the actual dimensions of your footings, and figure out how many cubic feet of volume they enclose. Since concrete is ordered by the cubic yard, you can simply divide the cubic footage by 27 to determine how much you'll have to order. Ask for a 2,500 PSI (pounds per square inch) mix, and be certain to have several assistants on hand when the truck arrives ... to help you move material to areas that may be out of reach of the truck and hopper. (Many builders create an earthen ramp on the uphill side of their excavation, allowing concrete trucks to back up high enough to pour belt beams and wall cavities directly from the hopper.)
You'll want to rod (that is, stir) the concrete with a shovel to disperse it in the trench and remove air bubbles, but be careful not to dislodge the reinforcement. Compare the footing's top with the lines as you work, and move material so that the surface is eight inches below the floor-to-be and is flat and level.
The concrete can be patted smooth with a wooden trowel, but it isn't necessary to be too particular when doing so. While the mix is still pliable, check to make certain that the starter bars are properly located, vertical, and projecting about 18 inches above the footing.
