830-515-2002

Blocks and Bricks for a Strong High Wall

Because of its solidity and weight, a masonry wall calls for very careful planning. It must sit on soil firm enough to support it, and not block natural drainage. If you have any doubts about the site of your wall, consult your local building authority. In many areas, codes prescribe strict standards for masonry structures more than a few feet high, specifying materials, dimensions, reinforcement, and depths of footings.

DIGGING TRENCHES FOR FOOTINGS

The footings must be at least 18 inches below grade and must rest on earth not affected by frost. Before digging, mark the borders of the footing trench and the centerline of the wall with stakes clear of the digging area to fix the marks. In loose soil, you may have to bank the trench walls back from the bed by as much as 45 degrees to keep them from caving in. Keep the bed as level and flat as you can, but do not smooth it off by filling loose earth back in: The footing must rest on undisturbed earth. If the virgin soil at the proper depth is loose, tamp it.

A footing’s width and height depend on the thickness of the structure it supports and local soil conditions; consult your building code.

TOOLS Tape measure 4' level Water level Hammer Maul Rebar cutter Square-edged shovel MATERIALS 2 × 4s for floats Wooden stakes Common nails (4½") Rebar Tie wire Sand Bricks or stones Ready-mix concrete Polyethylene sheeting

POURING CONCRETE FOOTINGS

POURING CONCRETE FOOTINGS

In any but the loosest soil, wooden forms are not needed to contain the concrete poured for a footing. In most soils, widen the trench on one side to allow for smoothing the concrete and laying blocks and mortar from the footing up to the surface (opposite, left). But if the soil is firm enough to keep the trench walls vertical for their full height, you have a convenient but expensive alternative: You can dig the trench no wider than the footing and fill it with concrete to just below ground level (opposite, right).

Both types of footing need strengthening with two lines of steel reinforcing bars (rebar) laid along the trench—and both probably need enough concrete for an order from a ready-mix firm. When the truck arrives, have plenty of helpers on hand. Pouring and leveling concrete is heavy work that must be done quickly.

SAFETY TIP

Check with utility companies about pipe and wire locations before digging a wall’s foundation. Wear gloves and a long-sleeved shirt when working with concrete.

Two types of concrete footing

A standard trench (above, left) is suitable for most soils. One of its walls is as steep as the firmness of the soil allows; the bottom is squared off to the width and depth of the footing, and above that the trench gets a foot or two wider to create working space. Reinforcing bars are laid, concrete is poured and leveled, then a block foundation is built up to within a few inches of ground level. A trench dug in very firm soil (above, right), has vertical walls separated by the width of the footing. Reinforcement is laid and the trench is filled with concrete almost to the surface; it needs no block foundation.

POURING A CONCRETE FOOTING

POURING A CONCRETE FOOTING

Marking the top of the footing

Dig the trench for the footing. If you must move a large amount of soil to dig beyond the frost line and you plan to build a high wall more than a dozen feet long, hire a professional. To move a relatively small amount of earth, consider renting a gasoline-powered trencher, which you can operate yourself. Along the sides of the footing trench, drive 12-inch stakes into the ground in a zigzag pattern every 3 to 4 feet. Mark the stake at the highest spot in the trench, 8 to 10 inches above the bed. Mark all the stakes at that height with the help of a water level (left).

Installing grade pegs

Make grade pegs by cutting a rebar into 18-inch lengths, one for each stake, with a rented rebar cutter. Drive a peg into the trench bed next to each stake so that the top of the peg is level with the mark on the stake (above). Do not drive the pegs too deep—pulling them up to the correct height will loosen them. Remove the stakes as you go, filling in the holes they leave with soil. Check the level of the pegs with a 4-foot level. If any is too high, tap it down.

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Laying reinforcing bars

Check your building code for the correct size rebar—usually between No. 4 and No. 8. Set lengths of rebar in the trench bed alongside each row of grade pegs, supporting the bars 2 to 3 inches above the soil with bricks or stones. Where two bars meet end to end, overlap them 12 to 15 inches. Cut bars to length if necessary with a rebar cutter. Lash the bars together with tie wire, then tie the bars to the grade pegs. Once all the bars are in place, remove the bricks or stones.

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Completing the footing

Working with helpers, pour concrete into the trench, taking care not to dislodge the grade pegs. Spread the concrete with square-edged shovels. Break up large air pockets in the concrete by pushing a shovel into the mix, again avoiding the grade pegs. Fill the trench so that the level of the concrete is at least ½ inch above the tops of the grade pegs. Level the footing with floats fashioned from 2-by-4s nailed together (right). Working on one small area at a time, even out the concrete with a patting motion. Continue, compacting the concrete and spreading it into the corners of the trench until the tops of the grade pegs become visible. Smooth out the concrete by sweeping the trailing edge of the float across the surface, pulling the float toward you in wide arcs. Again, continue until the tops of the grade pegs and footing are at the same level. Cover the footing with polyethylene sheeting and let it cure for seven days.

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