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Chapter Five: Top of Open Tank

Finish off the top of an open tank with an edge which turns outward. This gives extra strength around the top edge.

Notice that the horizontal wires are on the outside. This position gives maximum tank strength as well as a good place to wedge the 7.6 centimeter form board, which is used to bend the wires to horizontal. Center cut the top horizontal wire every forty-five centimeters, leaving three vertical wires to bend to horizontal together. These center cuts are a rare circumstance where the wire tails are not removed, there is a 7.5 centimeter tail on each side of each section. They will soon be covered by reinforcing bar.

Bend and trim vertical bars to fit the lip. Then place 0.6 or 0.9 centimeter bar around the outside of the lip. Sixty millimeters is adequate for the small tank pictured. Now cut enough fifteen centimeter wide strips of welded wire to go around the tank lip. The strip lengths should overlap at least 7.5 centimeters. Bend the fifteen centimeter wide strips over the same form board. Use the long wire as a catch to hold the wood board.

The board is of no further use when the fifteen centimeter strip has reached a ninety degree angle. Complete the bend by hand. Work along the strip two or three times to gradually complete the bend. Use a bouncing pressure with the palms of both hands. Listen for a click when the wires contact at the bottom of each downward push.

The short side of this piece is placed on the top. The long side goes down and forms the hypotenuse of a structural triangle.

Metal lath of thin gauge is painted black. It is much better for ferrocement work than the heavier gauge galvanized. Galvanized should only be used if the thin gauge is unavailable. Thin gauge is 4.5 mm.

The purpose of the fine metal lath and hardware cloth is to hold fresh plaster in place. If you find yourself in a location where these products are not available then use two layers of poultry netting inside and out, which is four layers total. Try to stagger the layers of poultry netting so that the holes of one layer are covered by wires of another, this will necessitate occasional cutting to reposition the wire layers. Poultry netting has a tendancy to produce high places so a vigilant inspection is required before applying the plaster.

The height of this open top tank is set so that a standard piece of metal lath can be cut into two functional pieces. One piece fits the inside wall up to the outward lip curve. The second piece covers the inside wall plus the lip; top and bottom (see last photo).

The bottom half of the lip is covered with lath. Thin welded wire is placed on top. Beware of sharp cut ends of metal lath. Flatten them inward with a tool.

Substitute two layers of poultry wire if you are in an area where metal lath is not available. The openings of poultry wire are too large, it is best to stagger the wire pattern to bisect the openings. This will require periodic cutting to reposition and start over. Chicken wire has been used successfully on many reservoirs.

Continue to Chapter 6