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Chapter Seven: Thin Steel and Hatch

Metal Lath (Thin Gauge Expanded Metal): Begin with the wall. Attach the lath to the wall with wire or hog rings. This is not the final tie. Securely in place is enough for now. Metal lath sheets are about 2.4 meters of length. Since the wall is slightly less than two meters, the metal lath will curve up into the interior ceiling. The radius is smaller approaching the center of the tank, this causes the top corners of the metal lath to overlap too far. It is nearly impossible to force plaster through two layers of metal lath. Cut the top corners to make overlaps of two centimeters. Finish walls and then do the ceiling. Use the temporary supports to help hold the lath in place. When the interior lath is complete, put a layer of poultry wire on the outside roof, extend it down the wall about thirty centimeters below the curve.

To repeat: 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.

Outer layer of fine wire: The outer layer of fine wire may be welded wire with 1.25 cm squares, metal lath, or two layers of poultry wire (with the wires of the second layer bisecting the holes of the first layer). If lath is used, try and keep all the opening directions going the same way. This seems a small detail yet examination of the openings will reveal that one direction is wider and will thus allow for easier plaster entry than the other direction. It doesnÕt matter structurally which way the lath is placed but it is convenient during the rush of plaster application if the holes match directionally.

Cut the outer layer of fine steel to reach the beginning of the roof curve. The poultry wire on the roof is sufficient to hold the wet plaster in place at the curve. A roll of 75 centimeters wide and a roll of 120 centimeters wide fine wire cover the outer wall, for example.

Finish Tie the Steel Armature: Loose outer layers of fine wire are exposed by sags and bulges of heavy plaster. Not much can be done at this point. There are two ways to secure the fine steel so it wonÕt bulge. One is with hog rings. Hog ring pliers or a pneumatic hog ring gun are required using this method. These may be obtained from an upholstery supply store. Tie the underlying welded wire, where it is loose, with loops that go all the way through the tank. If the outer steel is tied well to the inner welded wire and the inner welded wire is loose, there will be a large sagging thick spot where the inner wire is loose. Bulges absorb many kilograms of cement.

If hog rings are not available, stitch the seams of metal lath. Use a slightly thinner wire than tie wire if it is available. One person passes the wire through at an angle and another passes it back, also at an angle. Tighten every third or fourth stitch. After all the seems are tight, stitch around and around the tank. Work upward to the wall curve. Space each pass around the tank 20 to 25 centimeters above the last. Although this method takes the most time, the result is excellent when the stitching is tight.

Stitching also works well for the roof, reduce the spacing to fifteen centimeters or less. Another procedure is to tie a third layer of welded wire tightly beneath the inside ceiling lath. The metal lath is supported well by an extra layer of welded wire underneath it. Finish plaster the ceiling to cover this layer of welded wire.

The Hatch:

Locate the hatch near the inlet pipe to facilitate maintenance of plumbing parts and observe water flow. Make the opening large enough to remove the ladder. If the ladder base is 70 cm make the finished hole size 76 cm and the rough opening 80 cm. To accomplish the example size, start with a 83 centimeter circle of #3 bar wired to the roof. Trim steel and bend welded wire in a convenient way. Plan for at least a four centimeter cement curb around the finished hole.

A ferro cement hatch can be either hinged and locked with a hasp, or locked down with two hasps. If hinged, use a large hinge. Weld or tie extra welded wire and steel to the hinge where it attaches to the roof and hatch, do the same thing to the locking hasp. The hatch begins with a circle of #3 bar which is about ten centimeters larger than the completed roof hole. Then fabricate a small version of the tank roof with hinge and hasp attached. The dome shape of the hatch will accomodate the plaster curb around the roof hole. Tie hinge and second locking hasp part to the roof. Reinforce these areas with welded wire patches and reinforcing bar scraps. A ferrocement hatch is heavy, the hinge and locking hasp should be large. The hatch is plastered after the tank has cured for a few days.

The tank is ready for plaster when all the fine steel has been placed. Observe the stiffness of the armature. It is strong enough to fill with water. The plaster waterproofs the tank and protects the steel: when it shrinks during cure, the roof will lift upward and the supports will hang from the ceiling. This power marks the tank strength and illustrates why ferrocement is so strong.

A 45 by 60 centimeter piece of welded wire is the inside lowest layer (in the above photo). It has been darkened to make it more visible. A person on top of the tank ties wires carefully passed upward by a companion working below. It is an effective way to support the inside layer of expanded metal in the ceiling. This method creates a thin roof without sags, it can help save many kilograms of cement.

The hinge mechanism is made of plastic pipe cut in half. It is taped to surround #4 reinforcing steel as shown to directly the left. The hinge bar is shown bent into the ceiling in the above photo. A prefabricated hatch can be plastered and cured before building the tank; it would also be installed at this point of armature construction.

The stake of wood supports the hatch lid two centimeters above the curb. This distance simulates cement thickness (Approximately one centimeter over steel). Complete the hatch and curb similar to the pictures in the last half of chapter five.

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