purchase tank book
Tank Book Index
Website General Construction Guide
Continue to Chapter 9

Chapter Eight : Roof Support

Brace the roof to hold the wet plaster when everything else is finished. Five by ten centimeter posts pushing 2 x15 cm boards against the ceiling will hold the wet plaster when there are enough supports to make the roof rigid. There will be several people working on the roof to plaster it. The roof should feel hard, almost like standing on solid ground. If it is soft, it will sag when the plaster is applied. The roof supports also help hold the walls vertical and straight. Leave enough room between the outer supports and the wall for plaster work. Wire the supports up so they stay in place during construction.

Now is the time to make a strong center post if one was not cast into the floor concrete. A 10 x 10 cm post is strong enough to hold up wet plaster, 5 x 10 is sufficient to place the roof steel. A base is required to hold the center post vertical and steady. This is easy enough to nail together. Make the center post easy to remove when the tank is finished.

It is not uncommon to return to a tank the day after it has been plastered and find the supports hanging from the ceiling. This occurs because the cement plaster shrinks as it cures. The shrinkage around the walls can cause the roof to lift upward as much as five centimeters. Tie the support structure loosely to the ceiling so nothing falls and damages the fresh and relatively soft plaster wall. This effect is not so obvious in smaller tanks but is an indication of the live strength within the composite material known as ferrocement.

Place a circle of forty-five to sixty centimeters of steel or plywood on top of the center pole. For those who venture into construction of tanks as a business, this top piece for the center pole should be steel made in two halves which bolt together through a hole in the top of the center pole. Put holes in the circular plate for tie wires to secure the long rafter supports which extend as spokes from the wall to the center support. For smaller tanks or construction of a single tank, a 2 x 15 centimeter board will work fine. These boards also bend nicely to the shallow roof curve and make good support rafters when wedged upward against the roof steel with stronger support wood such as 5 x 10 cm. Be sure to secure them to the roof with loose wire ties so they donÕt damage the wall if they fall.

Ferrocement tank construction is a good business in areas where tanks are needed for water or grain. There is no way to make a tank better or less expensive in the size range of 50 - 500 cubic meters. This is especially true in areas where the water is highly corrosive and steel tanks last only a few years. Price your product near what installed steel tanks sell for and there will be enough profit to build a good business enterprise. Smaller ferrocement tanks are more expensive than other construction methods but are still competitive when the replacement cost of other tank types is included over their life cycle. One ferrocement water tank will last through replacement of several steel or plastic tanks even in areas with minimally corrosive water.

Note: A very thin ferrocement covering can be put on plastic tanks to shield them from the sun and they will last indefinitely.

Those who plan construction of numerous tanks or other structures with roofs will eventually make adjustable metal roof rafters and support stanchions which are removed and taken to the next project. Visualize a wire umbrella frame with the support stanchions extending straight down to the floor. Stanchions are made of square steel tubing which is supported by a screw jack. Such jacks are easily made from 2.5 cm threaded steel stock. Weld a seven to ten centimeter square of plate steel welded on the bottom for a foot. A nut and washer push upward against the bottom of the stanchion to make a tight fit of the rafter support spokes against the ceiling.

The center post for larger tanks is simply a larger version of the one pictured. The stanchions are made of a lower piece using three centimeter square tubing. A piece of 2.5 centimeter tubing slides inside for course adjustment and is set to the desired length with a pin extending through; this secures the adjusted length. For larger tanks use three centimeter square tubing inside 3.8 centimeter outer tubing. There are many combinations of outside measurement and tubing wall thickness such that a smaller size will slide inside a larger size, some steel supply stores have the right sizes and some donÕt (the oneÕs that donÕt often say there are no sizes that fit).

Square steel tubing is usually supplied in lengths of 6.5 meters ±. Short stanchions are made by cutting these lengths into quarters, cut the lengths into thirds or even halves to support higher ceilings at the center of large tanks. Combinations of these square tubing sizes are also used to construct the rafter support system. A pin of #3 bar at the top of the stanchion fits into a hole in the bottom of the rafter to maintain a secure lock on position. The roof weight can be quite large during plaster application.

A complete discussion of rafter support systems is too lengthy for this small book, yet it is worth mentioning that the rafter supports can be assembled with circumference chord connections. These can be straight pieces which slide over smaller stubs of square tubing welded to the rafters. One or two of these circumference connections should be of adjustable length. The pattern of the assembled support structure will look very much like the reinforcing steel bar pattern in the roof armature. All support rafters need not reach to the center when building very large tanks but it is simpler to do so on sizes below 200 cubic meters ±, assuming three meter walls in the 200 cubic meter range.

Notice that when the single rafters are connected to each other there is a structural integrity to the rafter supports which is independent of the tank armature.

Hinged tubing tails at the outer circumference of the roof support system are helpful for tying the rafters to the wall steel armature. This complete roof support design introduces a major cost saving since most of the roof armature can now be built on the ground and lifted as a single piece on top of the rafter support structure. Roof armatures which are pre-built on the ground are upside down because the ceiling metal lath is placed on top. The only remaining work when the pre-assembled roof armature is placed on the roof supports is to join it to the wall steel with short reinforcing bar pieces in the wall to roof curve. These pieces should extend forty-five centimeters into the roof steel and be tied securely. Cover the reinforcing bars inside and out with welded wire to complete the wall to roof curve and the union of roof and wall. Finish the inside of the curve with metal lath making sure lath overlaps are not too large and then the outside with poultry wire.

This roof construction technique efficiently utilizes the extra layer of welded wire beneath the ceiling metal lath discussed previously. The tank builder who has roof support technology is ready to build roofs for just about anything. Support stanchions are placed two meters ± apart around the outer circumference. Two meters is a good place to start for spacing stanchions along the rafter toward the center. All this depends entirely on how one designs the overall support system, how large the tank is, and how perfect one wishes the roof to be. Safety is also a consideration. No workers should be inside large tanks when the roof is being plastered. Additionally, for very large tanks, the roof plaster can be placed for about one third of the top at the center and then 60 cm wide plaster spokes can extend into the walls. This way requires a second plaster day but is safer if one has doubts about the support structure.

A final note about the roof rafters and building very large tanks involves the floor. Large tanks are quicker to build if the floor extends underground like a bowl. The amount of water in the bowl shape of the floor reduces the size of the tank but introduces the problem of a slippery slope under the stanchion feet. Eliminate this problem by casting flat topped steps which ring the floor at the proper radius measurement for the stanchion feet. It is easier to make entire circular steps on the proper radius than to figure out where to place a small step for each rafter stanchion to stand on.

Installing hinged rafters

This method illustrates applying the metal lath to measured lengths of the first layer of welded wire, while that wire is laid out flat on the ground. Then turning the now clipped together layers over, rolling the assembled layers into a roll, placing that roll on the rafters, and unrolling it in factory widths across the roof. ÀClaro?

Continue to Chapter 9