Chapter Six : Support Roof Armature Steel
Ferrocement roofs start with one steel bar. That bar is the first step toward a time just before placement of concrete plaster. At that future moment the one bar will have become a supported armature as hard as ground. The photo below illustrates how one bar becomes the grid for the kitchen ceiling portion of a roof.
One bar spans the kitchen. A second bar supports the first at either a visually or precisely measured arc. As the weight of the roof armature increases it becomes necessary to lasso a second vertical bar against the first to keep it straight. This kitchen roof is small and thus the supports required are also small.
Peter was the one responsible for most of the sink area. Jerry Gottsdanker may have been part of it. Both are great to be around and work with.
Notice there are no windows as yet. Window holes are cut in after the structural curves are fair and true. The curved wall on the left separates the pantry from the kitchen.
A sketch of the kitchen shape helps clarify how the pantry fits in the kitchen area. The counter top spirals into the pantry on a curve similar to a seashell. It gradually becomes narrower as it transforms to one shelf among floor-to-ceiling storage shelves or cabinets. This pantry is readily accessible to the kitchen work area, its partition wall also supports roof steel.
Placement of roof steel becomes easier after the first lonely bar is supported in the proper position. This first bar is a support rafter for reinforcement bars placed at a right angle, it may not be a permanent bar. The idea is to start somewhere and then easily and quickly layout a grid of reinforcing steel until it becomes a completed roof plane. Brace where the span is large enough for the reinforcing bar grid to sag. Small diameter plastic pipe, bamboo and light-weight wood are all fine for temporarily adjusting and supporting the roof plane position. This project seems more difficult than it actually is. Go for it. Someone will be on top tying wire knots quite soon. Use a temporary post if necessary.
Loosely place the first welded wire on the inside ceiling surface as soon as the largest roof span has a stiff and well supported grid. This first inside layer of welded wire and a 25± cm (10"±) lath strip follows the path of the first support rafter. Round water tanks have supporting rafters radiating outward from the center, as does this conical roof.
Other shapes employ parallel rafters, spaced at about one meter (39.5") Bending straight steel tubing is done by placing a new length of 3 to 4 centimeter (1 1/2"±) square tubing on top of the newly made roof plane and bending it every half meter or so with heat, space bends closer together to better match tighter curves. Old and trashed 2.5 cm water pipe has been recycled into quite a few water tank roofs (1"). Precision bends can also be made cold with a simple device made from a hydraulic jack as shown below.
The two parallel roof supports in the following photo are approximately in the center of the longest span, they have replaced temporary supports which held the roof curve while the grid of reinforcing bars was placed. A layer of welded wire and strips of expanded metal are placed prior to the rafters so that they are in a permanent location until concrete plaster is applied. The spacing and position of the two parallel roof rafters is determined by the width of expanded metal, which is about .75 meters (30"). Add half the 25± centimeter expanded metal strips above the support rafters to determine an approximate 1 meter separation between rafters.
The black rectangle on the floor is metal lath. Use the lath as a gauge to make an approximate measure of the support rafter stanchion spacing measured along the rafter itself; about 1.25 meters (50"). Rafter spacing of one meter with supports at 1.25± meters will provide solid support for a roof when a large crew of workers places the concrete on top. Acquire project experience before changing these safe minimums very much.
Standing between the rafters and springing on the balls of oneÕs feet will produce a slight flex in the roof armature. Slight flex means barely discernible; an obvious flex will sag. I know one intelligent person that experienced roof collapse during placement of concrete plaster and have heard of others. Follow this schedule of support to make either a small or very large roof.
A rafter can be seen in the background, kitchen area. It is at an angle to the parallel support rafters in the foreground. This is mentioned in order to point out that support systems are determined by the plane of the roof armature and comprise a complete subject. Topological mathematicians and three dimensional puzzle makers would have a great time building stanchion and rafter systems for imaginative architects.
For smaller roof areas one can use 2 x 15 centimeter boards (1 x 6") supported with 5 x 10 centimeter posts (2 x 4"). Boards of this size curve nicely when pressed up against the roof armature by the wooden support posts. This method is adequate for smaller water tanks and individual rooms like the kitchen area shown previously. Though the same rock solid feel underfoot as with the steel posts and rafters can be accomplished with wood, the lack of screw jacks to adjust the post is a disadvantage. Also, posts are cut to odd lengths and wood is so costly that some thought must be given to what will be done with the odd lengths after they are removed as temporary roof supports. Those who are comfortable with wood should not hesitate to use it to support wet roof plaster.
The following photos show the placement sequence for rafters which are plastered into the ceiling along with the actual reinforcing steel. Although the photos are of a water tank, the same technique works on any roof armature. The heat marks made during rafter bending are clearly visible, immediately below. Use of square tubing, in this region, began after available recycled water pipe was all gone. Cost of new steel led to re-useable hinged rafters, shown further on.
Imagine the forward portion of the wall is a garage or shed attached to a house. If the walls are to be plastered first, it is not necessary to tie the rafters to the wall steel as shown. However, this is a good technique to understand and be able to use when it is the most efficient way to build. Keep in mind that ferrocement does not isolate the structure interior from outside heat or cold.
The first layer of welded wire is placed above of the rafters on this roof. This tank was built so long ago that I do not remember why this particular order was chosen. It looks as though there may have been more jobs going on than there were stanchions available. Whatever the reason, the rafters were plastered into the structure yet the armature was built as if the rafters were going to be removed. The joint at the top of the center pole indicates it was removed. Perhaps this particular roof was built with a crew that varied from day-to-day and a mistake was simply worked with.
The order of steel application in this roof armature illustrates that the main job is to get the steel in place so that the concrete plaster can be applied. Here the reinforcing steel went on top of the welded wire and then the second layer of welded wire was placed on that grid. There are many ways to support the ferrocement roof.
Next, the second layer of welded wire sandwiches the re-bar grid. The reinforcing steel is #3 bar (95 millimeter, 3/8 inch). This lighter weight bar is adequate for final strength but slightly thin for fabrication. It may explain why the 15 centimeter square welded wire went on first.
Notice that a 15 cm strip of welded wire is clipped over the rafter for attachment of metal lath.
Support structures are as varied as the final ferrocement structure.
Examining the various methods of support fabrication provides increased freedom of structural design.
And. Yes. Ferrocement roofs really are strong. This roof party got to jumping and the roof was not harmed. Frank, wearing the white hat, always an interested engineer, positioned himself below to observe the jumping test through the front door.
The next rafter system employs hinged rafter tails. The house being built was constructed by a bronze sculptor who worked for his brother-in-law, Scott Morgan. Scott, a great life-long friend has gone to the great beyond. While he was here on Earth, he was known as the king of ferrocement. The man almost never measured anything.
Note the center pole lying on the floor above. Metal lath was clipped to the first layer/lower of welded wire and then put on top of the rafters. Experience revealed that a well tied third and lowest layer of welded wire served well to support the expanded metal, yeilding a thin roof. See tank manual for added discussion of this topic.
Though low cost and quick; never ever build a round dome echo chamber to live in. Use reversed curves and multi-faceted surfaces to disperse sound rather than focus it. Think about holding cupped hands to your ears. Have you ever seen reverse curve ears that disperse sound? Knowing the concept and building an acoustically pleasing home environment deserve design direction.