The Traditional Tile Roof
Throughout a large part of Honduras the traditional roof is fired clay tiles set on a wooden framework. Though relatively inexpensive and easy to construct, this type of roof has some serious drawbacks. In a heavy downpour, especially one accompanied by wind, small droplets of splashing rain can drift down and dampen the interior of the house. High winds and even small earth tremors can cause the tiles to shift and create a leak or, even worse, to allow a tile to fall in on the occupants. The most serious problems of traditional tile roofs are health related. The spaces between the tiles allow easy entrance of mosquitoes, the vectors of the parasite that causes malaria and the virus that causes dengue, and kissing bugs, the vectors of the parasite that causes Chagas' disease.
These problems can be mitigated by installing a ceiling. Many homeowners will tack sheets of plastic or cardboard to the undersides of the "tirantes." This reduces the entrance of insects, but prolonged splash from heavy rains or a shifted tile that creates a leak can result in a significant accumulation of water on top of this ceiling that will cause it to fail and dump its contents into the house. Boards or fiber-board panels nailed to the undersides of the "tirantes" offer a little more protection but can still be problematic in the event of a significant leak in the roof.
An effective solution to all of these problems that can be done as a retrofit to an existing house with a tile roof is to construct a domed ceiling of ferrocement. The domed shape sheds the water from any leaks, and it is strong enough to resist a falling tile and even to support the weight of someone standing on it to repair the roof. (The "egg shell" effect.)
Ferrocement
Ferrocement is a sand-cement-water mixture applied to a a wire reinforcing matrix. A strong ceiling about 1.5 inches (3 centimeters) thick can be made with a matrix of steel reinforcing rods and 3 layers of chicken wire.
The following will help to assure a durable concrete:
- Use washed, screened sand free of soil and organic matter. Do not use sea sand unless it has been thoroughly washed with fresh water to remove the salt.
- Mix 4 parts of sand to 1 part (by volume) of Portland cement. Mix the dry ingredients thoroughly before adding water. The preferred method is to use a cement mixer. If done by hand, the mixing should be done on a clean surface to avoid contamination of the mix with soil. A piece of 1/2-inch plywood, 4 feet by 4 feet, makes a good mixing surface. Alternatively, the day before one could pour a thin, flat mixing slab directly on the ground.
- Use clean, fresh water. Dirty or salty water will weaken the concrete.
- Add only enough water to provide a stiff, plastic mix. Adding too much water at this point can yield a weak concrete and one that will crack as it dries.
- Prepare only as much mix as can be used within about 1 hour. If the mix becomes too hard to use, throw it away or use it for some other purpose. DO NOT ADD WATER TO SOFTEN THE MIX AND DO NOT ADD OLD MIX TO NEW MIX! This will result in a much weaker and less durable concrete.
- Keep the hardening concrete continuously moist for at least 15 days. Spray the surface with water frequently throughout the day to keep it from drying out. The hardening of concrete is a chemical process that requires water. Drying before this curing process is complete will result in a weaker product.
| |
The Armature
The armature of the ceiling is made of arches of 3/8-inch (10 mm) rebar spaced 8 inches (20 cm) apart. The arches are crossed horizontally by 1/4-inch (6 mm) rebar, also 8 inches apart.
To mount the arches in the top plates, drill holes 1/2 inch (13 mm) in diameter and 1 inch (25 mm) deep at intervals of 8 inches (20 cm). To prevent rot, paint the insides of the holes with wood preservative before inserting the ends of the rebar.
In the corners of a hip roof or in the corners of a room where the ceiling will meet a partition, bend the 3/8-inch rebar at right angles and tie the corner to the hip arch.
If there is a skylight in the roof, cut the rebar and bend it upward 90o and fasten it to the framing members with staples.
Tie chicken wire to the rebar frame, two layers below the rebar and one above.
Staple the edge of the chicken wire to the side of the top plate.
Align the layers of chicken wire so that the wires line up as little as possible, making the open spaces as small as possible.
Applying the Mix
The cement mix is 4 parts of sand to 1 part of cement with only enough water to make a very stiff mix.
To the top of the chicken wire, apply a 1/2-inch (13 mm) layer of the mix, pushing it firmly into the chicken wire matrix. Lightly scratch the top surface of hardening concrete to enhance the adhesion of the second coat.
The application of the mix has to be backed up in some manner. One way is to have a second person beneath the chicken wire holding a board or a trowel at the point of application.
For large areas, it may be more convenient to back up the application with pieces of 1/4-inch (6 mm) plywood cut small enough to conform to the domed shape of the ceiling.
The day following the application of the first layer, apply a second half-inch layer of the cement mix, troweling it to a smooth finish on top. Once hardened, keep the concrete continuously moist for at least 15 days.
When the top two layers are complete, the ceiling is structurally strong, but the underside is still rough and aesthetically unpleasing.
Parge the underside with a 1/2-inch (13 mm) layer, forcing the mix into all the crevices and hollows. To keep the fresh mix from falling, it must be applied a little at a time, gradually building up the layer to the full thickness. Once the concrete is firm, lightly smooth the surface with a damp sponge.
The Finished Ceiling
After the concrete has cured for at least 15 days, it can be allowed to dry so that it can be painted.
Phil A. Arneson
El Ocotal
Municipio de Yuscar�n
Departamento de El Para�so
Honduras
El Ocotal
Municipio de Yuscar�n
Departamento de El Para�so
Honduras
Phil A. Arneson
El Ocotal
Municipio de Yuscar�n
Departamento de El Para�so
Honduras
El Ocotal
Municipio de Yuscar�n
Departamento de El Para�so
Honduras
It was a very good post indeed. I thoroughly enjoyed reading it in my lunch time. Will surely come and visit this blog more often. Thanks for sharing.
ReplyDeletehttp://clearviewsurfaces.com