A Ferrocement Dome Structure

 español

To make a relatively inexpensive small structure to serve as my workshop, I built a dome of ferrocement. Because I am using rainwater as my sole source of water, I designed it to function also as a rainwater collector. To maximize head space for a given footprint, I used a parabolic shape rather than a hemispherical one.

 

To calculate the height of the parabola at different distances from the center, I used the formula

 y = ax² + c

where y is the height above the floor, x is the radius of the circle at floor level, a is a shape parameter, and c is the height of the axis of the parabola (where x = 0).

(This is derived from the general equation for a parabola, y = ax² + bx + c, where we set b = 0 to make the parabola symetrical about 0.

To estimate the parameter a, set the value of x to what you want as the radius of the circle on the floor (where y = 0) and c to what you want for the height of the paraboloid (where x = 0). Then a = - c/x².

 








Using the formula y = ax² + c, I calculated the height y at half-meter intervals of x, beginning at the center (x = 0) and cut narrow boards to that height and nailed them vertically to a frame to make a parabola-shaped form. We then laid a 3/8" rebar over the top so that it touched the top of each board. The bottom ends of the rebar were tied to a short piece of rebar embedded in the concrete footing at each corner. 









 

With the first rebar tied in place, we turned the form 90 degrees, placed a second rebar, and tied it to the first.


We then continued around the circle to place 4 parabolic arches (8 ribs) and fixed them in place by tying horizontal circles of rebar at intervals from the top. To avoid stacking too many rebars at the center, the rest of the ribs were cut slightly short of the center and tied in place between the others. The bottom ends of the ribs were tied temporarily to rebar stakes driven into the ground in a circle of the same radius as the corner rebars. 











 



Arches of rebar were then placed parallel to the footers for each wall, the first one vertically directly over the footer, and the others leaning slightly outward.  








 



The ends of the parabolic ribs were then released from the temporary stakes in the ground, and the rods were bent upward and tied to the arches to form the "wings." 










 



With the form of the dome complete and all the rebar securely tied together, three layers of 1" chickenwire were tied to the rebar frame, two layers above and one layer beneath the rebar.







 


Care was taken not to align the openings in the chickenwire so that the holes remained as small as possible.













 









The rebar for the vertical walls was then tied in. Extra reinforcing was done around the windows and door.








 



















Three layers of chickenwire were also tied to the walls, two outside and one inside.
 

 





With all the chickenwire tied firmly to the rebar frame, the parging was begun, first the vertical walls and then the dome. The parging was done with a stiff mix of 1 part of Portland cement to four parts of fine sand. It was applied in four layers, each about 1 cm thick, two layers on the outside and two layers on the inside. Care was taken that the joints between batches on the bottom layer did not coincide with the joints between batches on the top layer. After it had hardened, the parging was kept continuously wet with a fine spray of water for seven days.

 









With the parging complete, the top ends of the rebar were welded to form a security grate, leaving a skylight for ventilation and to admit light.














 




The four corners were made into funnels to collect rainwater and direct it downhill through a 4-inch pipe to a 10,000-liter tank.







 




An acryllic plastic cap was placed over the skylight, leaving a space for ventilation. 






 



Notice the "rain shadow" on the wet concrete after a rain.








 

Since this is a workshop, steel doors and windows were installed for security to keep the tools from "walking." The roof was sealed with a Portland cement/acryllic sealer, and the exterior was painted with an exterior latex paint.