Roof Systems

There are a zillion styles and methods of building a roof, many of which can be adapted to sit on earthbag walls. Our job is to show some techniques with which to anchor the roof to the walls and share roof styles that we feel complement the earthbag system. We are big fans of Native American architecture as well as vernacular architecture worldwide. All we need to do is look at how indigenous peoples built their homes to suit their environments to see what design features we, too, would find practical. Native American styles vary dramatically from earth lodges and tipis of the Great Plains to the majestic timber and plank houses of the Pacific Northwest.

9.1: A colorful variety of asphalt shingles turns this roof into a work of art.

9.2: The bond beam must be continuous, covering the full perimeter of the wall.

The most obvious consideration is designing a roof that protects the walls appropriately for the climate. Longer eaves are called for in a wetter climate. Dry climates can take advantage of the use of parapets and vigas (log beams) commonly seen in the Southwest. Moist climates are natural watering systems for a living roof, while in a dry climate the roof can be used to harvest precious rainwater.

Due to historic use, building codes in the Southwestern United States include structural design standards that pertain specifically to earthen architecture, many of which we have adopted to earthbag construction. Modern adobe and rammed earth buildings require a continuous bond beam built of either wood or concrete installed on the top of the finished earthen wall. The bond beam acts as a tension ring that ties all the walls together into one monolithic frame.

overlaprebar around corners 3-4 feet with tie

6" high by width of ^ wall overlaprebar around corners 3-4 feet with tie

6" high by width of ^ wall

tire wires twisted in center 1 with nail and hammered into bag keeps tension in — between boards

9.3: To anchor the bond beam to an earthbag wall, 16-inch (40 cm) long, #4 reinforcing bar (rebar) is driven 12 inches (30 cm) into the green (uncured), tamped earth-bags at a maximum 20 degree angle, at least 4 inches (10 cm) in from the outer edge of the bag, and staggered at 24-inch (60 cm) intervals.

tacked on top of fofm boards. Wires around spacers suspend rebar at center of form

9.4: A minimum of two continuous #4 steel reinforcing bars are suspended in the form to provide tensile strength for the concrete.

A concrete bond beam is like a foundation on top of your walls. Codes vary from state to state, but typical dimensions are six inches (15 cm) high by the width of the wall. Most concrete bond beams are poured into wood forms that have been built on top of the walls for this purpose and then removed after the concrete has cured. The bond beam is secured to the wall by the opposing angles of rebar, thereby preventing uplift of the roof caused by high winds (Fig. 9.3 & 9.4).

Bond beams can also be built of wood in the same dimensions as a concrete bond, using either massive solid timbers or laminated lumber. A version of the adobe and rammed earth building codes can be obtained from the Adobe Builder, an architectural trade journal that publishes a book for adobe codes and one for rammed earth codes (Fig. 9.5, 9.6 & 9.7). (Check the Resource Guide in the back of this book).

install J-bot while concrete is wet to anchor 2"X 6" or 2"X 8" wooden plate install J-bot while concrete is wet to anchor 2"X 6" or 2"X 8" wooden plate

9.5: The bond beam also provides an anchor for attaching and distributing the individual weight of the roof members, whether they are rafters, trusses, or logs. In some cases it may double as lintels for the window and door openings.

9.6: A 2 x 6-inch (5 x 15 cm) woooden frame used as the concrete bond beam formwork can be left in place, and does double-duty as an attachment for the roof rafters.

9.7: For curved walls, thin, flexible Masonite can be used as a form.

An alternative to the heavy wood bond beam prescribed by code is a light wood ladder roof plate anchored to the top of the wall with poly strapping cinched tight with a tensioner device (Fig. 9.8). (Refer to "Velcro Plates" in this chapter for more information on poly strapping and tensioners).

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