Layers of a quality clay plaster built up to at least 6 inches (15 cm) thick make an excellent protective covering for an earthbag dome. Good quality clay has natural water resistant characteristics. It should be sticky and fairly stable. We mix this nice, sticky, stable clay with about a 60-70 percent sandy soil, like our reject sand (see Chapter 2 for more about reject sand) with a copious amount of long straw right out of the bale, 6 to 12 inches (15-30 cm) long (Fig. 13.2).
This base layer goes on a lot like cob. The main difference is the higher clay ratio. When clay gets wet it swells, inhibiting further water migration. The first one-half to one inch (1.25-2.5 cm) will soften and the finest clay particles near the surface will wash away exposing a gross network of deeply embedded straw.
Over this clayey four- to six-inch (10-15 cm) base coat, a second plaster coat gets smacked on about three quarters to one inch thick (1.875 cm-2.5 cm). A strictly natural earthen roof plaster will require periodic replastering, but the steeper the slope of the dome, the quicker it sheds water. The quicker it sheds
13.2: For a water-resistant earthen-base plaster, we increase the percentage of clay to sandy soil, pack it full of long straw, knead it into hefty loaves, and smack them onto the dome.
water, the quicker it dries out. Steeper domes translate into less plaster maintenance.
Something to think about when designing an earth-plastered dome is how to protect the windows from all the silty water that comes off the roof. We built dormers and sculpted rain gutters out over the arched windows and down the buttresses — this design also directs water away from the foundation. This functional aspect became its most pleasing esthetic attribute. Isn't it just like nature to combine function with form? (Fig. 13.3).
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