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www.northernarchitecture.us/earthbag-...-building/

In addition to lateral support, buttresses (as well as curves) follow criteria for determining their height to width ratio. For every foot (30 cm) of wall height, add six inches (15 cm) of width to the wall, either as total thickness, or as a curve or buttress. In other words, the height to width ratio is two to one expressed as 2:1, height to width (Fig. 5.5).