The Threat of Sweat: How One Earthen Roof Works
As a child growing up in northern New Mexico, I tended to overlook some things in my native environment that visitors found incredible. I remember cousins from Kansas who upon first seeing a stack of adobe bricks commented that nothing substantial could possibly be built with dirt. They had seemingly fallen asleep in high school during the part about sod houses and barns on the Great Plains. I also recall being offended when a relative laughed at the weeds and cacti growing out of the roof of our house. In retrospect I understand their incredulity and in the intervening years have myself wondered how an earthen roof with an Upper Sonoran garden growing out of it might function. Until a few years ago I had not taken the opportunity to investigate the technology. An extraordinarily wet year in northern Arizona, however, prompted an invitation in April 1993, to assess water damage at one of the Native American villages in the region.
At the invitation of the owner of a stone house that appeared to have been essentially unaltered in several hundred years, I excavated a small test pit on the roof. The results reveal some interesting data and lead to a rather surprising conclusion: It is not moisture from without that is the primary enemy of the building, but moisture from within.
The top 1-1/2 inches of the 8-inch system is composed of loose, granular topsoil that is sandy and drains well. Under it is a system of three clayey soils of different compositions which together comprise the next inch-and-a-half. The moisture content in the soils tells all: The well draining topsoil was nearly dry; the sandy clay beneath contained over 13% water and seems to have been placed where it is because of its water-retentive characteristics; beneath it, a layer of purer, less permeable clay retained less than 3% moisture. It would seem rain and melting snow penetrate the sandy soil until saturation occurs and runoff from the built-in slope to the rough-hewn canales occurs. The remaining water is slowly absorbed by the sandy clay, which, in a repetitive cycle, releases the moisture into the porous, rapidly drying, topsoil above.
So much for the weather. I was puzzled to find more than four inches of sandy soil beneath the almost impermeable clays. Had I not been there in the springtime, or had that soil been dry, I may not have the same thoughts. But over 9% moisture in the soil, and nearly 16% in the inch-thick brush mat supporting it, seemed to indicate that the real threat, (and an appropriate response) came from within the house.
Though semi-arid and generally warm, northern Arizona experiences moderate to severe cold for about five months out of the year. During that time people tend to be enclosed in their homes, enjoying life around the hearth. Trips outside result in a net importation of moisture; from the damp wood, the soaked clothes, mud on the shoes, the wet grocery bag, plus the humidity expelled from breathing, may localize more moisture than is produced by annual precipitation. Without knowing the various statistics, (average amounts of moisture produced by cooking such-and-such for so many people who make that many trips outside to feed so many children and fires) I would guess from the architectural response that the greatest threat to the building results from vapor from human bodies and its various support activities.
The sandy soil just above the brush mat is a manipulated version of the topsoil with just enough clay added so that it retains moisture. During the five "inside" months this sandy soil captures the warm, damp atmosphere that is the greatest threat to plasters, vigas and other wooden elements, and slowly disperses it over the other seven months when the windows and doors are open. The sandy soil's mass and absorbability are more than double that of the system designed to keep the weather out because the greater threat came from within.
Read about how Crocker Ltd used this technique at Hutmacher Farm in North Dakota.