Protecting the Crown
Earthen buildings that do not have overhanging roofs present a particularly difficult conservation problem.
Being three-dimensional, parapets require a more complex approach to preservation than mono-dimensional wall surfaces. The outside of a parapet is exposed to wind and rain; the top is subject to wind, rain, piled snow and occasional traffic; and the inside, adjacent to the roof, gets all of the above with the added liability of a junction with dissimilar materials. There are many forces that work to corrupt and usurp the crown.
Many inventive (and expensive) chemical solutions have been tried in attempts to solve the parapet problem, and none can really be termed successful. Years of research and experimentation by petrochemical engineers attempting to "stabilize," "preserve," and (my favorite) "impregnate" adobe have resulted in some spectacular effects.
Historic walls capped with multiple layers of carefully contrived designer muds have responded by rejecting the exotic mixes as vehemently as they reject Portland-based plasters. Caps have exploded in freezing weather, untreated walls beneath treated tops have eroded leaving mushroom pedestals, and vertical surfaces preserved like cheese have fallen off in sheets. Adobes impregnated with everything from polysiloxanes to asphalt emulsion have led to shotgun weddings between scientists and more traditional craftsmen called in to make the situation right.
If you opt for an exterior earthen or lime render for practical or aesthetic reasons and rightly reject the idea of petrochemical amendments, are you stuck with frequent maintenance? Fortunately not, here are a few examples that illustrate why that is so.
At the pueblo of Acoma, New Mexico, the builders understood the vulnerability of adobe parapets and responded in the most logical manner possible: They eliminated them. Many of the buildings at Acoma have extended vigas supporting a short overhang. The overhang is decked with lumber or latillas and supports a short, unplastered native stone parapet, which is naturally weatherproof. The overhang protects the vertical surface of the wall below. A practical solution to be sure.
At Jacona where the appearance of the historic buildings we worked on would have been substantially altered by an overhang, we selected a copper cap. Being malleable, copper molds easily to the wall, and shallow channels on either side can be adjusted and bent to dump water at designated spots. The result is a maintenance cycle that is at least 10 years (we are at 13 now) between minor touch-ups and who knows how long before a complete re-mudding. The cap is unobtrusive and reversible, important components when dealing with historic buildings.
Another good solution is the one we implemented on the church at Questa with our colleague Pete Mold from England who, among other projects, lime-plastered the reconstructed Globe Theater in London.
We imported Pete to design a hydraulic lime mix that was compatible with the adobe walls. "Hydraulic" means that the water used to mix the plaster does not actually dry out; it "sets" by becoming incorporated in the crystal of carbonated lime. Covered with a lime wash, the plaster chemically becomes stone and is resistant to weather while remaining vapor permeable -- a prized characteristic for plasters on earthen walls.
It is, of course, the resistant part that is key and we note that winters in Questa are nothing compared to the freezing, North Atlantic saltwater gales against which Pete customarily battles.
Finally, careful flashing at the junction of parapet and roof will help keep water at bay. We prefer a custom made, snap-together reglet system, which allows the removal of the flashing when it is time to re-roof.
Read a lively discussion of this article here.