Straw Bale Design and the Pueblo Style Home
By Chris Keefe
Straw bale builders, especially in the Southwest, are often asked to build Pueblo style homes. Owner builders seem to like the style as well and often site the flat roof as a reason why. It is often perceived to be a simple roof structure with little framing experience necessary. That is true, to a point. The roof of a pueblo style home is a bit misleading. In actuality, it is not flat. The roof itself has a slope to it, usually a 2/12 or something comparable.
The illusion of flat is created by the parapet walls around the roof. These are the small walls that rise up above the plane of the roof, and create the architectural detail of the “roof line,” i.e. the notched or fortress type look. Behind the parapets is a sloping roof that drains water to specially designed drains called “canales.” These canales extend out beyond the edge of the building by about one foot or so and direct water away from the structure. It is important that the canales have sufficient extension beyond the parapet to insure water is directed away from the walls and from the foundation. If possible, direct the water to a cistern and use it for irrigation when the rains subside.
Typically roofs with a slope less than 3/12 need special attention to ensure proper drainage. Regular roofing shingles will not work as the slope may allow water to blow up under the shingles. Years ago, the best option was a built up roof in which numerous layers of tar paper were torched down to the roof with alternating layers of melted bitumen or tar pitch. This practice is still used; however, it is not the most popular option available. Just about all of the options available today employ the use of a single-ply membrane. All of these membranes are fairly harsh when it comes to their environmental impact, so pick the one that will last the longest for your application. The options include EPDM (ethylene propylene diene monomer), CSPE (chlorosulphonated polyethylene), PVC (polyvinyl Chloride) or reinforced PVC, and MBR (modified bitumen, reinforced) membranes, among many others. The most commonly used are EPDM and CSPE membranes.
Follow the manufacturer specifications for the proper installation of the material. Pay great attention to the flashing around the edges of the parapet walls and any openings in the roof like plumbing vents or skylights. Be sure to cap the top of the parapet wall with a piece of the membrane run up from the roof AND a metal cap on top of that. Details of the flashing can be found in diagram A below. Finally, be sure to lay a water proof layer over the top of the bales or attach it to the bottom of the box beam. This can be 30# roofing felt paper or EPDM material. The idea is to direct any leaks that do get in (hopefully there will never be any) away from the top of the bales. The water will hit the barrier, run down the side and make a wet mark on the plaster, telling you there is a problem that needs to be addressed.
The biggest weak spot I see in a traditional pueblo style home is the place where the vigas project from the side of the structure. Vigas are the ends of the beams that extend about a foot beyond the plaster. In many modern pueblo style homes, the vigas are ornamental and are not actual structural beams. Whether structural or decorative, they carry the same drawback. The risk is that the wood vigas will eventually shrink and the plaster joint will no longer be tight, enabling water to seep in through the plaster joint. A lot of attention needs to be placed on water proofing these joints. Many new pueblo style homes do not have vigas at all. Instead, they have the typical parapet walls with architectural detailing at the top. This is probably the safest way to go; however, it is possible to run vigas if you really want them.
Be sure to completely flash the vigas at the wall with self adhesive flashing. Flash like it is a window: start at the bottom, then add the sides, then the top, overlapping each transition by at least three inches of material. Place metal caps on top of each viga to protect the wood. Finally, add a metal head flashing to protect the joint at the wall. Proper caulking may make all the difference in the World as the plaster joints shift. You will need to watch them over the years and make sure the plaster joints are as tight as possible and properly caulked.
As long as you create the roof with enough pitch to drain properly and you install the correct roofing and flashing, Pueblo style homes can be a great option for straw bale construction. As always, be sure to flash your window and door openings and any other projections through an exterior wall. With enough attention to detail, you can create a beautiful pueblo style home without having to worry about water damaged bales.
ABOUT THE AUTHOR
Chris Keefe, from an early age, discovered his creative spirit in art. In 1996, he received a B.A. in Liberal Arts focusing on drawing, Music and Philosophy from San Francisco State University. In tandem, he was actively involved in a grassroots environmental project for five years at the University of California in Berkeley. He became interested in the field of straw bale as he began his graduate studies in 1999 at the San Francisco Institute of Architecture. Focusing to integrate his work in the environmental and sustainability field with his creative imagination, he received a Master of Architecture and Ecological Design in 2001. Soon thereafter, he founded a company called Organicforms Design which offers ecologically and artistically based design utilizing natural and sustainable materials. Since then, he has worked and completed several exciting design/build projects in Southern Oregon. In 2002, he began to focus primarily on straw bale research and design as the lead designer on the innovative project, The Straw Bale Village in Jacksonville, OR. Please visit his website at www.OrganicFormsDesign.com
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