Building the Sun Cottage Part V: The Structural Frame

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Follow an architect as he leads us through the planning, pouring, and pounding involved in . . .

All too many of us have found that building an energy-efficient home seems to be a dream . . . a fantasy that's kept just out of reach by escalating prices and high interest rates. Of course, MOTHER has long been exploring ways of breaking this vicious circle of waste, and one inventor of solutions—whose work we've shown you before (starting back in issue 67)—is architect Angus W. Macdonald. Angus has developed a number of housing designs that apply low-cost building techniques to passive solar, earth-tempered homes . . . and he's agreed to relate, in a series of articles that will span at least six issues of MOTHER, much of what he's learned about planning and building such structures. The series is following the actual construction of one of the architect's standard designs . . . Sun Cottage.

As we advance toward the final construction stages of your earth-tempered, passive solar home, the choice of building techniques broadens considerably. In the last issue of MOTHER, Part IV of this series discussed the two ways of erecting earth-sheltered walls (from either block or poured concrete), but there are at least four good options for the structural frame of your home. A deck of reinforced concrete may be cast to form the roof . . . steel bar joists, with concrete or heavy plywood decking, can be used to make earth sheltered roofs with long spans . . . heavy-timber roof framing can be set on structural masonry partitions, allowing the use of wooden decking . . . or a heavy-timber, post-and-beam framework of oak may be used in conjunction with tongue-and-groove (T & G) decking to provide an attractive, rustic interior. In this installment, we'll explore some of the practical options for the owner-builder, in hopes of helping you decide which technique best suits your situation.

As you consider the design of your roof, bear in mind that earth sheltering imposes extreme loads on a structure. Not only must the dead weight of water-saturated earth (with a possible surcharge of snow and ice) be considered, but large live loads, such as animal traffic and the weight of plantings, also need to be taken into account. Engineers plan on a total possible load of from 230 to 250 pounds per square foot (PSF) when figuring earth-sheltered roof capacity. And a heavier structure, such as one with a concrete-slab roof, must actually be designed to carry at least 250 PSF because of the weight of the concrete itself.

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