Building a Passive Solar Home: Part V

Follow an architect through the planning, pouring and pounding involved in building the Sun Cottage structural frame.

| January/February 1984

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    The Sun Cottage is a standard design applying low-cost building techniques to a passive solar home.  
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    The concrete roof slab and long span roof design for the Sun Cottage passive solar home.  
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    The heavy timber roof design for the Sun Cottage passive solar home.
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    The post-and-beam frame design for the Sun Cottage passive solar home.

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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 EARTH NEWS has long been exploring ways of breaking this vicious circle of waste, and one inventor of solutions 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 EARTH NEWS, 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. 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.

Concrete Roof Slab 

Spans of cast-in-place concrete roofs are generally limited to a maximum of 12 feet, but aren't as difficult to form properly as you might imagine. I recommend a product called Epicore II (made by Epic Metals Corporation) to help owner-builders form concrete roof slabs. This deeply corrugated, heavy-gauge metal decking acts as both formwork and slab reinforcement for simple spans. The exposed metal ceiling can be painted, and has deep grooves for service laterals. During construction, Epicore II can support workers and wet concrete, provided that shores (supports) are placed every 6 feet. And it can generally span 12 feet, using a 4" slab reinforced only with wire mesh. This is a pre-engineered system that's fully documented by the supplier.

As you may recall from the Part IV discussion, the concrete belt beam at the top of block walls can be omitted if you use concrete for your roof, because a roof slab acts as a diaphragm to stabilize the top of the wall. [EDITOR'S NOTE: Many terms used in this article are defined in the Passive Solar and Earth Sheltered Home Building Glossary.] To link the walls and roof together, bend No. 4 steel bars so that they reach from grouted block cavities into the roof slab (as shown in Fig. 1). As you position wire-mesh reinforcement, you can also place horizontal service runs that you might want to cast into the slab.

Long Span Roof 

If you require a span longer than 12 feet to accommodate an open interior or for flexibility in locating partitions, a commercial system may be utilized to efficiently bridge up to 30 feet with earth loadings. Long-span, steel bar joists can go from the rear wall to the front facade, where they'll rest on a steel beam supported on steel pipe columns. This method requires welded connections but allows large window and door openings along the southern exposure. The steel joists are generally spaced on 2' centers (see Fig. 2) and must be sized according to span. (For most applications, though, you can expect them to be about 18 inches deep.) A thin concrete slab may be cast on corrugated or 3/4" plyform decking over these joists. When laying plywood decking, use H-clips and 1/16" clearance to allow for expansion between boards.



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