Mother's $10-per-Square-Foot (or Less!) Earth-sheltered House: Part II

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FRONT ROOF

We used homemade Hitch-plate (or "gluelam") beams for our rafters. Since they consist of a section of plywood sandwiched between two conventional boards (thereby adding the plywood's multidirectional strength to the more one-directional capability of the boards), they're quite strong. Such beams are approved for construction practically everywhere, but some building codes stipulate that they may be designed only by architects or engineers. Fortunately, we're without such restrictions, so we consulted load tables for Hitch-plate beams from Architectural Graphic Standards to determine an appropriate spacing for the rafters in our building. A good deal of conservatism is built into the tables, and we introduced a generous amount of our own by using 48" maximum spacing to allow for easy sheathing and drywalling.

Using glue and nails, we built up our Hitchplate beams from lengths of 2 X 10 secured around 3/4 ", or doubled 1/2 ", plywood cut to size. The main north-south beam on the front roof drops from the top of the east plate (over two jacks; in the central post) to a point just east of the front-door opening at a 1-in-12 pitch. This, and each of the other southwest rafters, was bird's-mouthed' approximately 3/4" deep and 8" wide where it rides on the sill. Once we got that initial beam in place, we removed our temporary bracing for the east-west bisecting wall to make the floor less cluttered.

Next, we prepared the cross rafter to which the three southwest jack rafters are attached. This was done by cutting out a section of 2 X 10, mitered` at both ends, long enough to penetrate the area above the west bedroom header and rest atop the 2 X 12 blocking. This board was notched to fit into the slot above the blocking. To this first cross rafter, we attached a ledger' strip cut from a piece of two-by lumber . . . then each jack rafter was notched to hang on this ledger.

All three jack rafters have different compound angles, requiring double cuts where they intersect the cross rafter. The additional notching needed to fit the ledger, however, increases the number of saw cuts on those ends to four. The portions of the jack rafters that overhang the exterior wall were then bird's-mouthed, the beams were set in place, and their inside ends were nailed through the cross rafter. When all three were in position, we added two more cross rafters for reinforcement, nailing them into the back of the first. Then we went ahead and installed some of the 2 X 10 bridging to keep the rafters square.

Each of the four rafters that span from the east bisecting wall top plate to the front of the building has a different pitch . . . and the result is that the roofline describes what is called a hyperbolic paraboloid as you move from the center to the southeast. Because all four of these rafters were to bear on the beam that would span the greenhouse opening, the next step in construction was to build up that beam from five 2 X 6's on edge, with 2 X 8's lying flat on the top and bottom. We used a pressure-treated 6 X 6 post in the center of the beam's span for support, and finished the horizontal member with rung oil to enhance its appearance.

Because the four southeast rafters bear flat (with appropriate bird's-mouths) on the east bisecting wall top plate, they arrive at the front wall tipped sideways at a 5° angle . . . representing the 1-in-12 east-west roof pitch. Consequently, the bird's-mouths where the rafters ride on the greenhouse beam had to be cut at compound angles, with the tipping-angle cut being more pronounced than the bird's mouths themselves. It's important to cut these compound angles into the ends bearing on the greenhouse beam, rather than the ends on the wall top plate, so that top and bottom surfaces of the rafters run parallel to the sheathing and interior finish materials.

Bridging these four rafters with 2 X 10's was a fairly time-consuming chore, since there were a number of different angles involved. For this job, our carpenters found a Squangle nearly indispensable. This tool, made by the Mayes Brothers Tool Manufacturing Company of Johnson City, Tennessee, allows one to accurately reproduce odd angles while maintaining plumb with a built-in bubble level.

Once we'd finished the bridging, we moved on to getting the front half of the roof overhead. On the southeast portion of the roof, full-width pieces of plywood could be used between the 4'-spaced rafters, but the area over the radial rafters on the southwest roof required a number of triangulated cuts. This proved to be a blessing in disguise, though, since we were able to find some salvaged plywood, which had been cut into parallelograms, for a very reasonable price. On our roof, working with the odd-shaped material was no disadvantage, and we saved at least $2.50 per sheet.

We used salvaged roll roofing (lapped 18" and sealed with cement) for the top layer, though we seriously considered using a sprayed-plastic coating suggested to us by architect Angus Macdonald. (See page 150 for more about Angus' work.) The roll roofing won out in the end, simply because it was so inexpensive . . . however, we had little doubt that the plastic would outlast it by a wide margin. An additional potential benefit to using the plastic is that polyurethane foam insulation can be sprayed on beneath it, thereby eliminating the need to install fiberglass insulation between rafters. Furthermore, the plastic can be continued on down earth-bermed walls as a structure's waterproofing, forming a continuous sheet over and around the building. In this aspect of construction, as in so many others, one must weigh savings against durability and the labor involved.

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