"Minimum Framing"

Our structural framing systems borrow ideas from various sources - the National Association of Home Builders' (NAHB) "optimum value engineering" framing system, the "Minimum Resource House" developed by Madison architect Bruce Kiefer in the 1970's, Wisconsin's Forest Products Laboratory, and the Illinois Small Homes Council's work in the 1960's. All share a common goal: to use only the minimum amount of wood necessary to achieve a strong structure. This is important as our supply of good-quality framing lumber dwindles and prices rise. Using the 'optimum' system diagramed below we needed 1,064 board feet of framing lumber for the four exterior walls of the Affordable Natural House. To frame the walls using conventional framing practice would have required 1,594 board feet, about a third more.

Conventional wall framing • More wall studs • Large headers • Windows mulled together • Meets building code

Optimum wall framing • Fewer wall studs • No headers • Windows between studs • Meets building code

 

Above: The north wall. Note the notches in the tops of the 2" x 6" (5 x 15 cm) wood studs, ready for the let-in ledger. Notches for the ledger are cut all-at-once while the studs are stacked in a pile.

Above: The south wall, raised into place. This wall will have 8 windows, yet there is no need for any window headers. Our carpenters learned the system readily.

In conventional construction, a double top-plate is used to tie together sections of wall, and as a bearing surface for floor and roof framing, We use a single top-plate, and instead turn the second plate into a ledger, notched into the wall studs. This allows us to span over doors (which of course are wider than 22 inches), and to occasionally "cheat" a bit and have slightly wider windows in selected locations, without the use of headers.

With this framing system we accept a limitation, namely that all our windows units must be about 20 to 24 inches (50 to 60 cm) wide. In return, we save most of the wood normally required to carry the loads over wide window openings, the extra studs and headers made from old-growth trees or expensive engineered wood.

Left: All the major structural elements are laid out to align with each other, spaced 24" (60 cm) apart. Each roof truss and floor joists sits directly over a wall stud.

It gets better -- we also avoided all exterior wall sheathing (the exterior cladding that covers the framing, underneath the siding). In conventional construction in our area, wall sheathing is usually either plastic insulation or a wood panel product such as plywood or oriented strand board (OSB). Instead, the infill of straw-clay insulation takes the place of the sheathing, bracing the wall studs laterally and providing a surface ready for application of stucco or siding. For squaring the walls during construction, we used 1 x 6 let-in diagonal boards. Using this system we avoided purchasing 27 sheets of wall sheathing, or 864 square feet (81 sq. meters) Finally, our other framing minimizes the use of wood too -- roof trusses made of small-dimension lumber, and engineered floor joists made from small trees.

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