The differences between high density attached and tract style detached housing extend to the shop floor. Though truss designs may be similar, the project size, location and degree of repetition differs, influencing unit labor costs and material requirements. High density means “maximum square footage on minimum amount of land.” Traditionally this has been achieved via loft spaces in steeply sloped roofs. Recently though, even more living space is gained by extending buildings to four or five stories. Height restrictions often necessitate flat or nearly flat roofs. Truss manufacturing is affected in the following areas:
Materials
Higher lumber grades and/or LVL chords are needed for larger attic rooms and for shallower depth trusses. Fire retardant material may be required in urban areas. Since higher grades of treated lumber are not usually stocked at treaters, extra lead time may be required.
Cutting
Linear saws are clearly favored to cut the many short members in shallow trusses. Nesting the cuts of the web members below and cutting them from optimal board lengths can reduce material cost by more than 5% compared to a component saw. Since many of these short pieces (w5 to w13) look nearly identical to truss builders, ink marking at the saw saves considerable table time. More advanced marking is also available that identifies the “face-up” of the board, the top-chord end of the piece, and the truss plate locations. This refined identification minimizes material handling – enabling pieces to be placed in the right position and orientation the first time.
Fabrication
Building large quantities of short roof trusses on a 14-foot table and conveying them on 14-foot exit rollers seems like an incredible waste of floor space. Given that height restrictions and contemporary architectural requirements continue to increase the volume of these low-rising trusses, alternate fabrication methods are worth exploring. For example, an 8-foot wide single pass truss table can fit into a narrow space against an exterior wall or column-line. Its two-man build crew can easily hand stack and band trusses on a cart in an aisle adjacent to this table, while staying under roof cover. This table can greatly increase the overall plant efficiency, as the main gantry lines focus on a more consistent mix of larger trusses.
Contrarily, attic room trusses, which utilize the full width of a gantry table, can be even more labor intense. Though automated jigging may help maintain level ceilings, manual jigging is often needed to insure that room walls are plumb. In addition, code-required insulation depths often require frame downs between collar-ties and room walls, as shown here. Additional studding is needed in the end truss. Note the skillful use of a “K-web” configuration at the intersection of the floor with the room wall. Though a solid 1.5 x 16” x 28’ LVL bottom chord member would eliminate 16 of the 28 pieces in this design, its added material cost would far exceed the labor savings.
Next Month:
Material handling opportunities