Now is the time to consider Spruce-Pine-Fir (SPF) as a substitute for Southern Pine (SP). As noted below, #2 SPF’s 19% higher bending value and only 10% lower compression value usually enables longer spans. Cull rates are generally lower with SPF, and it is lighter and easier to handle. [For chart and diagram, See PDF or View in Full Issue.]
Since the relative prices of SPF and SP often diverge, consider using hybrid lumber as shown in the box on the drawing. On this 40-foot truss, the lower of the structural values of SPF and SP are used in design calculations, so that either grade of #2 lumber may be used. On the truss chords (see chart), only Fc, the compression value, is lower for #2 SPF. However the considerably higher bending value more than compensates. On the web members, only Ft, the tension value, is lower for SPF Stud (by only 4%). However, Fc, the compression value, is 36% higher. One caveat applies – the truss plate design values for SPF are lower than SYP which may raise plate cost – but consider also whether smaller plates would be prudent on a 40-foot truss.