Calculate the required spacing for roof rafters supporting a metal roof system with 40 psf live load and 15 psf dead load, using 2x8 Douglas Fir rafters with 16-foot span.
Correct Answer
A) 16 inches on center
With total load of 55 psf (40+15) and 16-foot span, 2x8 Douglas Fir rafters require 16-inch spacing to adequately support the load according to span tables.
Why This Is the Correct Answer
Option A (16 inches on center) is correct. The total design load is 40 psf (live) + 15 psf (dead) = 55 psf. For 2x8 Douglas Fir No. 2 rafters spanning 16 feet under a 55 psf total load, standard span tables indicate that 16-inch on-center spacing is the maximum allowable spacing to keep deflection and bending stress within code limits. At 24-inch spacing the rafter would be overstressed at this load and span combination.
Why the Other Options Are Wrong
Option B: 24 inches on center
24 inches on center is too wide for this load and span combination. At 55 psf total load over a 16-foot span, a 2x8 rafter at 24-inch spacing would exceed allowable bending stress and deflection limits. 24-inch spacing is appropriate for lighter loads or shorter spans with this member size.
Option C: 19.2 inches on center
19.2 inches on center (a spacing used in some engineered lumber layouts and metric-based systems) is not the answer yielded by span tables for this specific load/span/member combination. While between 16-inch and 24-inch, it is not the code-table result for these conditions.
Option D: 12 inches on center
12 inches on center is unnecessarily tight and would be over-designed for this situation. While it would certainly be structurally adequate, span tables do not require this close spacing for 2x8 Douglas Fir at 55 psf and 16-foot span. Selecting 12-inch spacing indicates misreading of span table values.
Memory Technique
Step 1: Add the loads (40 + 15 = 55 psf total). Step 2: Use that total with the span (16 ft) and member size (2x8) to look up the span table. Heavier total load → closer rafter spacing. Think of it as: more weight needs more support points.
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