Structural and Framing Fundamentals
Loads, spans, and why it matters
Roofers must understand dead load (weight of materials), live load (snow, wind, maintenance), and deflection limits. Know that adding tile or a second layer of shingles increases dead load and may exceed the design capacity of the framing. Questions often test span tables - rafter size vs. spacing vs. span - and the snow load and wind uplift requirements in the adopted code.
Know the parts: ridge, hip, valley, rafter, collar tie, ceiling joist, purlin, and fascia. Understand rafter length calculation using the Pythagorean theorem or the slope factor: rafter length = run × slope factor, where slope factor = √(1 + (rise/run)²). For a 6:12 roof, the slope factor is about 1.118; for a 4:12, about 1.054.
Traps: confusing rise/run with pitch percentage; forgetting that collar ties are not the same as rafter ties (collar ties resist uplift in the upper third; rafter ties resist outward thrust at the bottom); and assuming a ridge board carries load (it typically does not - it's a non-structural alignment member unless designed otherwise).
Worked example: a 24-foot-wide building with a 6:12 gable has a run of 12 ft. Rafter length = 12 × 1.118 ≈ 13.42 ft, plus overhang. Two rafters plus ridge make the total span; verify against the span table for the chosen lumber and spacing.
Rule of thumb: never add load to a roof without checking the framing - and never assume the existing structure was designed for the new material.