Structural Loading, Framing, and Roof Geometry
Loads that matter here
Roof live load (typically 20 psf for ordinary roofs, IBC Table 1607.1), dead load of the covering, and snow. Ground snow load varies across the state - roughly the mid-30s psf near the coast, higher inland and highest in the Berkshires - and must come from 780 CMR Chapter 16, never a rule of thumb. Add drift loads at step roofs, dormers and parapets, and unbalanced or sliding snow where an upper roof sheds onto a lower one.
Dead load is where roofers get caught: a second shingle layer, tile instead of asphalt, or a later solar array can push a marginally sized rafter past its limit.
Framing and geometry
- Rafters are sized from the IRC Chapter 8 span tables for the species, grade, spacing and load.
- Rafter ties or ceiling joists resist outward thrust; collar ties in the upper third do not substitute for them.
- The bearing cut at the wall plate is limited to a notch no deeper than one quarter of the rafter depth, with minimum bearing.
- Slope factor converts plan area to sloped area: multiply plan area by the square root of (144 + pitch squared), divided by 12. At 6:12 the factor is 1.118; at 9:12 it is 1.25; at 12:12 it is 1.414.
Worked example
A 2,000-square-foot plan footprint at 6:12, with 10 percent waste. 2,000 x 1.118 = 2,236 square feet of sloped area, or 22.4 squares; with waste, order about 25 squares. Ordering from plan area alone under-buys the job by more than two squares.
Traps
Reading span from the ceiling-joist table instead of the rafter table; ignoring added dead load from a second layer or tile; and cutting a rafter notch deeper than one quarter of its depth just to make it sit flat on the plate.