An attic with limited storage is designed with an additional 10 pounds per square foot live load compared to an attic without storage. Verify the storage capacity of truss-built attics with the truss engineer before using the attic for storage.
Roof Live and Snow load is less than (<) 30 PSF. Use the rafter snow load tables in areas of the country with ground snow loads of 30 PSF or more. Verify the design ground snow load with the local building official.
Rafter dead loads Use the 10 PSF rafter dead load columns when using one layer of roof coverings such as fiberglass shingles and wood. Use the 20 PSF rafter dead load columns when using roof coverings such as tile and slate.
Ceiling Joist and Rafter Deflection
1. All ceiling joist spans in the tables use L/240 deflection, where L is the length of the joist or rafter in inches divided by 240.
2. All rafter spans in the tables use L/180 deflection.
Ceiling Joist and Rafter Span Tables
1. Use the following tables to determine the maximum unsupported horizontal distance that ceiling joists and rafters can span.
2. Refer to the IRC or to the AF&PA Span Tables for Joists and Rafters to determine spans for lumber widths, species, grades, and snow load factors not in these tables.
3. These tables apply to roof systems framed using standard 2 inch (nominal thickness) dimension lumber. An engineer must design roof truss systems.
Rafter Span Measurement and Span Adjustment for Ceiling Joist Location
1. Measure common rafter spans horizontally beginning at the exterior wall and ending at the ridge board. Measure jack rafter spans horizontally beginning at the hip or valley rafter and ending at the ridge board. Do not measure rafter spans along the actual length of the rafter.
2. The rafter span tables assume that ceiling joists or rafter ties are located at or near the wall top plate. If the ceiling joists or rafter ties are located higher on the rafter, reduce the rafter spans in the tables using the following method. Measure the height of the ceiling joists or rafter ties above the top plate. This is the (HCJ). Measure the height of the ridge board above the top plate. This is the (HRR). Divide (HCJ) by (HRR) to calculate the ratio of the vertical height of the ceiling joists or rafter ties (HCJ) to the vertical height of the roof ridge (HRR). Reduce the rafter spans by the factor in the following table.