Rafter Span: Spans may be increased by using brace board, see below.

 

Cap Only System: Use 24-1/8" or 48-1/4" spacing with 4 ft. wide poly. 36-1/8" for 6 ft. wide. Load shown in lbs./sq. ft.

Base & Cap System: Use 24-3/8" to 24-1/2" or 48-3/4" to 49" spacing with 4 ft. wide poly. 36-3/8" to 36-1/2" for 6 ft. wide. Load shown in lbs./sq. ft. .

• Photo of Greenhouse with Brace Board

 

Source: Add-On Solar Greenhouses & Sunspaces by Andrew M. Shapiro

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Spans for #2 Douglas fir/larch. Roof slope (7/12 pitch) or less. For steep roof slopes see bldg. dept. for deductions in snow load and rafter size. Other woods, see Notes. 

 

 -Reload Sundance Supply Site