Select Polycarbonate.-.Quote & Order.-.Guide & Samples

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

• Photo of Greenhouse with Brace Board • Layout for 48" wide Standard Poly
 
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