How far can I span a girder or rafter without an intermediate post?
There’s no official answer. Lumber span charts are for weight-bearing structures like floors and decks. A pergola must only hold itself up and resist sagging. Nevertheless: • I’ve seen a solid 4 x 8 spanned about 18 ft sag a full 3 in. in the middle. No joke, it looked like a smiley face. I’ve also seen double 2 x 6s spanned 16 to 18 ft in the same condition. Over time gravity wins. To defy the pull, however, I always use a double girder and bring as much beef to bear as I can—within aesthetic reason. A double girder creates solid structure and it looks good. • Good looks and visual composition matter. You can stretch a double 2 x 6 across 20 ft, but it looks spindly. Go for a balance of muscle and grace. • The Southern Yellow Pine Council’s ceiling joist chart makes some sense for pergolas. It shows a single 2 x 10 (which is part of a 16 OC ceiling system) spanning 26 ft, so two of them should cover a pergola no fuss, no muss. • The Western Red Cedar Lumber Association—Peters’ is ceda
