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 ... more.

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