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Pergola Rafter Span: Sizes That Hold Without Sagging

A 2x6 cedar rafter spans around 8 to 10 feet, a 2x8 up to 12 to 14. Span tables, spacing, beam sizing and when to get it checked.

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New softwood rafters and a ridge beam framed against the sky
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Spans for an open pergola follow comfortable rules of thumb. Add a roof, a swing or snow and those rules stop applying entirely.

Rafter spans, open pergola

rafterspacingtypical maximum span
2x616 in~9–10 ft
2x624 in~8 ft
2x816 in~13–14 ft
2x824 in~12 ft
2x1016 in~16 ft
2x1024 in~14 ft

Cedar, redwood and treated southern pine all behave broadly similarly at these sizes; Douglas fir is stiffer and will do a little better.

Beams and posts

beampost spacing
Doubled 2x8 or single 2x108–10 ft
Doubled 2x1010–12 ft
Doubled 2x1212–14 ft
  • Doubling two members with a spacer is easier to lift than one large timber and lets you sandwich the posts
  • 6x6 posts for anything above about eight feet tall, 4x4 only for small low structures
  • Metal post bases on concrete footings, not timber buried in soil

Depth runs vertically

A 2x8 on edge is several times stiffer than the same board laid flat, because stiffness scales with the cube of depth. Rafters sit on edge without exception; flat boards belong only in a decorative top layer carrying nothing.

Spacing for the shade you want

spacingshadelook
12 inDense at middayHeavy, substantial
16 inUseful midday shadeThe common default
24 inLight — mostly sunAiry, less timber

Adding a perpendicular top layer of 2x2 or 1x2 slats over the rafters at close spacing is the usual way to increase shade without changing the structure.

When to stop using rules of thumb

  • Spans beyond about 14 feet
  • Any solid, polycarbonate or louvred roof
  • Shade sails, which load the structure in wind
  • Snow load of any significance
  • Anything hanging — swings, heavy vines, hammocks, lighting rigs
  • Attached to the house, which almost always requires a permit

Common questions

How far can pergola rafters span?

As a working guide for an open pergola with no solid roof, a 2x6 in cedar or treated pine spans roughly 8 to 10 feet, a 2x8 around 12 to 14 feet, and a 2x10 up to 16 feet, at 16 to 24 inch spacing. Those figures assume no snow load and nothing hanging from the structure. Add a solid or louvred roof, a heavy vine, a hanging swing or real snow and the spans come down sharply — at that point it is a structural calculation, not a rule of thumb.

How far apart should pergola rafters be spaced?

Between 12 and 24 inches on centre, chosen mostly for the shade you want rather than for strength. Sixteen inches is the common default and gives useful midday shade; 12 inches gives dense shade and a heavier look; 24 inches is airy and uses less timber but admits most of the sun. Closer spacing also allows slightly longer spans for the same rafter depth, because the load per rafter drops.

What size beams does a pergola need?

Beams carry the rafters and span between posts, so they are deeper: a doubled 2x8 or a single 2x10 typically spans 8 to 10 feet between posts, and a doubled 2x10 or 2x12 handles 12 to 14 feet. Doubling two members with a spacer is both stronger and easier to lift than a single large timber, and it lets you sandwich the posts rather than relying on end-grain fixings. Keep post spacing to 10 to 12 feet unless the beams are sized for more.

Does rafter orientation matter?

Very much — depth must run vertically. A 2x8 laid on edge is many times stiffer than the same board laid flat, because stiffness rises with the cube of depth. Rafters therefore always sit on edge, notched or hung over the beams, never flat. Flat-laid top slats are fine as a decorative upper layer carrying nothing but themselves.

Do I need a permit or an engineer for a pergola?

Often a permit, and occasionally an engineer. Many jurisdictions require a building permit for any freestanding structure above a certain size or height, and almost all do for one attached to the house, since it becomes part of the building's load path. An engineer's input is worth having for spans beyond about 14 feet, for a solid or louvred roof, in a region with meaningful snow load, or where anything will hang from the structure. Check locally before buying timber.

What about snow and wind loads?

An open-slatted pergola sheds snow and takes little wind uplift, which is why generic spans work for it. The moment you add a polycarbonate roof, canvas shade sails or louvred panels, the structure collects both snow and wind, and everything changes — including post footings, which then have to resist uplift as well as carry weight. Treat a roofed pergola as a different structure from the start rather than as an upgrade to an open one.

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