Pergola Post, Beam and Rafter Sizes for a DIY Build

A freestanding pergola looks simple: four or six posts, two beams and a row of rafters. The hard part is that it stands alone in the open, with nothing to lean on. This guide covers the size of each part, how the frame stays upright and stays down in the wind, and why our plans leave the roof open.

• Is there a span table for pergolas?
• What size posts does a pergola need?
• How far can pergola rafters span?
• How big should pergola beams be?
• Why does a pergola need knee braces?
• How do you hold a pergola down in the wind?
• How big and how deep should pergola footings be?
• Why do our pergola plans have no roof?
• How do you plan your pergola?

Is there a span table for pergolas?

No. There is no official span table for pergola rafters or beams. So our plan engine borrows the closest official tables we know: the ones in the AWC deck guide, DCA 6.

Those tables size deck framing for 40 pounds per square foot (psf) of live load plus 10 psf of dead load. That is people walking, chairs and the deck boards themselves. An open pergola roof carries far less day to day, so a frame sized for deck loads is a conservative frame.

We also read the weakest species row in each table: redwood, western cedars, ponderosa pine and red pine. That way the same plan works with pressure-treated lumber or with cedar.

What size posts does a pergola need?

Our plans use 6x6 posts. DCA 6 sets 6x6 as the smallest post for a deck, and we keep that minimum for a pergola. A 6x6 is stiffer than a 4x4, and it gives the knee braces and post caps a solid base.

You pick the clear height under the beams: 7 ft, 7 ft 6 in or 8 ft. The beams and rafters sit on top of that, and the whole frame stays under 10 ft tall.

Each post stands on a post base bolted to the concrete footing. The Simpson Strong-Tie ABU66Z is one example: it holds a 6x6 post 1 in above the concrete and carries a rated uplift load. The gap keeps the post's end grain out of standing water.

How far can pergola rafters span?

The rafters run across the two beams. Our plans size them with DCA 6 Table 2, the deck joist table, for the weakest species group:

The span is the clear distance between the beams, not the full rafter length. On an open pergola, our rafters sit 24 in apart. When you add shade slats, they move to 16 in apart so the slats have support.

How far can rafters overhang the beams?

The overhang past each beam has two limits in DCA 6. It can be no more than the overhang in the table, and no more than one quarter of the span. Our plans draw a 12 in overhang and only pick a rafter size whose limits allow it. That rules out the 2x6, whose table overhang is under 12 in.

How big should pergola beams be?

Each line of posts carries one beam, built up from two or three 2x boards. The beam sits on the posts in a post cap. DCA 6 allows a notched post or a post cap, requires a post cap for every 3-ply beam, and does not allow a beam bolted to the side of a post without a notch.

Beam size comes from DCA 6 Table 3A. A beam's span depends on its size and on the span of the rafters it carries. Our plans read the Douglas fir-larch, hem-fir, spruce-pine-fir and redwood row:

This is the table that sets the number of posts. Our plans run from 8x8 ft up to 16x12 ft. The 16 ft pergola uses 6 posts, three on each beam line, so each beam span stays inside the table.

The beam may also run past the end posts. DCA 6 allows that overhang up to one quarter of the beam span. Our plans also cap it at 24 in, for looks.

Why does a pergola need knee braces?

A freestanding pergola has no house to lean on. Wind pushes it sideways, and the posts want to tip like a stack of dominoes. Something has to keep the corners square.

The post bases cannot do that job alone. Simpson's product data says post bases do not give enough resistance to stop a post from rotating about its base. It says they are not recommended where the top of the post is not supported, and it names fences and unbraced carports as examples. A pergola's posts are only as steady as the frame on top.

So our plans add knee braces. Every post gets a 4x4 brace at 45 degrees in both directions: one up to the beam and one up to the rafter above the post. Each brace has 24 in legs and one 1/2 in lag screw at each end. DCA 6 uses the same idea on decks: corner posts taller than 2 ft get a diagonal brace, lag-screwed to the post and the beam.

How do you hold a pergola down in the wind?

Wind does not only push a pergola sideways. It can also lift the top. So the frame needs a solid path from the rafters down to the ground.

Our plans build that path one link at a time:

1. Rafters to beams: a hurricane tie at every place a rafter crosses a beam. DCA 6 asks clips like these to hold at least 100 lb up and sideways where joists sit on a beam.
2. Beams to posts: a post cap at the top of every post.
3. Posts to footings: an uplift-rated post base bolted into the concrete.
4. Footings to ground: deep concrete footings whose weight helps hold the frame down.

Simpson's data puts the last link on the designer: the concrete and its anchorage must be designed for the uplift. That is why our footings go deeper than the minimum.

How big and how deep should pergola footings be?

Our pergola footings are round concrete, at least 12 in across and 36 in deep. A larger pergola gets wider footings: our plans size each one with DCA 6 Appendix B, Table B3, by the area of roof each post holds up.

The 36 in depth is our own conservative choice. DCA 6 asks deck footings to bear on undisturbed soil at least 12 in below the undisturbed ground, or below the frost line, whichever is deeper. We go deeper than 12 in so the concrete's weight helps resist uplift.

Frost depth is a local question. Ask your building department whether a freestanding pergola must reach the frost line where you live. If it must, and your frost line is deeper than 36 in, dig to the frost line.

Before you dig, DCA 6 has two reminders that apply here too. Keep footings off septic systems, leach fields, utility lines and enclosed meters. And contact your local utilities (call 811) first.

Why do our pergola plans have no roof?

Our pergolas are open frames on purpose. The rafters carry no roof panels, only optional 2x2 shade slats set 1-1/2 in apart. Those slats cut the sun but let rain, snow and wind pass through.

A solid roof would change the structure. It adds weight, holds snow, and turns the top into a sail in the wind. The deck tables we borrow do not cover that: DCA 6 does not apply where snow loads exceed 40 psf. A roofed frame needs its own design, so it falls outside these plans.

For the same reason, our plans keep the beams and rafters for the frame alone. The sizes in this guide are checked for an open pergola and nothing more.

How do you plan your pergola?

You now know how a freestanding pergola is sized: 6x6 posts on uplift-rated bases, a built-up beam on post caps, rafters from the span table, knee braces at every post, and deep footings. Check frost depth with your building department before you dig.

Our free pergola preview turns your yard size into a 3D model sized with the rules in this guide, before you pay anything. You can also compare pergola styles on the pergola plans page. Still deciding on shade? Our pergola vs shade sail guide compares cost, build time and weather limits.